BBC monthly outlook
Summary
Unsettled and cold at first. Turning warm later
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Wednesday 5 May – Sunday 9 May
A cool, showery week and warmer weekend
The remainder of the first week of May will see cooler than normal weather persist through the working week as winds slowly ease. Scattered showers are expected in the afternoons, and these may bring hail and rumbles of thunder to some areas. In Northern Scotland, showers may be wintry too. As we head into the weekend, another low pressure system will approach from the west, but this will bring a significant change to the temperatures. Saturday and Sunday will see much warmer air move in from the south as bands of rain push into western and northern areas.
Monday 10 May – Sunday 16 May
Staying cool and showery for a time
The very warm air is not likely to stick around for too long, with a cold front bringing temperatures back below normal early in the second week of May. The jet stream, a ribbon of fast-moving air in the upper atmosphere that drives weather systems, will tend to be nearby or just to the south as high pressure retreats into the tropical Atlantic. Low pressure should be more dominant throughout the week, although we do expect high pressure to build back into the UK later on, towards mid-May.
As the high builds in from the south, northwest or north winds will develop on the northern edge of it, and this will continue to feed a polar air mass into the UK. Temperatures will stay a bit below normal for the time of year throughout the week, but around mid-May we should begin to see some drier conditions. This is due to the high pressure system from the south pushing the jet stream north of the UK, which will take the weather fronts with it.
The one potential hang up in the forecast is over high pressure to the northwest of the UK. If this high instead builds stronger to the north of us, it will set up a drier pattern with warmer air feeding in from the south. This would likely create a distinct north-south split, as Scotland and surrounding areas are still feeling the polar air while southern England and Wales get tropical Mediterranean air. There is perhaps a 25% chance of this developing instead.
Monday 17 May – Sunday 30 May
Dry weather at first; warmer and wetter later
The second half of May will increasingly be dominated by a strong area of high pressure from the southwest. This high, often called the "Azores high" due to its tendency to strength near the island chain, is a prominent feature in the Atlantic in late spring and summer as the northern hemisphere warms. This year there is also unusually warm sea surface temperatures in both the Pacific and Atlantic which helps promote high pressure development around the Azores and west of Europe.
Our computer models are not having a very successful time forecasting the long range pattern this month, so we are leaning a bit more on the historical analogues. These are statistical analyses of previous years with similar large-scale weather patterns to what we are seeing now. Many of these historical analogues suggest high pressure will have a frequent presence in western Europe, but they differ on exactly where and how extensive it will be.
For the forecast over this fortnight, we expect a lot of dry weather with high pressure nearby or overhead. Temperatures will likely moderate and even begin to warm up as there are some encouraging signals for the high to finally shift eastwards towards the end of the month. This would also come with some wetter weather as low pressure develops to the west of Ireland. The main risk for this forecast, with perhaps a 35% chance of developing, is that colder air continues to feed in from the north with a more unsettled pattern returning later in the month.
Further ahead
With better signs for a warmer, but wetter, end of the month, we will look into early June to see how the start of meteorological summer is trending.
https://www.bbc.co.uk/weather/outlook