Archives For north atlantic oscillation

2015-04-27_20-21-59

cold plunge of polar air to end April

The newspapers have this colder than average week billed as a “polar plume”.  Cold air cannot really be said to “plume” like warm air (spanish plume).  Nevertheless, it is certainly cold up North with settling snow over the hills and wintry precipitation elsewhere too.

The GFS ensemble (several model runs combined at once) chart below shows that the colder than average (upper air) temperatures will last until the weekend at least.  Thereafter temperatures rise but note the rainfall spikes indicating unsettled conditions.

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upper air temperatures

For us here in the sheltered SE it is just cooler than average with patchy overnight frost and bright days with light showers Tuesday pm. A blustery cold front on Wednesday is likely to bring more purposeful rain for a while, as could further active fronts on Thursday when the trough axis moves through our region accompanied by the jetstream nearby to the south.  Friday is likely to see a transient high pressure ridge bringing settled weather and then a change in wind direction by the weekend.

The overall cool set-up is due to “northern blocking” which is when pressure builds over Greenland and the Pole and the, previously strong, Azores HIGH slackens off and nudges south. At the same time the usual Icelandic low pressure weakens or is dominated entirely by a HIGH pressure (see chart below).

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In this situation the jetstream works its way south of the UK.  The result is that cold polar air is able to leak south out of the polar regions and into the mid latitudes.

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The index shown above summarizes the overall pressure pattern in the Atlantic. It is called the North Atlantic Oscillation. The NAO measures the pressure difference between Iceland and the Azores.  It has been mostly positive this winter: meaning that pressure is LOW over Iceland and HIGH over the Azores.  Typically a positive NAO indicates a strong zonal jetstream and mild often wet conditions for Europe with relatively fast moving LOWS passing through.  The chart below shows how the NAO has gone negative recently and this indicates that pressure has built over the Pole, creating a blocking situation.

The charts above show theta-e temperature which shows cool airmasses over the UK clearly.  These are 850hPa temperature charts which represent temperatures at 1500m (1.5km) above the “boundary layer”.  This height is used to avoid disruptive temperature changes which occur nearer the surface caused by day and night, mountains and water bodies etc that upset the overall temperature pattern for analysis.  The situation shown below by this weekend is quite different, though still unsettled.  Note the warmer flow from the SW.

After the transient ridge on Friday it looks like pressure will fall into the bank holiday weekend as a low pressure nudges in from the Atlantic.

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pressure falls into weekend

The milder humid air brought from the SW by the Atlantic low could potentially cause some significant rain at times around the weekend and into early next week as this meets cold air over the country.  Despite the milder upper air arriving from the SW, it is unlikely the “milder” temperatures will be noticeable in such wetter and windier conditions. In winter this set-up could have brought big snow events but in early May it will simply bring rain. For the detail on timing and amount of weekend rainfall we will have to wait and see, but it certainly looks potentially quite wet, though models suggest pressure building briefly thereafter.

2015-04-27_20-10-05

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.

2015-02-26_19-17-13

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.

2015-02-28_09-19-25

 

link to accuweather take on this system

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

 

The ensemble charts above show that we can expect a week of change ahead.  At the start of the week fronts will bring modest episodes of rain and swings in temperature to the SE followed by a notable change mid-week as both temperature and pressure fall (see charts above) with the possibility of snow for the SE.   By next weekend there is a risk of some proper cold into the start of February.  As usual Reigate and the SE will be sheltered from most of the action but the weather will take interesting and notable swings in a predominantly downward direction nonetheless.  Models agree on how this transition will happen and it is largely based on the evolution of a LOW starting life off Canada near Newfoundland …. here are charts from the ECM that shows the story of our Canadian LOW and how it is likely to bring another taste of winter to the UK.  (Update: worth explaining that the Canada LOW mentioned here is not the same as the Nor’Easter LOW that caused blizzards in NE USA Monday-Tuesday this week: it is a LP preceding it.  If you look below at Chart 2 for 28 Jan you can spot the infamous Nor’Easter bashing the NE coast well modelled on this ECM chart from back at the weekend. The US Nor-Easter Blizzard2015 storm is responsible for building the Atlantic BLOCK helping to push Arctic winds our way but it is not travelling to the UK. The NYC storm looks to travel North up the Canadian coast, filling near S Greenland, unable to break through the Atlantic high pressure ridge extending north that it, in part, helped to build. Hope that helps!)  In any case, the weather set-up gives Reigate another flaky chance of some snow.

2015-01-24_15-47-35

1. The story starts now with the Azores HIGH pressure giving a dry and pleasant weekend for the SE, Saturday has seen brilliant blue skies as a result.  The Azores HIGH has been dominating the Atlantic recently and our Canada LOW, deepening rapidly off Newfoundland on Monday, will be forced round the HIGH to the north west to Greenland. This LOW will deepen rapidly because of the great temperature difference between frigid air pouring off Canada and humid sub-tropical air fed up from the south courtesy of the Azores high.

For us in Reigate this period sees a ridge of the Azores high pressure ebb away slightly during Sunday and this will nudge a mild SW flow to raise temperatures temporarily high into the UK and SE overnight into Monday morning.  This minor weather episode (marked by the ups and downs in temperature on the 850hPa ensemble chart top of page) will be heralded by increasing cloud on Sunday as fronts bring some patchy rain in on a warm front overnight into Monday.  Polar air will follow a cold front later on Monday and a ridge of high pressure will build quickly overnight turning the winds into the NW with a dry chilly night in store for us in Reigate and SE into Tuesday.

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2. Later on Tuesday the Canada LOW is forecast to move NW to near southern Greenland where it will feed on a brisk northerly wind of freezing cold air from the Greenland ice cap.  This freezing cold air will create a vigorous cold front that will reach the UK early on Wednesday.  This active cold front will usher in an unusually cold Polar Maritime NW’ly wind across the country reaching the SE late Wednesday with the possibility of heavier rain or even snow. One to watch carefully.

A point to note from the charts above is that the central Atlantic is anomalously COLD at the moment and so the brisk NW’ly wind will not warm up as much as usual on its journey over the Atlantic to the UK, increasing the likelihood that it will bring wintry stormy gales to the NW, with some models even showing wintry precipitation for us in the SE too, perhaps reaching us by Thursday, though these are likely to be sparse unless the cold front stalls in which case more significant falls are possible. (Update: GFS 18Z suggests this cold front could be active and bring snow across entire UK in its wake…)

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NW airflow 2m temperature anomaly

Check the temperature anomaly chart above which shows how unusually cold this NW’ly wind is going to be.  Usually NW winds do not impact SE England with snow unless fronts stall or there is an especially active undercut of unstable cold polar air.

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3. Our Canada LOW that started life just off shore from Newfoundland is forecast to cross Iceland mid-week and then slide SE into the North Sea, around the blocking high extending north through the Atlantic to meet rising pressure in Greenland, a good scenario for a COLD Europe!  As the LOW transits SE into the North Sea it will bring down Polar and then Arctic northerly winds on its back and snow for the North, NW and NE coasts.  Arctic air rarely brings snow to Surrey or Reigate … it usually runs out of moisture and lift on its transit across the cool land and instead the south and SE usually gets azure blue skies with frosty nights.  NE and E coasts can get snow showers.  Daytime temperatures everywhere may well stay near freezing if this comes off.

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It’s too far ahead for any detail but some model runs show Polar LOWS sliding round the edge of the trough on the left exit side of the jetstream (where lows develop rapidly) as the trough moves east .  These daughter lows can bring snow to the SE but this is too far off to be certain.

So how long might this cold snap last?

There are indications both for and against a more prolonged cold snap but, on balance, the peak of any Arctic cold looks likely to be shortlived as the high topples east, ebbs south and allows gradually more westerly winds back across the UK.  For enduring cold we really need the LOW to move south into Europe and pressure to build to the north bringing in a blocked situation allowing cold easterlies into the UK (beast from the east).   However, this scenario looks unlikely because the Azores HIGH is likely to remain relatively dominant.  This is shown by the generally positive North Atlantic Oscillation chart above.

2015-01-24_19-28-39

North Atlantic Oscillation

The NAO is a forecast measure of the sea level pressure difference between Iceland and the Azores.  When the NAO is positive it usually indicates a strong pressure difference with a big HIGH pressure over the Azores and a LOW over Iceland.  A positive NAO correlates with a fast zonal westerly jetstream and mild wet winters for the UK.  A negative NAO indicates colder winters in which pressure rises to the north (Iceland) and allows easterly or NE winds to bring cold airmasses from the Arctic or more commonly Siberia into the UK.

2015-01-24_19-27-51

Arctic Oscillation

The Arctic Oscillation (AO) is a related cousin of the NAO.   The Arctic Oscillation is a bigger scale comparison of pressure at the Pole compared with that in the mid-latitudes.  A positive AO usually correlates with lower than normal pressure over the Poles compared to the higher pressure further south in the Atlantic.   This brings mild wet conditions to Europe (zonal westerly winds).  A negative AO sees pressure rise higher over the North Pole and this acts like a balloon to push polar air out into mid-latitudes… a cold winter scenario.  The AO forecast shows it going negative by early February and this correlates with the cold snap.  Unfortunately it looks like the AO will go positive again thereafter but this is too far off to be certain.  On the other hand…. !!

The charts above show a Polar view of the Northern Hemisphere.  They show that pressure is forecast to rise over the Pole disturbing the zonal westerly flow of the polar vortex by a displacement of the polar vortex away from the Pole (see below).  This might increase the likelihood of a prolonged cold plunge of Arctic air reaching the mid latitudes including the UK  This is good for cold weather enthusiasts in Europe!

2015-01-24_19-39-11

The other longer-term chart shown above in favour of a cold late winter is a sudden stratospheric warming forecast in early February.  A SSW can lead to pressure rising over the Polar regions a few weeks later, disrupting the upper westerly winds, potentially upsetting the jetstream and bringing cold to mid-latitudes.  This is well correlated and was significant in bringing a late winter in 2013.  So… much of interest at the moment as we enter the final third of winter 2014-2015.

note: for all decision making purposes and forecasts please consult professional agencies: e.g. MetOffice at http://www.metoffice.gov.uk/public/weather/forecast/gcpg7rs0t#?tab=fiveDay&fcTime=1422057600

http://www.nc-climate.ncsu.edu/climate/patterns/NAO.html

So far December weather for Reigate and across much of S England has been much the same for a while: breezy westerly winds and exceptionally mild on occasions with December temperatures breaking records in Reigate: the highest December temperature of 12.7C was recorded on Dec 18, the highest average and highest minimum were also records broken this month in the town. The December CFS chart for Bournemouth temps show how much time they have been above average this month so far.  Whilst it has been mostly relatively mild this month so far in the UK, large parts of the rest of the N Hemisphere has been even warmer with much of N America and parts of Russia being anomalously warm too.  On the other hand E Siberia has been very cold with Verkhoyansk falling to -50C at times.

Whilst a colder spell is expected to arrive soon after Christmas this will not be sufficiently extreme or long enough to move 2014 away from a record breaking warm year.  The Central England Temperature records go back before 1780 and 2014 looks almost certain to be the warmest ever recorded CET with an anomaly of +1.49C above the long term average.  This matches 2014 as a warm year globally, if not quite the very warmest.

So, apart from the rather underwhelming “bomb” depression earlier in the month, why has the weather been so mild, uneventful and unchanging for much of December, so far?  An immediate cause of our quieter weather this December is the jetstream being less active than last year.  On the charts below spot how much more active the 2013 jet was at the same time last year across the Atlantic.  A succession of Atlantic storms were driven headlong at the UK by a powerful jetstream which sat further south.  This year the jetstream has mostly been further north than the south of the UK and has been more meandering (meridional) and less powerful: hence fewer and less powerful storms for us in the south, most of the storms have tracked to the north of Scotland which has had an “ordinary” December of high winds and heavy rain!

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The reason for our milder than usual December is that much of the time our winds have arrived from a warm SW direction round a persistent HIGH over Europe.  We have had few incursions of Arctic northerlies, as yet.  Nevertheless, for parts of the month especially for the NW of the UK, there has been a sustained NW wind associated with high pressure over the Mid -Atlantic and LOW pressure over Iceland and this has pegged down our overall temperature anomaly to 1C above long term average.  This pattern of broadly westerly winds is called zonal and is indicated by a positive North Atlantic Oscillation (pressure low over Iceland and high over the Azores).

While the jetstream controls our synoptic scale pressure patterns, a more remote driver of our overall weather in the Atlantic mid-latitudes is the rather more geographically far-flung “Madden-Julian Oscillation”.  The MJO is a cycle or wave of weather disturbances (clusters of convective storms) that start in the Indian Ocean and move east through the tropical oceans into the Pacific and beyond.  It usually peters out across the Atlantic but can be observed to continue as an unbroken oscillation into the Indian Ocean where it starts another Phase 1 again. The MJO connects up with observed known weather patterns in the Atlantic and has therefore got an impact on UK weather, especially apparently, in years of weak or neutral ENSO (el-nino southern oscillation).  This year is currently a weak El Nino.  With the MJO in Phase 3-4 then a positive NAO is usually favoured but is not fixed of course, yet more drivers are likely to influence proceedings in the new year, not least “sudden stratospheric warming”.
MJO Phase 3-4 = +NAO

MJO Phase 3-4 = +NAO

DIY forecast for the possible storm 27-28 December

When upper winds (including the jetstream at 300-250hPa) match the direction of those at the surface, the weather tends not to change very much over quite long periods, like recently.

winds the same at height: no change

winds the same at height: no change

Wind direction and air masses can tell us a lot about how the weather will change and sometimes this is helpful when forecast changes are uncertain or simply fail to arrive when expected.  To an observer on the ground clouds at different heights can sometimes appear to be moving in different directions.  If on such occasions, the observer sees the clouds moving perpendicular to each other then a rapid change in weather is likely to be on the way, either good or bad!  The so-called “crossed-winds rule” is handy if you want to know how quickly expected weather events will arrive, especially if you are out and about on a day when expected changes are afoot but the timing is uncertain from forecasts.  This could prove handy later this week when a large storm might upset our run of benign conditions but the exact timing of fronts and events is uncertain, especially at this range!

2014-12-22_21-38-47

The storm, expected to intensify rapidly in the Mid-Atlantic during Boxing Day (yes, a possible rapid cyclogenesis bomb depression) could take a track modelled by the GFS that runs from the N of Scotland down through the N Sea. This SE track could pull in strong Arctic winds and, combined with high spring tides and high waves, this could cause a storm surge event for E coasts of the UK.  Note this is too far off to be certain at this stage.

Other models, such as the ECM and UKMO have different tracks that take the LOW on a less potentially damaging course.  Nevertheless, the models are worth watching because on some tracks the synoptic situation is similar to the 1953 storm which caused a lot of coastal flooding.  Of course, sea defences are much improved since 1953 so, even if the track was similar, it would be unlikely to cause anything like the same impacts.

compare 1953 storm surge with models 2014

compare 1953 storm surge with models 2014

It is also noticeable how similar the zonal pattern preceding the 1953 storm is to the current synoptic pattern: a strong positive NAO was present with both (see below).

2014-12-22_16-48-17

Back to the present… a weak cold front moving south during Christmas Eve will introduce a cool N/NW polar air flow across the country by Christmas Day. No snow is forecast for the SE with this cold front and it will be noticeably colder but brighter on Christmas Day.  A high pressure ridge on Boxing Day could start the day bright and frosty but, depending on the final track of the RaCy depression, things could go downhill rather quickly thereafter.  If you are out for a walk on Boxing Day then this is where the Crossed-Winds rule will come in handy to do some of your own forecasting!  Here’s how:

Bozing Day crossed-winds rule

Bozing Day crossed-winds rule

It will be necessary to see clouds at different heights – especially those wispy cirrus ones to check high altitude jetstream winds.  If it is completely overcast then forget this unless you happen to see breaks in the low cloud.

Find the surface wind direction: look at lower cumulus clouds or just local wind vanes or weather stations. It’s important to get the correct surface wind direction though so don’t rush this bit.

Stand with your back to the surface wind : the simple rule means that any low pressure will be on your left.  You can also tell how quickly weather will change for you locality thus…

Watch the direction of any cirrus clouds: check this with an aerial or static object.  If they can be seen to move at all then they will be shifting fast, because cirrus can be 10km up! The best cirrus for this is hooked mares tales or jetstream cirrus which appears in long impressive streaks along the core of the jetstream or similar upper winds. The more fuzzy edge is on the tropical warm side and the firm sharp edge to cirrus, if this can be spotted, is on the polar air side.

 

Now, the rule: stand with your back to the surface wind and if the cirrus (upper winds) come from the left then the weather will get worse.  The more perpendicular the winds (right angles) the more rapidly the weather will deteriorate.

If the surface and upper winds are going the same way, then there will be be no immediate change, or just very slow change.

If the upper winds come from the right then the weather will start to improve.

On the back edge of the storm, chilly Arctic air could bring down an active cold front with snow showers on the back edge into the SE, but more likely these will be restricted to NE/East coast.  It’s ages away so no details are possible but this storm looks one to watch.

Finally, the longer range outlook is rather uncertain!  Signals such as stratospheric warming, forecast to take place into early January, favour a cool January with the break-down of westerly flows into occasional cold Northerlies / easterlies.  In contrast, the MJO seems to favour more of the same unsettled westerlies for a while at least.

Quite possibly it will probably be a mix of the two!  This is suggested by lastest output by the CFS showing some zonal westerly weeks and some potential cold northerlies with the subtle position of HIGH and LOW pressure being critical to the outcome.  Lots of interest over the Christmas period and beyond, keep watching the weather!

 

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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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Post-Tropical storm Andrea arrives in the Atlantic this week and sets the scene for a significant switch back to a more Atlantic dominated weather pattern (called “zonal”: where a stronger west to east flowing jet stream brings in Atlantic weather from the west). Andrea is the first tropical storm of the 2013 Hurricane season and soaked the east coast of the USA from Florida northwards last week.  Her arrival in the Mid-Atlantic coincides and perhaps helps to nudge our weather back to a more “zonal” flow and a positive North Atlantic Oscillation (HIGH over the Azores and LOW over Iceland = “normal”!). Andrea is ageing and much-weakened since thrashing the US east coast.  Whilst she is unlikely to storm ashore by herself, she is set to merge forces and deepen a slack LOW in the Atlantic and gather strength ready to sweep fronts across the UK, especially the NW which will get most of the rain this week.  The “Andrea” system will arrive in SE England on Wednesday with windy conditions as isobars tighten.  Reigate and SE is likely to escape the worst of the rain, at least early in the week, as HIGH pressure builds, at last, over Europe (giving them a drier week after record breaking floods and rain in Germany, Austria and Czech Republic). update Mon: some rain arr Tues with fronts. Most rain Wednesday.

The plus side of this switch back to a westerly weather pattern is that the east will see the back of the “spoiler” cool NE wind which brought some gloomy cloud cover in the mornings and colder than expected “warm dry days” this weekend and last week where promises of 25°C never happened in the SE.  With a warmer flow swinging around from South and SW temperatures should climb away this week to a real 20°C and actually feel properly warm as humid air is fed over the SE from the balmy South West rather than the cold North Sea.  Update Monday: temps will fall away to mid-teens Tmax as rain spreads more quickly east than first models predicted. For Reigate and the SE the heaviest rain is likely to be at the end of the week as heat builds over Europe and possibly sees convective storms associated with a front over France bringing in heavy rain for Friday / Saturday.   This warm feed of air has the potential to be a Spanish Plume event causing significant thundery activity in the SE.  Check back later for updates on this exciting potential! (looks less likely on latest runs; but wetter weather mid-week more likely now!)

uncertain forecast

Thereafter, things look to be set in a cooler unsettled cyclonic pattern with LOW pressure systems storming off the Atlantic bringing wetter conditions for the rest of June but still much uncertainty on this one. 

In summary: this week the wind will clock round each day from a cool easterly (from North Sea) through Southerly to humid SW and weather is likely to become more mobile but with dry weather hanging on longest in SE and feeling warmer too!  What’s not to like?  It’s Summer, enjoy!

links Andrea on NOAA http://www.nhc.noaa.gov/refresh/graphics_at1+shtml/143437.shtml?gm_track#contents