Arrival of the Queen of Freezer? December low to usher in cold end to the warmest year on record

1. early stage set-up Weds

2. Sat deepening cyclogenesis

3. occlusion stage

4. moving south

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.

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

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.

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.

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.

London cool dip

jet relations

drop in surface pressure

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.

classic depression

wind directions in a low

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.

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!

Wet Wednesday as Tropical warm waves goodbye to Polar cool!

rainfall on warm front

500m air temps showing front

upper air shows rising warm air over cold

surface chart shows warm sector

Quick resume of some wet weather approaching on Wednesday for the whole of the UK. The current cool pool is being nudged out of the way East on Tuesday by a vigorous return of an Atlantic jet stream blowing across the UK W-E during Wednesday and ushering a wedge of Tropical air.  As the warm humid Tropical air from the SW meets the cool Polar air the warmer less dense tropical air will be forced to rise, cool and condense over a wide area and produce persistent and occasionally heavy frontal rain. This is a classic active warm front and occluding from the NW.  The warm sector will brush across the SE during the day raising temps from 8c to 15c during the afternoon despite the rain! Rain totals for the SE may approach 10mm for the day but the heaviest rain will fall further NW along this extensive warm occluding front.  

A point to note is that, despite the classic text-book mid latitiude depression having a warm and cold front, it is usual for one front or the other to be more vigorous.  In this case, it seems the warm front is the more active, although the front is occluding as the cold fronts catches up the warm and lifts the warm sector up.

On Thursday, a much drier and more pleasant day, any sunshine could raise Tmax to near 20c in Reigate! By that stage despite being a relatively warm W/SW flow, the air mass is probably more accurately described as a modified polar maritime, coming in behind the occlusion, nevertheless, the SE will sit in warmer air than of late.

Here is an animation, courtesy of NMM netweather that shows the transition from cool polar pool to warmer SW flow in the next few days.

Interesting warm front Friday!

strengthening front

warming uppers

light surface winds

8000 feet strong upper winds

lifted index LOW for a time

arctic to surrey: a long warm front

Warm fronts do not usually grab the weather headlines. Warm air usually glides gently up and over cooler air in front and causes light rainfall. So it is tomorrow with a pencil thin line of warm front precipitation crossing the UK, slowly from west to east during the day.  

A complicating factor is that the significantly warmer air advecting behind the front will meet the COOL POOL of air lingering in a trough over Northern Europe and the North Sea. This could intensify the warm front as temperature gradients and lift increase. Another factor of interest is the strong upper winds behind the warm front with only light winds at the surface,  Strong wind shear can encourage lift which causes more condensation, thicker cloud and more persistent rain.

The warmest air is kept in the SE where thundery showers are a low risk in the afternoon and evening but possible for a time after the warm front passes and introduces warmer air throughout Friday evening. CAPE values and lifted index values match a low risk of thundery outbreaks at any time afternoon through to early evening.  Temps will rise, unusually continuously during the course of the day from 12ºC at 9am to 20ºC still at 9pm: MUGGY! These factors indicate a warm front that may yield more interest than usual.  Eyes up, Brollies at the ready!

Clouds associated with a warm front

As a warm front approaches the cloud thickens and decreases in altitude from a high cirrostratus veil through to altostratus and finally stratus. Fracto-stratus are the low level, ragged fragments, of stratus which appear dark and forbidding, below rain clouds such as nimbostratus. Despite its’ angry looks, this particular warm front is weakening on its way across the UK and will not bring any significant rainfall to Reigate. News on the exciting cold weather this week coming soon!

Forecasting rain using the clouds

Today illustrated how observing changing cloud types can help predict an approaching front and rain. If you look up, you can often have a good shot at forecasting the weather several hours away. Today, a warm front approaching Reigate rapidly from the west was heralded by a series of cloud formations. In case you missed the drama, here is a round up of the best bits:

A bright blue sky and a frosty calm morning quickly gave way to a milky sky with the appearance of high cirrus cloud building into a thicker blanket of milky cloud called cirrostratus. These indicate increasing moisture in the upper atmosphere, which is often a sign of deteriorating weather and frontal rain approaching. A “halo” might appear around the sun with thickening cirrostratus: again, a sign of deteriorating conditions aloft. Cirrostratus clouds continued to thicken and lower into a blanket of middle altitude clouds called altostratus. These turn the sun into a fading white disc. Altostratus clouds are rarely thick enough to cause rainfall. As the front approaches low level stratus clouds appeared scudding above Reigate Hill in the 50mph winds aloft. Persistent rainfall started in Reigate mid-afternoon when thicker featureless nimbostratus clouds moved overhead, marking the arrival of the front proper. The temperature rose throughout the day, from 0°C at 8am to 10°C by 10pm, another indication of an approaching warm front.  The front travelled 200 miles in about 4 hours, a 50mph race across the UK and, despite the gathering wind and gloom, brought a wonderful lesson in meteorology, hope you had a moment to enjoy it!

Sleet and you shall find!

“Sleet and you shall find” is the likely scenario for Monday in Reigate! Deep and crisp and even snow is not looking on the cards for us on Monday despite it snowing almost everywhere else right now (see map) and despite low temperatures overnight and tomorrow morning.
Light snow might lay for a while am but temperatures are due to creep above freezing for Monday as a warm front pushes “warmer” air across the region. This will deliver sleety rain on a southerly wind rather than snow through the morning. Nevertheless, snow is notoriously tricky to forecast and the cold front following closely behind might still deliver moderate snow showers as temperatures fall Monday pm and overnight to Tuesday heading in on an increasingly brisk and cold Northerly / NE.  Why is snow so difficult to forecast accurately?  There are good reasons for this… but this is a question to tackle later.  Meanwhile, latest model runs suggest cold will hang on for at least a week, snow still very much on the cards… so perhaps “ask and it will be given to you” might still hold true for snow lovers? (Matthew 7:7)

Text book low pressure approaching but drier for the new year

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.