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Weather news and views from Brian Gaze.
Posted Mon 9th September 14:25
After an unsettled start to the week high pressure will build from the south and have more influence on the UK's weather. By the weekend it probably becomes quite warm, particularly in southern and central regions. What about the longer term?
Recent GEFS ensemble model updates have favoured above average upper level air temperatures as we head through the second half of the month. That can be seen on the London plot below and some of the runs suggest temperatures at the ground level could reach the middle 20Cs.
On the precipitation part of the plot (lower half) there is very little activity during the same period. In fact many of the runs keep it completely dry. Comparable plots for more northerly locations show a greater risk of rain but they look quite dry too.
So is that it? Probably yes but possibly not. The GEFS also includes some colder runs as I have highlighted.
The colder runs in the GEFS still look dry on the whole. What they show is high pressure becoming centred more to the north and west of the UK. The pattern allows for cold arctic air to feed southwards. Today's GFS 06z operational run goes in this direction.
The GFS 06z chart below is from Friday 20th September. High pressure is centred to the northwest and relatively cold air is feeding southwards across the UK. In Scotland 850hPa temperatures dip below -5C. It is very early in the season but that is just about cold enough to produce some wintry precipitation over higher ground.
On the precipitation type chart below the pink shading is used to indicate snow. Therefore the GFS 06z run is showing the risk of a few flakes of snow falling (no accumulating) over a large part of Scotland on Friday 20th September! It probably won't be correct and as I said earlier most of the GEFS runs don't go as cold as this. But it is something to keep an eye in the coming days with the main interest being whether a pattern for high pressure to build northwestwards gains momentum.
High pressure is expected to play a big role in the UK's weather during the next couple of weeks. The most likely scenario is for warmish and dry days with quite cool nights. However there is a low chance of colder arctic air sweeping southwards later on. If that happens a few early flakes of the white stuff may fall over high ground in Scotland.
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