Archives For depression

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 cool LOW pressure tracking across from Greenland and Iceland will move over the UK on Friday and sit over Reigate for the weekend and into much of next week. Atlantic depressions usually have some warm tropical air circulating with cold polar air but not this one!  With high pressure to the west any warm SW air flow is being blocked out, leaving Reigate on a “cold-washing-cycle” with mostly swirling cold polar air circulating around the low. Cool air coupled with low pressure even in this cool Spring weather causes unstable airmasses.  Instability means that big showers can develop as thermals rise from warming surfaces even in the weak Spring sunshine we are experiencing.  Convective thunderstorms are a remote but interesting potential risk for Reigate over the weekend.
Various fronts and troughs will circulate around the LOW pressure which is blocked by a HIGH over Scandinavia and E.Europe and won’t move much. The LOW will bring showery rain, some of it heavy with the possibility of hail and even thunderstorms, especially over the weekend (50% chance of convective thunderstorms on Sunday over Reigate).  Temperatures will never climb much above 7°C and, with gusty winds of over 30mph, it will feel cool, but nothing like what we had earlier this week. Some models predict a return of cold Easterly winds as the LOW slips south over the UK and drags in cold continental air again. There is still uncertainty about this but it will certainly be an unsettled week. Other models see glimmers of spring after around the 23 March as winds turn more SW and a ridge builds over the country bringing drier conditions.  More on this later, keep your fingers crossed!

A text book depression / low pressure system approaching the UK from the west might signal the beginning of the end for this long run of bad weather. It’s a big low pressure system tracking NW rather slowly and will take all weekend to clear off but most of the rain will fall in the west and not much will reach Reigate. The warm front is due to pass over Reigate around midday on Friday bringing some brisk SW winds and cloud and some light rain. The “warm sector” following the warm front comprises an air mass called “Tropical Maritime”, bringing balmy +10°C temperatures to Reigate over night Friday through to Saturday morning. The cold front is due to pass over Reigate after lunch on Saturday when temperatures will drop by several degrees to 3°C overnight . Despite the slow movement of the depression as a whole, winds will be pretty strong especially as the cold front passes. The cold front will bring gusty winds, possibly up to 50mph on Saturday morning, and heavier showers on a cooler NW wind. The air mass following a cold front is called “polar martime” and will feel significantly cooler in the wind, even in sheltered Reigate.
There are signs that a HIGH pressure will build up over the south of the UK by the new year week bringing drier weather, at last! The ensemble forecast below shows a drier spell to start off 2013. An ensemble forecast is one which combines several computer weather forecasting model “runs” and sees how well they match.  The closer the match, the more confident the forecast and more probability that it will be correct.  Each “model run” is “perturbed” which means tiny differences in starting data are used to simulate the error and vagaries of real chaotic weather systems.

27-12-2012 22-58-26 dry spell in january

This is a high resolution satellite image taken today 15 December at 20:15 by the NOAA weather satellite on its most recent pass over the UK. It shows a wonderful swirl of cloud over Shetland – the centre of a depression which formed over the North Sea from the front that caused the rainfall over Reigate on Friday. This LOW pressure formed on the “left-exit” region of the jetstream: those high speed winds high in the atmosphere which are so important in controlling the development and movement of depressions and anticyclones on the surface. The synoptic weather chart below matches the same period of time and shows the LOW pressure feature associated with this swirl of cloud and the attendant fronts. More on fronts and jetstreams later!

depression over shetland 15 dec