Archives For cold front

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!

The weather has been mercifully quiet, with mostly warm and sunny conditions for Reigate over the last 2 weeks. This is gradually about to change but no particular drama is afoot other than some perky and windy frontal rain arriving in Reigate on Thurs night /Fri am, followed by showers and cooler breezier weather with hopefully some interesting clouds over the weekend!

Our two week dry spell (well, 13 days) ended briefly today with a light shower.  As the blocking high sinks away to the S, a cooler NW flow will swing by over the weekend ushered in by an active cold front on Thurs/Fri which will mark the arrival of some cooler days and frosty nights through to early next week.

The charts show a typical early spring flip-flop in temperatures as warm air from the SW on the back edge of HIGHs is replaced by cool NW and even N winds as LOWS sweep across the North of the UK.  The back-edge of the LOW moving over Scotland in the next few days will drag down chilly Polar air especially on Saturday and Sunday.

Daytime temps will struggle to double figures which, when last weekend hit 20c might feel a bit of a shock!  This cool airtstream, fresh from Polar regions, will also be warming through from below and therefore get rather unstable: this means thermals will have a tendency to rise, and keep rising through the chilly upper air, creating towering cumulus clouds and showers.  The chart below shows a typically unstable temperature height diagram with some ingredients for a showery day capable of building some nice cumulus clouds: steep lapse rate, saturated airmass through a large column of the atmosphere, some wind sheer (change of wind speed or direction with height).

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By the afternoon especially on Saturday cumulus congestus clouds might even be tall enough for some sharp showers, possible hail in the cold upper air and the odd clap of thunder is not impossible.  Nevertheless, total rain predicted is low and there will be sunny intervals. April showers, but a month early!  Next week sees a continuation of flip-flop spring weather.  April has the possibility of some high pressure returning with sunny and dry weather but the exact timing of this remains uncertain.  There is also some indication of unsettled pattern in April too, so take your pick!

LOW over Scandinavia

LOW over Scandinavia

Update Friday: showers likely but heavy showers & thunderstorms unlikely Saturday as upper air temps rise and build pressure rapidly during the day inhibiting convection.  Warming uppers means lower lapse rates, less perky convection, thermals rise less vigorously and less high and so cumulus clouds build  less height.

trough splits

A classic intensifying COLD FRONT passing over Reigate on Friday 26 April is set to bring some heavy rain and then a chillier feel to the weather after temperatures soared to over 22ºC today.  Reigate has been bathed in warm Tropical air brought by south westerly winds this week and night time temperatures have barely fallen below 10ºC. The arrival of cold polar air is set to push maximum temperatures below 12ºC during the day over the weekend, so it will feel decidedly less balmy.  Expect a cool, bright and showery weekend with cold nights and a frost risk. Sunday looks dry and bright with light winds, so this will be a pleasant day.  So Polar air is not all bad news with sparkling visibility as well as possibly some splendid cumulonimbus or cumulus congestus clouds on Saturday! 

This typical cold front, albeit initially lacking in perky thunderstorms, shows how relatively dense cold Polar Maritime air advancing into warmer Tropical Maritime air still hanging on in the SE over Reigate causes moderate to heavy rainfall.  The warmer, less dense, Tropical air mass has temperatures at 1500m (5000 feet) of +8ºC. The advancing cold Polar airmass from the North has upper air temperatures of -5ºC and this air will push the warmer air up. Rising air containing moisture condenses, forming clouds and rain. The more rapid and vigorous the lift, the more condensation takes place and the heavier the rain. Reigate can expect moderate to heavy rain in the morning as the cold air undercuts the warm. Rising air also causes atmospheric pressure to fall and this occurs in advance of COLD fronts. There is often a rapid RISE in air pressure as cold fronts move away and this often leads to skies clearing and a temporary lull in showery activity before surfaces warm, lapse rates and instability increase and cumulonimbus clouds form bringing heavy showers. Saturday has a heightened risk of heavy thundery showers arriving on the northerly wind in easterly building belts of convective clouds: some big cumulonimbus clouds are possible. Belts of moving showers moving down a trough on northerly winds are called “scoopers”.

may recovery planThis cooler snap looks fairly short lived with a gradual recovery to warmer temperatures in early May with a HIGH pressure predicted on some models, so possibly a dry and bright bank holiday!  Check back for updates.

Cold front crosses UK

December 30, 2012 — Leave a comment

A distinctive cold front showed up on the rainfall radar today 29 December as it tracked east across the UK and brought a sharp period of heavy showers. The thread-like line of (red=heavy) showers on the radar opposite shows the cold front very distinctly.  A drop in temperature also occured as the front passed over. In Reigate, temperatures dropped rapidly from steady highs of 11°C in the morning to 7°C within hours of the front passing through. Blustery winds of 30mph also accompanied the front and then dropped off quickly as pressure rose after the front passed over. The satellite photo shows the line of frontal cloud and the bubbly cumulus clouds bringing showers in the polar-maritime air behind the front.  A classic text-book passage of a cold front!

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