Archives For pressure

Imogen is the ninth named MetOffice storm this winter.  She formed in the Atlantic in an area of steep temperature gradients under control from an active jetstream.

 

Storm Imogen is deepening rapidly today to 953mb, though on arrival in the UK she will be occluding and filling gradually to above 960mb on her track over N Scotland into the North Sea on Monday. The exact track makes a big difference to where the strongest winds are.  Current trends are for the storm to pull wind fields further north so impacts could be less than expected. Keep an eye on the MetOffice forecast as things are likely to change. Below is an outline of Imogen’s likely activity:

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Unlike the previous eight named storms, Imogen has a more southerly track, guided by a more southerly tracking jetstream, and the field of strongest winds and heavy rain are possibly set to impact the densely populated southern part of the UK, including the SE. High waves are also expected on the Channel coast.

 

Strong winds on Sunday night will be associated with Imogen’s fronts running ahead of the depression.  The cold front is an active kata-front, associated with descending cold dry air from the stratosphere running ahead of the surface front and enhancing lift and potentially generating heavier rain and gusty conditions (image and info courtesy UKweatherworld).

On Monday gusts up to 80mph on the Channel coast are possible, while inland the MetOffice consider 60mph possible in exposed places.  Around Reigate and sheltered parts of Surrey, 40-50mph gusts are more likely.  The North Downs could see gusts approaching 60mph. The strongest winds for the SE are likely to be through midday and in the afternoon.

Yellow warnings apply to inland parts of Surrey and SE England while the entire Channel coast has an Amber MetOffice warning. The first impact will be frontal rain tonight.  Fronts passing through overnight into Monday could drop over 20mm of rain in places particularly linked to the occluding “triple point” forecast to cross the SE overnight.

 

Monday is likely to see showers, some heavy, appearing through the day.  Warm sea surface temps in the Channel are likely to cause more on the coast but the brisk winds could bring them inland as the day progresses.

The cause of the strong winds behind the cold front on Monday is a steep pressure gradient.  On Monday tightening isobars show the steep pressure gradient bringing gusty showery conditions in unstable Polar Maritime air behind the cold front later on Monday.

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The Wight-Wash Oscillation (WWO) measures the pressure difference between The Wash and the Isle of Wight and is designed as a guide to wind speed in the SE corner of the UK. The WWO on Monday shows a significant 16mb gradient between The Wash and the Isle of Wight on the WRF model.  The Euro4 model has a more modest 12mb WWO.  16mb would be the largest WWO pressure gradient recorded and greater than St Jude, which was 12mb.

On Tuesday models show a wave depression bringing more rain to the SE, some even show fleeting wintry precipitation on the back end of this low as colder air ingresses from the north.  This is unlikely to be significant, at least on Tuesday, as upper air temps remain mostly too high for snow in the SE.

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ECM colder flow mid-week

Colder conditions are preferred by the ECM as northerly winds bring cool polar air further into the country through mid-week.  The Arctic Oscillation is again going negative which shows pressure rising over the Poles trying to push Arctic air south into mid-latitudes.  However, the NAO remains positive so Atlantic depressions will continue to bring frontal depressions for this week.

The 8-10 day mean shows a deep trough over the UK meaning low pressure and unsettled conditions remain likely into half term.

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The ECM builds heights over southern Greenland which links with higher pressure over the Atlantic, a more northerly feed of cold polar air is likely in this scenario into half term . The ECM has been outperforming the GFS so the more Atlantic driven GFS chart would be the less favoured option.

The Sudden Stratospheric Warming going on over the Pole is another astonishing feature of weather at the moment.  Today (Sunday) temperatures in the stratosphere over Siberia has got up to an amazing +12C from a more usual -70C.  SSW events often build pressure over the Polar troposphere a few weeks later which can cause cold incursions into mid-latitudes.  This is by no means certain but is perhaps our last chance of any sustained cold this winter… if it were to happen it would be late Feb/March. One to watch!

A system stirred up by a low pressure tracking out of Mexico into the Gulf of Mexico this week is on track to bring unsettled conditions for part of the weekend to the UK.  It’s nothing too severe for most but is an interesting feature that will bring some wind and rain everywhere.  Below is a satpic showing the development of this system as it interacted with a lively jetstreak on Saturday 28/02/2015.

development of LOW on jetstreak

development of LOW on jetstreak

 

This LOW illustrates nicely how extra-tropical systems can rattle clean across the Atlantic in a few days if they are picked up by an active jetstream.   This one does precisely that.  Spot the system leaving Florida on the chart below for today and the sat pic.  This system started as a low pressure crossing from Mexico into the Gulf of Mexico mid-week, so for weather systems it will be an aged fellow on arrival here in the UK.  Its’ longevity is partly due to the exceptional COLD over NE US which interacts with the warm tropical air and causes further deepening.

The Gulf low pressure is tracking quickly NE skirting the US east coast before being picked up and deepened further by an active jetstream.  The jetstream itself is particularly powerful at the moment due to intense cold spilling out of an exceptionally wintry NE USA meeting warm tropical air issuing from a strong subtropical Azores HIGH pressure converging with the moist Gulf airmass.  A result of the powerful jetstream is a positive North Atlantic Oscillation: the NAO is a measure that indicates the difference in pressure between Iceland and the Azores.  In positive NAO conditions the jetstream is often active, producing a strong westerly zonal flow keeping Europe mild and unsettled especially in winter, or early spring!

Our Gulf LOW is due to pass over Reigate fairly rapidly through Saturday pm and overnight into Sunday am and bring some moderately wet and windy weather, likely to go unnoticed because of the nocturnal transit.  Winds gusting in excess of 40mph are possible for Reigate into Sunday am in exposed places.  Notably, due to the TROPICAL origins of this airmass the temperature overnight Sat-Sun could climb to double figures in Reigate.  Tropical air crossing the Atlantic also picks up a tremendous amount of moisture so attendant fronts are likely to bring a lot of rain too, possibly exceeding 10mm overnight, which is a moderately wet night.

Here are the synoptic charts showing the Gulf low progress across the Atlantic, deepening and occluding into the North Sea.  Note the secondary wave which could bring additional rain later on Sunday to the South.

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The additional rain later Sunday afternoon / evening looks potentially heavy for the South and SE.  It’s a rapidly developing wave feature that needs attention as, on the northern edge, it looks to raise the possibility of snow across the Midlands.  Heavy rain is possible for the SE and #Reigate with a period of gales on the south coast.

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The outlook for next week is for the Azores HIGH pressure to extend a ridge to the north and cause a NW then northerly airflow for the UK.  This will bring cooler temperatures to the UK.  Whilst it is likely to be mostly dry for Reigate and the SE with pressure rising, wintry showers especially on east facing coasts of the North Sea could be possible depending on how the HIGH develops.  Frost is likely with temperatures dipping below freezing at night from mid-week.

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Atlantic ridge builds to the west and brings in northerlies

How long this Atlantic block persists, and the cooler weather, is uncertain.  The coldest scenario would depend on the HIGH moving north and east and building over Scandinavia to pull in easterlies from a cold continent.  This scenario is preferred by the ECM by later next week whilst the GEFS topples the high to the SE and brings back a zonal mild westerly flow from the persistent Icelandic LOW pressure that erodes the edges of the HIGH from the NW.  The charts below show the uncertainty as a wide spread of possible pressure and temperature towards the end of the first week in March.

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Gulf low for the weekend, then pressure builds

The cool start to March is shown below.  The overall outlook is for a persistent positive NAO and Arctic Oscillation to persist and this would suggest a brief cool epsiode without the formation of a persistent Scandinavian High.  Models have flirted with possible easterly winds by the end of next week but the outlook is for the positive NAO to persist and this rather suggests a quick return to milder zonal westerlies.  As the high builds in early in the week various troughs and fronts could even push some wintry precipitation as far as the SE on Tuesday (spot the pink on the rainfall chart below for Tues)

First week of March starts cool: MetOffice synoptic chart for mid-week shows Azores high briefly ridging north to block mild zonal westerlies and usher in a cold polar airmass, albeit briefly as this ridge looks to topple SE and by next weekend we could be in a pretty mild SW flow hitting mid-teens possibly.  So a cool, mostly dry middle part of the week for Reigate and much of S England but precipitation, some even wintry, pulling in on NW / N winds is not ruled out with a North Sea low possible.  As the Atlantic is likely to push westerlies back in later in the week we can expect more purposeful frontal rain pushing east across the whole country.

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link to accuweather take on this system

http://www.accuweather.com/en/weather-news/winds-to-whip-uk-north-sea-coa/43063930

 

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HIGH pressure dominates but is it all calm?

High pressure is known for calm, clear conditions, with little wind, cold frosty and foggy nights especially when there is little cloud. Pretty unexciting weather.  However, HIGH pressure is not as unexciting as all that.  Anticyclones can sometimes be surprisingly windy especially round the edges.  We spend a lot of time learning about LOW pressure, with associated storms and gales and torrential rain but understanding the inner workings of HIGH pressure is important to get the full picture of mid-latitude weather.

So… buckle up for the ride and let’s get super-geostrophic!  Wind blows from HIGH pressure to LOW pressure.  The wind speed and direction is the result of two forces: the pressure gradient force (PGF) is the difference between high and low pressure and sets up the strength of the wind and the overall direction which is for winds to blow directly from HIGH to LOW pressure.  Coriolis force (or Coriolis Effect) is a result of the spin of the Earth and deflects resultant winds to the right of their intended path in the northern hemisphere.  Here are some video links to review these forces before proceeding with super and sub-geostrophic winds. Skip below these videos if you already know about PGF and Coriolis.

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The winds do blow from high to low… but get pushed to the right by that Coriolis fellow!

The pressure difference between high and low pressure determines the speed of wind.  Winds do blow from high to low due to the pressure-gradient but are deflected to the right by another force called the Coriolis effect! Below is a chart showing upper winds at 850hPa (1500m) blowing round the same HIGH pressure shown on the synoptic chart at the top of the post.  Note the relatively high wind speeds circulating round the HIGH in the north of Scotland, the North Sea and across France and Biscay especially.  Winds obviously blow faster across the ocean but remember this is an upper wind chart so is above the boundary layer of most frictional forces upsetting the wind.  In any case, none of these locations is associated with a trough… it is all anticyclonic super-geostrophic wind.  So why is the wind blowing so strong when there is no LOW for miles?

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Given the same isobar spacing the wind speed aloft round high pressure ridges is often greater than the wind flowing around troughs and low pressure. This is surprising because we associate gales and windy weather with “storms” and low pressure systems.  The chart above illustrates super-geostrophic winds circulating around the Azores high across Europe.  These look pretty strong at 850hPa (1500m), the level above frictional effects of the surface.  The chart also shows the trough of low pressure over the Mediterranean where, given some of the locations with similarly spaced and even tighter isobars, the wind strength is not especially any greater and perhaps even less than that circulating freely around the HIGH.

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Wind is a result of pressure differences across the planet surface.  Wind wants to blow from high to low pressure.  This is called the pressure gradient force.  Due to the spin of the Earth winds in the northern hemisphere are deflected to the right of their intended path.  The two forces, pressure gradient and coriolis force, actually balance out to produce a theoretical wind that flows parallel to the isobars called the geostrophic wind, shown above. Unfortunately, isobars are almost always curved so the geostrophic wind hardly ever actually blows.

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Assuming a constant isobar spacing.  Around troughs of LOW pressure the wind is sub-geostrophic. This means it blows less than the expected geostrophic wind.  In the chart above the wind is shown as a black arrow.  In addition to the coriolis force, the centrifugal force acts to “push” the wind away from the low centre and is acting in the same direction as the coriolis force.  Note that the resultant wind is pointing slightly away from the LOW towards the HIGH, which is of course not possible because the wind would be moving into and against increasing pressure.  As the pressure gradient force cannot change, the coriolis force must weaken to allow the wind to return parallel to the isobars.  This means that the wind flowing around troughs of LOW pressure has reduced force acting on them given the same isobar spacing of a similar HIGH. These winds therefore blow slower than geostrophic wind and are called SUB-GEOSTROPHIC.

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Here is the HIGH pressure situation.  This time the centrifugal force is acting with the pressure gradient force to push the wind into low pressure.  As the pressure gradient cannot change the coriolis force must INCREASE to pull the wind back parallel to the isobars.  This means that the wind flowing around ridges of HIGH pressure has GREATER forces acting upon them than winds flowing round lows with equivalent isobar spacing.  These winds therefore blow faster than geostrophic wind and are called SUPER-GEOSTROPHIC.

Usually, of course, low pressure cyclones and depressions exhibit tighter isobar spacing than HIGH pressure and so resulting wind speeds round LOWS are most frequently higher than the HIGH pressure feeding them.  Nevertheless, assuming the same pressure-gradient force, winds exiting anticyclones can produce higher wind speeds than those entering depressions.

useful reference

http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/fw/grad.rxml

http://www.metoffice.gov.uk/learning/wind/what-causes-wind

http://education-portal.com/academy/lesson/factors-that-affect-wind-pressure-gradient-forces-coriolis-effect-friction.html

Is this it for winter? Read on…

Reigate stays cool for the first week of February after a couple of modest wet snow non-events overnight this weekend.  Snow thawed in the morning each day as temperatures climbed above freezing.  Whilst the air temperature was comparatively high at 5.8C Tmax at midday, the stiff northerly wind in Reigate was gusting to 30mph in town which brought wind chill as low as -5C at times.

The cause of the current cold weather is a northerly wind set up due to HIGH pressure ridging north in the Atlantic and a LOW over Scandinavia. This sets up a northerly flow, called an Arctic airmass, albeit modified by its significant journey over relatively mild seas.  Also, this particular Arctic airmass is not direct from the Pole, if you follow the isobars back from the UK you can see the air originating from southerly areas in SE Europe, so not truly polar.  In any case, it’s usual for Arctic airmasses to bring dry weather to Reigate and the SE: the long transit over land means it lacks moisture, usually dumping any significant snow over NE facing coasts well before it gets here.  More locally, our sheltered location beneath the North Downs, facing south, also affords good protection from Arctic airmasses and N or NE winds.  So, either way, Reigate rarely gets lots of snow from this airmass.

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arctic airmass

This week is likely to see further cold weather continuing as the HIGH slowly nudges in from the west by mid-week.  With HIGH pressure not far away and a relatively dry northerly airmass, a major snow event or indeed much precipitation at all is unlikely.

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So as pressure rises we can expect a cool mostly dry week with frosty nights and possible fog on occasions in lighter winds (fog not really ’til end of week though as wind remains significant running round high). In this set up a big snow event for Reigate seems most unlikely.  Nevertheless, an easterly / NE’rly wind for a time is a strong possibility, initially Tues-Weds as a front moves south, and so modestly disruptive snow showers reaching us cannot be ruled out.  Also, don’t forget icy roads and fog are arguably the most risky of all winter hazards so this kind of high pressure wintry weather should be handled with care if travelling.

At the moment  GFS, Ensemble and ECM models are agreeing that the HIGH pressure is likely to land up sitting somewhere to the north / NW of the UK by the end of the week.  With unusually LOW pressure in the south of Europe, this could set up a cold easterly wind especially across the south east, albeit this might yet not come off as other runs show the HIGH right over the top of the UK cutting off any easterlies very quickly.

The exact position of the HIGH will therefore make a big difference to whether we get much or any precipitation.  NE’rly or easterly winds, depending on their strength and track, can bring snow off the North Sea and inland into Kent and Surrey as well.  There is already a MetOffice yellow weather warning for the possibility of such an event mid-week, although these often don’t come to much they should not be ignored as the potential for perky snow showers causing local traffic problems has already been experienced with one minor brief thundery shower wintry episode last week.

The time-averaged charts show the overall story for the next week as being dry and colder than normal.  Throughout this episode the jetstream is wrapping round to the north of the HIGH, actually building it with milder upper air from the SW, and, eventually, it could help to deliver our easterlies at the end of the week.  As usual, for exact details check the twitter feed and consult official sites like metoffice for decision making.

refs

http://www.metlink.org/wp-content/uploads/2015/01/arctic_maritime_jan15.pdf

Update #2 25/12/14: update: cold weather arriving after this LOW, heavy rain overnight Fri-Sat; snow marginal for SE early Sat am, more likely for Midlands and EA, cold weather arrives in lee of this system.  MetOffice warnings updated:

Update #1 25/12/14 latest MetOffice chart lifts pressure and pushes track further south, with low moving SE across our area.  This reduces wind speed, still brings in colder air flow though with risk of snow increased for back northern edge of the system with NE winds. For SE possible sleet/snow on Downs early Sat am. Evaporative cooling could yield more snow for SE if rain sufficiently heavy (drags down cold uppers). Gale risk gone but replaced by some heavy rain, marginal snow risk and retaining the cold easterlies in the aftermath on Saturday with pressure building to dry bright frosty conditions.

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After a pleasantly cool bright and dry Christmas Day, an interesting depression due on Friday and through Saturday is likely to usher in a period of colder weather for the UK and SE in particular. The situation is a little uncertain still but the run of warm mild gloomy temperatures lately this December, already pushed aside gently by a weak cold front passing south through the country today, are likely to be pushed further down into some “proper”cold after the storm passes through by Sunday. This storm, forms in the Atlantic along the polar front and quickly races east towards the UK on Boxing Day Friday.  Storms tend not to deepen much if they move fast, which this one does at first: crossing half the Atlantic in a matter of 24 hours. The storm is mixing some airmasses with contrasting temperatures: cold polar air in the north is about to get up close and personal to mild warm Tropical air from the south west.  They are due to meet in the LOW pressure over the UK soon, so expect some interesting weather!  You can spot the impact of the storm on the upper air temperature chart below but also see the steeper drop to colder conditions thereafter.

GEFS shows cooler days ahead

GEFS shows cooler days ahead

The ECM charts below show upper air temperatures at around 1500m. These “850hPa” charts are commonly used as guides to airmasses because air at 1500m (850hPa pressure level) is not affected by changes day and night or surface characteristics, it is therefore a good guide to true airmass characteristics.  Note the really cold airmass to the north meeting comparatively warm air to the south and SW in this LOW.

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For the South of England the LOW will initially push warmer tropical air ahead with rain arriving for us in the SE on a warm front sometime Friday pm (top diag above Sat 00hrs).  The warm sector is likely to be windy with gusty SW winds and a considerable accumulation of rain, 10-20mm overnight into Saturday.  The warm sector tropical air mass (upper air +5C) could have temperatures near double figures whilst the polar air bearing down from the north is a much more frigid airmass (upper air -6C).  The contrast between these two airmasses could make the frontal rain particularly heavy while the cold front contrast could even have an odd rumble of thunder as cold air undercuts the warm and forces it aloft.  The skew-t diagrams below show the contrast in these two airmasses.

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The LOW centre crosses the North of England and into the North Sea overnight into Saturday when, due to it’s location under the left exit of the jetstream, it is forecast to deepen to possibly around 980mb. quite low especially for a depression located so near the shore.  Deepening occurs as the jetstream aloft encourages air to rise off the surface because air is diverging aloft.  So air is rising off the surface quicker than it can be replaced by air arriving: hence falling surface pressure. This commonly occurs when lows interact with jetstreams on their left hand side, near the exit of a jetstreak.

The classic frontal depression with cold and warm fronts separated by a warm sector only lasts for a matter of hours before the cold front, pushing forward more dynamically than the warm, catches up the warm front and pushes the remaining warm air into the upper atmosphere.  This is an occlusion and signals the end of the development stage of a depression.  The central pressure usually starts to rise after occlusion has occurred.

Whilst the situation is still uncertain, it is likely that Friday afternoon and Saturday will be windy and increasingly cold as the winds veer clockwise from the SW through to North and finally NE and E.  It is the latter NE and E winds that will bring the colder air to the UK and the SE especially.  Continental Europe is currently very cold so any air flowing from this direction will be chilly.  Cold crisp continental air will stay with us for a while as high pressure builds to the west and pushes north over Scotland while the LOW moves over Europe.  This setup allows easterly winds to flow over the UK.  Dry cold is expected as the pressure is likely to rise and stay high.  Expect some frosty nights. The duration of the HIGH varies between models but certainly should keep things cold and crisp through to the New Year.

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cold new year

Further ahead a split in the polar vortex and stratospheric warming are dominating weather chat and these are set to possibly bring colder conditions through January.  On the other hand, Phase 3-4 of the MJO (Madden Julian Oscillation) is usually associated with a positive North Atlantic Oscillation which brings milder westerlies to the UK.  So, it’s interesting times ahead, stay tuned and Happy Christmas!

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The MetOffice charts above show the formation and life-cycle of a December 2014 “weather bomb”, involving the process more properly called rapid cyclogenesis. There are reasons why meteorologists dislike the term “weather bomb” but perhaps the most obvious is that the sensationalist short-hand use of the term “bomb” detracts from the complex processes and variable scale and location of impacts.  The term “bomb” tends to hype stories in the press that can cause over-reaction and unnecessary concern. On the other hand it gets people reading about the weather, which is a good thing (like this post, ahem!).

Nevertheless, a “weather bomb”, a term borrowed from the US and New Zealand, is short-hand for a potentially extreme event.  Bomb depressions are deep low pressure systems that form by the process of Rapid Cyclogenesis (RaCy for short).  RaCy is the rapid formation of a deep depression when the central pressure falls more than 24mb in 24 hours.  Such RaCy depressions are usually of marine origin. About 12 such RaCy bomb depressions hit the UK in the exceptionally stormy winter last year 2013.  Although by no means the most powerful, the first and most famous RaCy depression of last winter was the St Jude storm that hit Southern England with moderate force in October 2013.  Pictures below are from that event and can be compared to the enormous scale of the more recent Atlantic bomb depression of December 2014.

The “bomb” depression that struck this December 2014 seemed to catch media attention, despite the impressive weather impacts being almost wholly restricted to the less populated NW, especially Scotland, where people are entirely used to coping with such lively weather.

http://www.stornowaygazette.co.uk/news/local-headlines/weather-bomb-cuts-off-power-to-18k-homes-1-3630321

December 2014 rapid cyclogenesis: the weather story

The December 2014 “weather bomb” was a depression (low pressure system) which formed rapidly far out west in the Atlantic between SE Greenland and Iceland.  The formation was associated with a fast moving jetstream and the surface convergence of sub-tropical air from the south west meeting a frigid NW polar airstream from Canada and more local air direct from the Greenland ice cap.  The big temperature differences between these air masses accelerated uplift and the lowering of central pressure.

impressive but not the day after tomorrow

impressive but not the day after tomorrow

Descending dry stratospheric air is another defining feature of RaCy systems.  Cold dry air from aloft turbo-charges the depression as it is injected into the depression.  The cold air aloft increases lapse rates in the surface airmass and causes air to rise more purposefully creating a dramatic fall in central pressure.  Descending cold dry stratospheric air can be spotted on the water vapour satellite images as a dark dry slot ingressing into the depression circulation over time and following hard on the heels of the cold front as it is blasted across the Atlantic.  The water vapour images below show the rapid development of the system during Tuesday 8 December.  In later images it is possible to see the speckly cumulonimbus clouds emerging in the unstable cold sector following the cold front. Such instability was caused by the descending dry air.

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Rather than going through the rather measured development stages of a Norwegian Model cyclone, a RaCy depression usually follows a life cycle more like the Shapiro-Keyser model below (though at the time of writing I am not certain as to whether the December 2014 RaCy depression formally fitted all aspects of this model).  Several key characteristics of the December 8 cyclone fit the S-K model fit and this is the usual model associated with RaCy depressions.

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The Shapiro-Keyser depression life-cycle model often features a cold front that is blasted rapidly ahead.   so rapidly that it “fractures” from the wrapping warm front further north. This is known as a T-bone fracture and experts can identify the moment of fracture using satellite photos. Additionally, cf course, upper air moves faster than the surface wind that suffers frictional drag even across relatively smooth ocean.

satellite features of emerging RaCY depression

satellite features of emerging RaCY depression

This meant that the cold front moved so rapidly that it split vertically into a fast moving upper front and a slower moving surface cold front. The cold front literally had its head ripped off!  The frigid upper cold air travelled over a shallow moist zone of warmer sub-tropical air and it is this that increased lapse rates and caused immense instability in the polar air stream that eventually arrived in Scotland.  Instability can be seen on the visible satellite pics as speckly masses of cumulonimbus clouds shown best in the satpic above.  In the charts and sat pics below note the wind speed associated with this polar air and the tropical air preceding it in the warm sector.

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In the S-K model the cold front is sometimes weakened during the formation process while the warm front remains active, wrapping itself in knots around the central “eye” of the storm.  The 850mb chart below shows temperatures of this cold upper air at 1500m above Scotland. The bomb depression this December seems to have matched this because, while the cold front was relatively weak (narrow squall line) the exceptionally unstable polar air behind it was arguably the defining characteristic of this system, bringing persistent convective storms and an outstanding 5000 lightning strikes and thunder-snow blizzards across higher ground in Scotland during the advection of this exceptionally cold and unstable air for an Atlantic NW airstream.

In the S-K model depression life-cycle the warm sub-tropical air is eventually left “sequestered” as a warm pool trapped in the middle of the mature depression which is called a “warm seclusion”.  The usual process of occlusion is bypassed as the centre of the low fills with warm air.  Meanwhile, the rapidly overshooting upper cold front causes S-K cyclones to often elongate in appearance on surface pressure charts, a feature associated with the rapid forward acceleration of the cold front in relation to the tightly wrapped, almost stationary, wrapped warm front. It is this tightly wrapped warm front (sometimes shown as occluded on weather charts) that shows another defining feature of S-K depressions.

As our initial bomb LOW pressure moved due east and filled and decayed offshore near Norway, a wave depression further south on the Polar Front also “bombed-out” to the SW of the UK and swept across Southern England on Thursday-Friday 11-12 Dec.

This was a separate small scale system but technically another rapid cyclogenesis as central pressure fell more than 24mb in 24 hours, but only just.  This illustrates the varying scale of bomb cyclones: some cover vast areas, some a small.  The 11-12 Dec RaCy depression was much smaller in size and intensity, max wind speeds were much more restricted and the whole system several magnitudes smaller in scale than the “mother” cyclone further north. Charts below show the evolution of this storm.

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Finally, the North Atlantic Oscialltion is a measure used to describe and forecast the mean pressure pattern over the Atlantic. A positive NAO indicates “normal” conditions with low pressure over iceland and high over the Azores. This is associated with a zonal west to east flowing jetstream and fast moving cyclones moving rapidly west to east bringing generally mild conditions to the UK in winter. Note the recent positive pattern matching the westerly flow and active zonal jetstream causing the RaCy depressions.  When the NAO turns negative the jetstream is often more wiggly and flows between latitudes in a more meridional flow potentially bringing cold air from the north when pressure patterns are more slow moving and even “blocked”.  A negative pattern is often associated with cold winter weather for the UK. The NAO is not a driver of weather, merely an indicator of pressure patterns.

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For a bit of fun we invented our own local Wight-Wash Oscillation (WWO) which is a measure of pressure across the south of England between The Wash and the Isle of Wight.  This would give an approximately similar local version of the NAO but just for fun!  We noted a WWO difference in pressure of 10mb during St Jude and only 9mb during the recent bomb wave depression.  The WWO particularly suits the passage of wave depressions across the Midlands which tend to yield the highest wind speeds for the SE.  It would also work in negative conditions which would give cold easterly winds in winter. Note this measure is just for fun!

Positive NAO remains likely on the run-up to Christmas 2014 so chances of a White Christmas is much reduced. Remember that a White Christmas for us in SE England is the rare exception to the rule.  On a brighter note, the earliest sunset has just passed and we can at least look forward to later sunsets from now on!

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A dedicated team of students from Reigate Grammar School weather station successfully launched a high altitude balloon (HAB) and retrieved the payload on Tuesday 25 November 2014.   After months of meticulous planning and with close cooperation and assistance from the scientists at the Met Office, Met Research Unit, Cardington Boundary Layer Research Facility and in collaboration with Chris Hillcox Near Space Photography, our students launched the 800g balloon and then used GPS trackers to chase and locate the payload as it fell to earth after an extraordinary 3.5 hour flight collecting some of the best amateur high altitude balloon photos of near space.  The students and staff were delighted by the success of their mission which took careful planning and preparation to reduce the risk of failure, which always remained a possibility.  The balloon took 2 hours to ascend 100,000 feet (30km) into the stratosphere, through temperatures as low as -70C in the tropopause, before the balloon burst at pressures 100x lower than at the surface.  The descent took over an hour.  Videos of the story can be found below.  See below also for “Why did we do this?”

Planning

Students spent weeks prior to the launch designing and building a lightweight box (150g) that would carry the payload safely to an incredible altitude up to 30km into the atmosphere.  The payload consisted of several HD cameras, spot GPS trackers and a GSM locator with meteorology sensors which were weighed to ensure the balloon would be able to lift them to the target burst height at 30km (100,000 feet) with 3.5m cu of helium.  All equipment was thoroughly tested (several times).  GPS trackers were tested to ensure phones would pick up the track reliably.  A laptop was enabled to operate on a mobile connection.  GoPro cameras were tested and checked, as were HTC cameras.  A school logo was laser cut onto a small plastic arm that would sit in front of one of the cameras during the ascent.  CAA approval was applied for and received.  Scientists at the Met Office research unit were also closely consulted regarding gas type (99.5% helium, not party grade!) and the practicalities of the launch location such as trees and masts.  Risk-benefit assessments based on MetOffice launch assessments were completed and approved. Various risks needed to be considered beyond allergies to latex and the dangers of helium inhalation,  particularly regarding student involvement in retrieval that might require travel across land that lacked nearby public access.  We checked and adjusted school insurance to ensure full coverage for the flight.  However, in the end, it was the good will and enthusiasm shown by the individuals involved with the student launch that drew the threads together and allowed us to proceed with this unique educational challenge.  Acknowledgements are listed below.

Predicting the right weather 

To avoid the balloon payload shooting off across the North Sea we had to choose a day when the jetstream and higher altitude winds in the stratosphere were relatively slack.  Winds in different directions with height would also be useful to ensure the balloon did not travel too far for retrieval purposes.   A single date could therefore not be fixed in advance due to changing autumnal weather conditions.  Parents of students were sent an open letter with details regarding the potential for this expedition and ways we would contact them to ensure students were available as any possible launch date approached.  The weather at all altitudes was studied carefully and a potential launch date emerged for Tuesday 25 November as a weak high pressure pushed the jetstream and high altitude winds further north.  A suitably safe landing site was also critical to mission success and the HABHUB website landing predictor was regularly consulted in the run up to the launch.  Conditions improved throughout the period until Tuesday looked the most ideal.  We were GO FOR LAUNCH!

 The launch

The launch day started early as the team had to travel well beyond the flight paths of Gatwick and Heathrow.  Our launch site was in Bedfordshire with a predicted landing site NE of Cambridge.

Amanda Kerr-Munslow, a Met Office Boundary Layer Research Scientist, gave the group a very interesting tour of the site, including lasers that measured cloud height.  Dave Bamber the Met office facilities manager then helped students inflate the balloon.  The payload box had last minute modifications completed and was then prepared and carried to the launch bed.  The balloon was carefully transferred, inflated, to attach to the payload at the launch site.  Cables checked, final preparation and then the countdown!  The pictures below show some screenshots captured by the movie cameras on the ascent to 100,000 feet.

short film: (FULL FILM scroll below)

(please refresh the page if video is from another playlist)

Payload retrieval

The chase and retrieval were very exciting!  Despite predictive forecasts showing a broadly good landing area (open farmland, flat, few forests, rural, few pylons etc) it was not certain that it would avoid landing in problem locations like hedges or trees, pylons, main roads, rivers, ditches, inaccessible fields remote from a road or someone’s roof!  It was an exciting moment for the team when the tracks stopped at Wicken Fen in Cambridgeshire and the maps appeared to show a landing site in an open field which appeared to be accessible from the road.  Reflective vests on and trackers in hand we rushed off to search and found the payload with the shredded remains of the burst parachute precisely on target.   A wonderfully memorable moment after an exciting and highly rewarding adventurous day for everyone involved.

Why did we do this?

It has been a long-term aim of the RGS weather station to launch a HAB.  Reasons are myriad but the experience of this has been a high quality challenging learning and teamwork experience for students and staff. Practical skills, meteorology and an understanding of the atmosphere were all critical for students to grasp to increase the chances of a successful launch and, importantly, retrieval.  The students more than rose to the challenge!  At RGS the sky is not the limit!

A driving force behind this enterprise has been to raise climate and weather awareness amongst students and more widely.  The images and story brought back beautifully illustrate the fragile atmosphere in which we all live and upon which the human race depends.  From our balloon view the atmosphere was shown as a thin blue veil of gases held precariously against the surface with the blackness of space directly above and not far away.  It is of course widely accepted that humans are forcing climate change which might potentially threaten our existence because we have altered the workings of the delicate atmospheric envelope.  The site and sound of planes cruising far below our balloon demonstrated how we might dominate a tiny hospitable part of the planet but that we reside perilously close to utterly hostile environments.  In this respect it is no surprise that we have probably damaged this delicate life-protecting atmosphere with our activities.  We would like anyone using our pictures to ensure that climate change is explicitly mentioned and that our balloon launch post is linked to any material used.

What next?

This project has proved that the stratosphere is closer than ever and increasingly accessible. School exploration of the stratosphere is now within the budget of schools or groups of schools that can collaborate (rather timely considering recent political announcements).  Currently it costs around £200 to launch a payload to 30km, to launch a further 30km above that would cost £200 million (Antares rocket cost)!  The exploration and science that schools can do at 30km is breathtaking! Floating balloons are also possible for longer experiments.  The IPCC consider that our climate is on the cusp of irretrievable change that will have great impacts in the next 100 years.  It is therefore of the utmost importance that schools engage their students with weather and climate and that exam syllabuses include meteorology and climate across a variety of disciplines.

costing the earth: different missions

This video of the flight has been edited from 6 hours of footage from 3 cameras pointing in different directions (GoPro Hero3, 2 HTCs) over the course of the 3.5 hour flight.  It also includes screenshots and photos showing the story of how students put their payload into near space.

(please refresh the page if video is from another playlist)

11 video questions for classes: (ask “why” / “how” for extension)

  1. Why is the flight path not straight?
  2. At what height do clouds stop?
  3. It rains ice particles from blue sky at some points in the flight.  Why might this happen?
  4. Does all rain that falls out of a cloud reach the ground?
  5. Are any clouds man-made?
  6. Listen to the wind and noise picked up by on-board camera mics during the flight: how does it change?
  7. How does the colour of the sky change with altitude?
  8. How “near space” actually is 100,000 feet (30km)?
  9. Which was the coldest part of the ascent?
  10. As the balloon burst there was a puff of “smoke”, what was this?
  11. Shreds of burst balloon were seemingly left hanging above the descending payload.  What do you think happened to these?

RGS launch team:

  • Piers Rex-Murray
  • Tom Tatham
  • Jasmine Hull
  • Louis Chambers
  • Edgar Povey
  • George Beglan
  • Harry Persand
  • Chris Meredith
  • Fraser Cadman
  • Matt Taylor

RGS staff:

  • Simon Collins
  • Peter Klein
  • Vanessa Ramsden

Acknowledgements and sincere thanks to the following:

Huge sincere thanks to the following for helping us to get our HAB project off the ground.

  • Near Space Photography : Chris Hillcox Chris was incredibly helpful in giving students ownership of their launch in all stages whilst offering any amount of technical know-how we needed to increase chances of success. Chris gave his time extremely generously throughout the enterprise and his expertise was pivotal in the success of the mission.  Many thanks Chris!
  • Royal Meteorology Society RMetS : especially Sylvia Knight who encouraged us to pursue the project and put us in touch with Amanda. Thank you Sylvia!
  • Met Office, Met Research Unit, Cardington : Boundary Layer Research FacilityDavid Bamber and Amanda Kerr-Munslow were on the launch site at Cardington.  Amanda gave us a thorough tour of the scientific research unit and assisted with all practical aspects of getting it organised at Cardington.  David was a hero for staying up all night on an IOP and then staying on to manage students in inflating the balloon.  Many thanks Amanda and David.
  • RGS DT department: thanks to DT department for their workshop space and the free use of their tools and being so patient when we left polystyrene all over the place.  Thanks to Martin especially for helping us with the lazer logo cutting.

Interested in a HAB launch or weather stations for schools?  Contact RGSweather!

short video version

news:

http://www.surreymirror.co.uk/Reigate-school-sends-weather-balloon-near-space/story-25116859-detail/story.html?ito=email_newsletter_surreymirror

Things have changed a bit since the previous post so a quick update is needed for race day.  Expect a final update Saturday night!

The unseasonable humidity and thundery showers for the end of the week and Saturday are set to be “blown away” on Sunday and replaced by ideal weather conditions for runners.  The charts below show a big difference in Tmax temps between 26C 22C (updated) for parts of the SE on Saturday (itself depending on cloud cover and showers) and a cooler 16C on Sunday.  The reason for this marked fall in temperature is a re-building HIGH pressure to the north which is introducing cooler air from a northerly direction to replace the humid continental air flow of the previous week. This change was expected but has arrived a day earlier than modeled previously.  So Sunday is drying up and cooling down, perfect weather for the day! More details below:

Race day is likely to start with lingering cloud but it will brighten up slowly, albeit remaining cloudy through the morning.  Some models keep light rain for a while but any remaining lingering showers should move away early.  Humidity will also fall, from 90% RH at race start to around 70% RH by midday,as a drier cooler clearer northerly air mass quickly replaces the humid continental air of previous days. The fresher upper air will be wrapping around the HIGH pressure building to the north and arriving in Reigate as a Northeasterly breeze.

Early on, temperatures of 13C will make for a fresher start for runners.  Sedentary early-bird spectators might begin to feel a little cool at the start after the relative heat of the previous days, especially in places open to the light NE breeze.  An extra layer might be a good idea for spectators. The day is set to warm up steadily to a pleasant Tmax 16-17C, ideal running conditions.

Though never a real nuisance inland, a gentle breeze is set to pick up to 11-12mph from the NE by mid-morning and there might be low “gusts” reaching 15-20mph for open parts of the route away from trees and exposed to the NE.  Reigate is sheltered from northerly / NE winds by the North Downs and any drying breeze is more likely to be welcome by sweaty runners! Another benefit is the air quality will be excellent compared with previous days of high pollution before the northerly moved in.  So a healthy run too!

Overall, the day is set to be great weather conditions for runners: a cool start, cloudy but drying out and warming up pleasantly with a drying breeze.

A great way of illustrating the air mass change this weekend is to look at Theta e and CAPE graphs below.  Theta E basically measures air mass instability (the higher the temp value the more “unstable” showery the air mass can be, similar with CAPE (convective potential energy)… spot how these two dramatically change on Sunday: from relatively unstable, energetic and showery conditions, to much more stable cooler air mass in a short space of time.

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HIGH sits over UK

HIGH sits over UK

High pressure continues to dominate September weather in the UK. In Reigate it hasn’t rained for 8 days and temperatures during these sunny days with broken cloud have regularly reached the low 20’s and even Tmax 23c on Tuesday.  The HIGH is edging to the NE and this will encourage light easterly winds over the SE which often brings more cloud off the North Sea and a risk of more dreary anticyclonic gloom at times perhaps building up by this weekend.  Cloud at night will maintain temps easily into double figures for Reigate but depress them somewhat into high teens in the day.  Nevertheless, it will remain dry into the weekend and any sunny spells will feel delightfully warm for late summer.

 The blocking anticyclone is called an omega block which is named as it looks like the Greek letter.  Traditionally these are persistent blocks which can last for a long time maintaining dry settled conditions, as it is now.  The jetstream remains far to the north taking stormy weather over Greenland and Iceland (where the Holuhraun fissure eruption is taking place and the larger Bardarbunga volcano threatens to blow as the caldera sinks by a remarkable 20 metres).  Winds over Iceland are generally blowing away from Europe at the moment and taking the SO2 eruption cloud over East Iceland and Norway.  Last weekend there was a brief spike of SO2 over England and Wales which was brought down by a cold front that carried some sulphur dioxide from Iceland over Ireland and into Scotland, Wales and England.  The spike short lived and unlikely to cause any health impacts. 

The models (GFS and ECM) have been predicting the end of this HIGH pressure for a while but it has persisted and bucked the computer models.  Currently, models see the HP (high pressure) drifting slowly further to the NE next week and pressure will fall first across southern UK creating conditions for showers to drift up on a slack Southerly flow: some models bring the first light showers as early as Sunday and Monday, however this is probably too early. 

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Another factor that is worth watching is a developing tropical disturbance off Cape Verde in the Atlantic which model runs are predicting will arrive in the Mid-Atlantic in the middle or end of next week.  The track is not certain yet, of course, but it could have an impact on how this high pressure eventually either falls apart or even builds back in again after a possible unsettled spell from mid-week next week. 

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worst case scenario is only one possible outcome of many… direct hit of extra-trop on UK

Quick update #2 02/09/2014

Several models (GFS, GEM, ECM) are getting wobbly with the high pressure this weekend and bringing in a trough or upper low pressure down the North Sea over the weekend or up the English Channel early next week.  Either way such scenarios will bring less settled weather to the SE this weekend and early next week: cloudy, some showers, but probably light mostly.  This is not a collapse of HP (high pressure) because pressure remains generally high and the jetstream well to the north of the UK.  Most models also suggest a building back of pressure next week, probably further to the north leaving the south vulnerable to unsettled conditions at times.  Best of the weather by Week 2 of September is likely to be Scotland. Point to note is that UKMet models do not see such an unsettled weekend, so things not certain… but seems to hint at vulnerable nature of the anticylcone.  HIGH building back stronger than ever second week sept.  

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High pressure… getting the wobbles

 

 

Quick update #1 31/08/2014

More agreement amongst models as to firmness of the High pressure lasting into second week, to around 9/10 Sept. ECM latest run shows nice firm blob of HIGH NW of UK.  Only snag with this is IF the subglacial Bardarbunga caldera in Iceland decides to erupt it would be when upper air flow is northerly, direct to UK and Europe.  However, this is all speculative and of course the most unlucky scenario. Check this website for volcano updates http://www.ruv.is/ and the Icelandic Met Office http://en.vedur.is/earthquakes-and-volcanism/earthquakes/

Forecast:

Anticyclonic conditions are building over the UK in the coming week or so courtesy of ex-hurricane Cristobal which is tracking NE across Iceland this weekend. The ex-hurricane has morphed into a deep low pressure that is dragging warm tropical air behind it into the Mid-Atlantic thanks to the jetstream and this is helping to build high pressure across the Atlantic and over Scandinavia.

Ex-hurricane Cristobal leaves trail of construction

Ex-hurricane Cristobal leaves trail of construction

 

Whilst the black and white UKMet synoptic chart (below left) shows surface pressure, the colourful chart (below right) shows important UPPER air flow as well as surface pressure on a so-called “500hPa” chart.  Upper air charts often represent conditions at at 500hPa (hectopascal) and are commonly used by meteorologists to get a better idea of how the atmospheric conditions will develop over longer periods of time.  This is because upper air is less disturbed by surface features such as oceans and mountains and by the influence of day and night that can upset surface charts and make them awkward to interpret predictions over longer time periods.  Upper air charts most commonly show temperature at 500hPa, or 5km altitude, which is about “half way” up through the troposphere, well away from surface upsets.  Upper air temperatures usually correlate with surface pressure, but not always.  See below to compare 2 charts, one surface and one upper air, for the same time period.

Warm upper air, usually of tropical origin, is shown on the 500hPa charts in orange and red colours.  Cool polar air at height is shown in green and blue. Where the colours are tightly packed together it shows a steep temperature gradient and this is often associated with the jetstream.  Warmer air is less dense and, rather like fluffing-up a thick duvet,  takes up more “space” in the atmospheric column compared to the the thin “blanket” of cold Polar air.  As warm air increases the 500mb “height”, fluffing up the atmospheric duvet, more pressure is exerted on the surface which increases the air pressure.  Cloud formation is inhibited as air sinks, warms and dries out.  The HIGH being built early next week, initially over the Atlantic, is formed by a warm upper flow of Tropical air pumped into the Azores high by an upper SW jet following on the heels of ex-hurricane Cristobal.

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So HIGH pressure will bring settled weather across much of the UK, including here in Reigate and the SE, for about a week with some warm temperatures and light winds and generally pleasant weather with some sunshine.   However, not all models show a uniformly “clean” high i.e. an anticyclone that is free from cloud and blocking all incursions of cool or moist air.  Note the UKMET chart (up) shows some weak fronts wriggling across the HIGH and this could mean cloudy skies for some and even odd showers at times.  Rainfall charts also do not show completely dry weather throughout next week (see below).  The ECMWF model (above right) shows a weak trough / cut off low feature to the west of the UK mid-end of next week and eventually forming a low feature in Biscay.  If this scenario comes off it could temporarily spoil the HIGH and possibly send showers into the South as it drags in warmer humid air from a southerly direction later next week.  So the exact duration and nature of the high is still a little uncertain so check back and also check UKMO of course for updates.

 

 

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By later in the second week of September and beyond an active Atlantic with the possibility of more extra-tropical cyclones will probably start to breakdown any remaining high pressure. The chart above shows the chances of tropical cyclone formation (TCFP) in the Atlantic and, whilst % figures are low, this still represents a fair chance of cyclones forming and ultimately entering the mid-latitudes later in September.  So this might not be a durable blocking high or one that lasts.   Nevertheless, it is good enough to keep active Atlantic weather systems at bay for a week.

The positioning of the HIGH is also critical to how “warm” we will get in the UK.  This time of year the continent is still warm so high pressure over Scandinavia to the NE or East of the UK brings in some warm continental air on an easterly flow.  This is how the GFS and UKMO sees things developing initially  with a  high positioned over Scandinavia and Europe through mid-week.  This flow could yield temps by Thursday in the mid-20’s, up to 25c is possible for SE.  With sea surfaces at their warmest this time of year it could be a good time to head to the beach.  By next weekend most models (above) see some movement of the HIGH to the west and NW of the UK where it could introduce a cooler Atlantic flow from Iceland.  So peak temps are probably reached before the end of this week.

Further ahead several models suggest a breakdown mid-September to a more unsettled regime.  The CFS temperature chart shows a rather sudden decline Mid-Sept and then again in October.  Whilst  this is not surprising for the time of year, such steps in the CFS can indicate frontal systems and depressions as different air masses arrive across the UK.  Warmer than average Atlantic sea surface temperatures, in some places nearly 5c above normal, is likely to encourage the formation of active depressions as is usual for Autumn.