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Provisionally, up to the 17th December the Central England Temperature (CET) is +6.7C which is 1.9C above the 30 year average. Therefore the first half of the month was very mild on the whole, although temperatures dropped for a time towards the end of the period as high pressure built over Scandinavia and colder air filtered in from the continent. As Storm Deirdre moved in from the Atlantic and bumped into the colder air, some places saw a period of snow; but for most of the country the first part of the meteorological winter was green.
In the short term a continuation of the rather mild and unsettled weather is likely. However there are signs of a change taking place during the Christmas period. Computer models show high pressure building northwards, at least across the southern half of the UK. That pattern would lead to drier weather in southern and central regions but in the north it may remain unsettled as disturbances push in from the Atlantic.
Temperatures to a large extent will be dependent on where the high pressure becomes centred. If it remains to the south, a mild southwesterly flow is likely. If it centres over the UK, it could become chilly and frosty under calm conditions even though the air mass aloft remains a mild one. Another possible outcome is for the high pressure to build farther north and open the gate to colder upper level air from the north or the east.
The GEFS postage stamp chart below is for Friday 28th December. Each stamp represents one run of the computer model with the starting conditions tweaked to help account for uncertainty. The majority of the runs show high pressure having an influence on the UK's weather, although there is still a low chance of unsettled weather winning out in the south and it increases the further north you head.
At the moment it does not look like high pressure will build far enough northwards to allow cold upper level air to move over the UK. Things could change due to the possibility of a Sudden Stratospheric Warming (SSW) event disrupting the circulation around the Northern Hemisphere, like the one last winter did.
The first half of December was mainly mild and green in much of the UK. In the short term it remains unsettled but high pressure could start to quieten things down during the Christmas period. That leads to the possibility of the frost risk increasing as it turns chillier under the high pressure area. However it is not certain and the predominantly mild theme may continue into the New Year.
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