The combination of a warm and humid plume spreading north over the next few days and low pressure with fronts moving in from the Atlantic will trigger heavy downpours and thunderstorms through the weekend.
It’s been changeable this week, with outbreaks of rain spreading across many areas on Tuesday, heavy and persistent in the north, with showers in places since then, but also some dry and sunny conditions.
Although the start of the week saw temperatures return closer to average by Tuesday, after a prolonged period of above or well-above average temperatures, today and tomorrow will see increasingly warm and humid Tropical maritime air spread north across the UK ahead of low pressure moving into the west.
The air literally feels tropical in London this evening, despite mostly cloudy skies and some scattered shower passing through. Temperatures here and other parts of southern England are reaching the mid-20s Celsius, but the dew point is reaching as high as 19-20C too, bring a rather humid feel, with the ‘feels like’ temperature more like the low 30s Celsius.
This very warm and humid air will stick around over the next few days and will be the fuel for an increasing threat of heavy downpours and thunderstorms, as low pressure and fronts move in from the southwest and interact with a plume of very warm and moist air spreading north across NW Europe.
The Met Office have issued Thunderstorm warnings for northeast England for Friday afternoon and for much of England from tomorrow evening through Saturday.
A cold front with a band of rain pushing east across Ireland this afternoon and also across western Scotland trails all the way southwest to the east of the Azores. This cold front will slowly push eastwards across mainland UK on Friday. Breeze convergence ahead of the cold front, aided by sea breezes developing as the land heats up, could trigger some heavy showers and isolated thunderstorms across eastern England Friday afternoon and evening, fuelled by over 1000 j/kg CAPE forecast - created by surface heating of warm and humid plume with wet bulb potential temperatures (theta-w) of 16-18C and steep low to mid-level lapse rates.
Fax chart for noon Friday shows cold front position, also a twig-like symbol, which is a convergence zone, over NE England - that may trigger storms in the afternoon.
Large amounts of CAPE towards North Sea coasts of E England Friday afternoon around convergence zone could fuel some storms by evening
Meanwhile, a wave is forecast to develop along the cold front across northern France in response to a thermal low developing over France as temperatures reach the low 30s and also increasingly cyclonic gradient developing in the flow ahead of a shortwave trough moving NE from the Bay of Biscay. The frontal wave developing along the boundary of high theta-w plume and cooler air to the west and increasing ascent of air with approach of shortwave trough from the southwest will create lift of warm moist plume – with heavy rain and embedded thunderstorms developing over Normandy and the English Channel during the evening, before spreading and expanding north across SE England, East Anglia and the rest of eastern England along with East Midlands early Saturday morning. There is a risk of frequent lightning and large rainfall totals in a short space of time leading to flash-flooding from these storms.
Shortwave trough arrives from the southwest and engages high theta-w plume over NW Europe, generating storms on the nose of the higher theta-w across N France
Heavy downpours and thunderstorms spreading north then northwest across the UK on Saturday, away SW England and Wales - which will be drier until later in the day
Heavy rain and thunderstorms then spreading further northwest across northern England and Scotland through Saturday, drier and sunnier conditions following from the southwest for a time, but further showery rain, perhaps locally heavy and thundery, looks to spread in from the southwest across Wales and southern England Saturday night, as an area of low pressure south of Greenland at lunchtime today, tracks southeast then east into the southwest approaches or English Channel by early Sunday afternoon. Showery rain, perhaps locally heavy and thundery, spreading further north and northeast across England and Wales on Sunday, some sunny spells in between showers too. The remnants of Saturday’s thundery rain and showers affecting Scotland too. So all-in-all, an unsettled day, and turning quite windy across the south too, as that low moves into the English Channel. But not a washout everywhere.
More heavy showers and some storms spreading northeast across England and Wales on Sunday
Further heavy showers and thunderstorms spreading in from the southwest across southern areas Sunday evening and night, as low pressure moves in across the southwest. Showers elsewhere, but also some drier and clear spells.
Slack low pressure looks to become slow-moving across the UK on Monday, so sunny spells but also slow-moving heavy showers and thunderstorms breaking out quite widely, with a risk of hail and localised flooding.
Low pressure will tend to pull further north on Tuesday to be across northern UK, so sunny spells and further showers likely for many. Low pressure moving towards Scandinavia mid-week, so showers perhaps becoming confined to eastern areas. Then later in the week, we may see high pressure build in from the west, heralding a drier, warmer and more settled end to the week.
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