Weather news and views from Brian Gaze.
Published 17th May, 15:10
A well known piece of weather lore says, "Never Cast A Clout Until May Is Out". Clout is an old English word for cloth, so the warning in the saying is that chilly conditions are still possible until the end of the month. That has been the case at times recently and there have been some chilly nights. Nonetheless, it looks as though the rest of May will bring a lot of settled and increasingly warm weather. However, it won't be completely dry and regional variations are expected.
In the short to medium term high pressure will probably have a lot of influence on the UK's weather. The details are uncertain though and at times more changeable interludes are possible as high pressure is eroded to an extent.
The GFS forecast pressure chart for Saturday, 27th May indicates that high pressure will be centred to the west of the UK. It suggests mainly dry, settled and rather warm conditions.
Spring often brings mixed weather, but this year that tendency has probably been increased by the Sudden Stratospheric Warming (SSW) which took place in late winter. It is likely to have been one of the factors leading to high pressure being centred further at times than is usually the case. In turn, areas of low pressure have pushed towards the UK and brought very wet periods to the UK.
The GFS chart shown above is just a snapshot from one computer model which updates every 6 hours. Therefore, it shouldn't be considered on its own when looking more than a few days ahead.
The key is whether the ensemble data supports the scenario. An ensemble model is run many times, with the starting conditions altered slightly in each instance to help account for uncertainty about the current state of the atmosphere.
The graph below for London is generated using data from the GEFS ensemble model. It shows forecast air temperatures at about 1500m above sea level and precipitation rates from all of the individual runs for the next 16 days.
The key points to note are:
1) Air temperatures are increasing. Through the first few days they are forecast to be close to the 30 year norm which is shown by the thick black line. By the last week of May they are well above it.
2) Precipitation amounts remain low. There are spikes on the lower part of the chart (each one indicates a forecast of rain from an individual run at that time) but they are small as well as few and far between
Even ensemble based forecasts can change quite quickly. Nonetheless, the trend visible on this chart has been showing for several days now.
As the air mass warms over the UK how will temperatures at the ground level respond? Quite positively if the GEFS is correct, particularly in southern Britain.
The data table below shows the forecast daily maximum temperatures for London from all of the GEFS runs for next 16 days. The different colours represent temperature ranges. Initially the light orange dominates, it is used to show the percentage of runs forecasting between 16C and 20C. However, during the last week of the month the amount of darker orange increases. By 29th May 61% of the runs are in this category, which is from 21C to 25C.
Therefore, it's not yet looking hot. Nonetheless, maximum temperatures of between 21C and 25C would be pleasant for many people. Also, temperatures in locations further north will generally be lower.
The combination of drier and warmer conditions suggests a promising picture for those looking forward to enjoy outdoor activities, gardening, or festivals later this month. After an uncertain start the barbecue season may finally find its mojo! Despite the promising signs it is worth remembering that when looking this far ahead at the weather prospects in the UK there is always an element of uncertainty.
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