High pressure likely to remain in control until the middle of next week, when milder, windier and more unsettled weather looks to return from the Atlantic. How long will the milder weather last? Read this blog
Early December last year, the weather patterns were quite different, with deep areas of low pressure passing close to Iceland, a strong and moist southwesterly flow and slow-moving weather fronts bringing exceptionally prolonged and heavy rainfall to northwest Britain between the 4th and 6th. Severe flooding affected parts of NW England, especially Cumbria, exacerbated by already saturated ground following the second wettest November across NW England and north Wales since 1910.
The culprit for the unsettled, windy and, in places, very wet weather, was a deep mid-to upper level polar vortex to the northwest containing deep cold air bottled over Greenland and Iceland. The steep north-south temperature and pressure gradient at the base of this vortex drove a strong jet stream across the Altantic and toward the northwest of Britain – spawning deep low pressure systems to batter northern areas with rain and wind.
Pressure pattern late Nov - early Dec 2015 - blues and purples lower pressure:
However this year, the last 9 days of November, since Storm Angus and another deep low moved away by the 22nd, has seen high pressure dominate the UK weather. And anticyclonic conditions look to prevail through the weekend and into early next week too.
Surface pressure anomaly since 22nd November - oranges/reds are higher pressure, blues low pressure
The central core of high pressure dominating our weather has shifted south from Scotland since the weekend, hence southern Ireland, south Wales and S England, have seen calm, clear and chilly conditions with hard overnight frosts. While tighter isobars further north away from the high’s calm central core has brought a milder, cloudier and breezier westerly to northwesterly flow to northern areas. The centre of the high will drift around western and northern areas on Friday, while a weakening cold front sinking south across the Northern Isles today drifts southwest across mainland UK later on Friday and into early Saturday, bringing more in the way of cloud for all and some light rain towards the east.
Synoptic evolution into the weekend, images courtesy of UK Met Office
However, over the weekend, clearer and slightly colder conditions will return from the east this time, as the high re-positions to be centred over the North Sea this weekend, which will drag in a colder, drier and hence clearer continental flow off the near continent. So, turning colder again over the weekend - with the return of frosty nights and fog patches forming in places.
Colder and drier air (lower dew points on chart below) move in from near continent at the weekend
High pressure will become centred over Germany early next week, but will extend far enough northwest to feed a southeasterly flow from the near continent bringing dry, settled and chilly conditions on Monday and Tuesday.
However, during the second half of next week, signs are from the models for high pressure to reatreat over the near continent and for low pressure systems to move towards Britain, with a milder southwesterly flow developing. The last few GFS operational runs have been more bullish than other models, notably ECMWF, at pushing frontal rainfall east across the UK mid-week, before more rain and a strengthening southwesterly wind arrives from the west to end the week ahead of a deepening low in the Atlantic. 00z ECMWF waits till Friday to bring rain across all parts from the southwest.
Milder southwesterly flow sourced from past the Azores muscles NE across UK by mid-week removing colder air
But, fairly high confidence now that high pressure will retreat south and southeast by middle of next week to allow a milder southwesterly flow in across the UK, with spells of rain moving through too. As early as Wednesday, GFS has mid-afternoon temperatures in double figures across many parts, as high as 13C across the southwest.
How long will the return of milder weather last? Obviously beyond next working week confidence will greatly drop. But some of the longer range ensembles, suggest a mean drop in temperatures toward mid-month, as the 00z ECMWF ensembles mean temperature for London indicates below.
And GFS ensemble mean ...
So this maybe a hint that the milder more mobile and unsettled Atlantic weather arriving next week may be short-lived before more amplification in the upper flow develops to allow cold air from polar regions to the north or continental regions to our east to move in across the UK. Certainly the upper patterns in the northern hemisphere are different to last year, as I mentioned in my last blog, with a weaker and displaced stratospheric polar vortex compared to this time last year where it was strong and nearer the north pole. However, the tropospheric polar vortex is fragmented over the coming days, with deep cold vortex over Siberia still but also signs of deep cold air vortex strengthening over northern Canada and Greenland for a time – hence the strengthening westerlies over the Atlantic, flattening out and sinking of the high pressure anomaly over northern and western Europe. But with the tropospheric vortices waxing and waning this last few and coming weeks, probably linked to the weaker displaced stratospheric vortex, the milder more mobile westerly incursions may not last all of December.