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Weather news and views from Brian Gaze.
Posted Mon 12th November 15:36
The weather has been unsettled recently but a big change is afoot. During the next few days high pressure centred over the continent will start to block the eastwards flow of Atlantic weather systems. In the short term that means unseasonably mild weather but there are signs of it turning colder.
The key to the weather during the next few weeks is the role of blocking areas of high pressure. As I've mentioned several times there seems to have been an increased incidence of them since the Sudden Stratospheric Warming (SSW) in February. The Beast From The East and the hot summer were two of the end results. As we head towards another meteorological winter things need watching closely because there is the potential for a cold pattern to become established.
In the short term the UK becomes sandwiched between high pressure to the east and low pressure to the west. That leaves us under a very mild south or southwesterly flow. Daytime temperatures on Wednesday, Thursday and Friday could approach 18C (64F), leading to the possibility of date temperatures records being challenged.
By the weekend high pressure begins to build northwards. That allows a continental flow to develop and temperatures begin to dip. Nonetheless, it still won't be cold for mid November.
The real interest from a cold weather perspective begins next week. There are signs of colder air filtering from the east, but there is uncertainty. At this time of the year deep cold pooling to the north and east of the UK is limited, so the chances of it drifting across the UK are lower than they would be in January. It's very much a case of hitting the bulls eye if we are to end up with anything other than chilly days and frosty nights. Nonetheless it can happen as 2010b illustrated.
During the first half of next week cooler air probably moves in from the east or southeast. The postage stamp plot below shows 850hPa temperature forecasts for Tuesday 20th November. Quite a few runs of the show a rather cold picture and a few have the blue shading (-10C 850hPa or lower which is very cold) over the near continent.
In the longer term the question is where will the high pressure go. If it sinks southeastwards a southwesterly flow gradually returns and possibly becomes established. On the other hand if it stays to the north and migrates towards Greenland we could be looking at a lengthy spell of below average temperatures and a cold start to the meteorological winter.
The 500hPa and pressure postage stamp plot below is for Saturday 24th November. It's not conclusive but a lot of the individual plots have high pressure centred to the north or west. Most of the runs are not indicating particularly chilly conditions in the UK, partly because as I mentioned the cold pooling isn't yet that extensive. Nonetheless, based on this data I would say a cold start to winter 2018/19 is more likely than not.
Settled weather returns this week and it becomes very mild for mid November. Next week high pressure is expected to remain in charge and builds northwards. That allows cooler air to filter in from the continent and it may turn quite cold and frosty. In the longer term high pressure probably remains the dominant feature and it may often be centred to the north of the UK. If that happens there will be a good chance of a cold start to the meteorological winter.
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