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.
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.
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.
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!