Archives For warm front





GEFS shows cooler days ahead

GEFS shows cooler days ahead









cold new year


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.  

warm 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.

ana front

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.

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.  

warm front friday cool poolA 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.

cape and li fri 28 juneThe 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!




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)

