Warm and humid air for this weekend with bursts of heavy rain. Uncertainty in the forecast for the late August Bank Holiday weekend. No heatwave guarantees, whatever you may read elsewhere.
The second half of August still allows summer warmth with its strong sunshine and long hours of daylight. The daytime temperatures so far this month haven’t been that special but often it has stayed on the mild side by night.
Later this week there will be warm air surging up from the south, with temperatures lifting into the mid 20sC, even up to 27 maybe 28C by Saturday. However, low pressure will slowly edge its way over the UK this weekend. The frontal bands reach over Ireland on Friday afternoon, then western Britain and most of Scotland by Saturday morning. As the low centres over the UK for Sunday, there will be heavy frontal rain and hefty downpours with the risk of thunderstorms. For the gaps in between, sunny spells will appear and the air temperature will manage to shoot up.
August 21st and 22nd - warm air but swirling bands of heavy rain, and thundery downpours by Sunday
These downpours will bring the risk of localised flooding but the air will feel warm and humid.
The ongoing social media shrieking about an upcoming heatwave seems to just be the usual thing. There is a bank holiday weekend coming up so the weather “forecast” has to include extreme weather. “Fair, warm with sunny spells and a few showers” just doesn’t cut the mustard to get people to click on the Facebook article or newspaper link.
We’ll await the Bank Holiday washout headlines next and then the misdirected blame. There is a lot of uncertainty in the forecast at the moment. The probabilities edge towards warmer than average, particularly for SE Britain by the end of the month. SE England average temperatures for August are around 21/22C. So London might get a few days of 25C but it's not the low 30sC, which some parts saw in July. A heatwave is defined in the UK as 3 consecutive days equal or above a threshold temperature that varies around the UK. That would need 3 days in the high 20sC across a region.
So the heatwave claims are giving quite a forecast from two weeks ahead and it's not the Met Office, however the wording is spun. The synoptic situation still looks quite mobile. The variations in track of a low pressure can result in fine, sunny, warm weather for the sheltered east coast to wet and blustery weather in the west. A low might move through faster, or reach further north and so the rain would be in a different place. If a high pressure looked to be establishing itself for the end of the month then that would anchor and steady the situation and give more confidence. There are more Atlantic low pressures on the way including what will have been Hurricane Henri adding into the north Atlantic energy mix, hence the uncertainty.
And that variation is all part of the British weather’s charm. In 2019, London saw 33.2C in London for the late August Bank Holiday yet last year it was unseasonably cool. A cold northerly flow meant that nowhere in the UK reached over 20C for the whole of the long weekend in 2020. There was even a frost in places to start BH Monday.
The ECMWF model shows high pressure building over the UK for the middle of next week with an Atlantic low trying to edge in from the west but then getting deflected up towards Iceland. The Azores High then builds in towards the UK which would bring settled, fair weather. The GFS model has high pressure further east over Europe with cooler air aloft which could bring more showers over the UK. Differing setups and air flows.
Maybe the UK will see sunshine and heat at the end of August, but trusted sources will share that if and when it is a steady forecast, not a clickbait punt. Looking at the situation in southern Europe this month, heatwave headlines can share a grim reality. With Sicily recently passing 48C and a new European highest temperature being investigated. Spain seeing its highest ever temperature recorded at 47.4C last weekend and terrible ongoing wildfires from Turkey and Greece across to southern France and Iberia. After the IPCC’s Climate Report earlier this month and with CoP26 coming up in Autumn, it’s fine to wish for some warm sunshine for a Bank Holiday break but heatwave headlines might need more care in a time of “Climate Change – widespread, rapid and intensifying.” IPCC
