The analysis chart above shows a weakening ridge of HIGH pressure over the UK being edged out north by a slow moving Atlantic LOW to the W/SW. Reigate is still currently (Saturday am) in cool dull easterly winds generated by the HIGH but a significant switch in wind direction will take place over the next 12 hours into the bank holiday period as a wholly different mild and humid Sub-Tropical Atlantic air mass, with a source region round the Azores, takes hold from the SW.
A mild moist S/SW wind drives in from the Atlantic as the LOW edges north east tonight. The first mass of rain is edging onto radar from the SW and is expected to arrive in Reigate by around mid-late pm today. Most rain is likely for places further north and west but the SE is still likely to pick up plenty of wet weather overnight with low cloud and rain into Sunday morning when it could turn heavy and showery for a while in the early morning as the trough passes directly overhead and pressure continues to fall. Things are expected to clear to brighter conditions later in the afternoon as pressure rises and winds turn more westerly. Cloud cover will hopefully break and cloud height will lift during the afternoon becoming more cumuliform.
If the sun comes out then there could be a low risk of an odd heavy shower Sunday afternoon, possibly thundery, but these are more likely further north of our area where more unstable air makes progress across the Midlands and East Anglia.
During Sunday winds will be occasionally blustery with moderate convective gusts possible, especially on hills and nearer the coast, and make the mild temperatures Tmax 16C feel considerably cooler. Temperatures overnight could hold up to a balmy 12-13C.
Winds turn from SW to more southerly through Monday and pressure should up-tick slightly giving a mostly dry and warm day and less windy as things stand currently. Troughs could progress east during Monday and build cloud and produce some showers. More importantly there is a looming threat of something special for later Monday-Tuesday night.
As the northern block (high over Greenland) holds on, the Atlantic LOW just west of Ireland will usher in a mild and moist S/SW flow of air from the continent. An unstable LOW brewing in the topical Atlantic today (Saturday) is forecast to sweep up and intensify from Biscay later Monday and into Tuesday and this might bring heavy rain and winds to the south and SE and a possible thundery episode later Monday but more likely overnight into early Tuesday for SE. The jetstream is dipping well south and is forecast to perk up and approach the UK from an unusually southerly direction by Tuesday. If this happens the jetstream could deepen this low considerably, as modelled by some charts (latest UKMet shows 980mb).
Depending on the evolution we could find ourselves in the unstable left exit region of a jet where divergence aloft enhances convective action and creates heavy rain. Warm air from the south will also contain more moisture. A dry slot at mid-levels might also enhance instability (rising dry air cools more quickly increasing lapse rates and CAPE, enhancing lift). High dew points near the surface temperature also encourage condensation and indicate extremely moist warm surface flows.
So all these ingredients stirred up could be a good recipe for some briefly moderate-severe weather in our region especially some briefly torrential rain, though totals are unlikely to amount to more than 10mm. Gusty winds and gales near coasts could also accompany this system. Latest metoffice chart shows pressure dipping to 980mb in the North Sea which is significantly LOW pressure for the time of year.
However these episodes have a habit of tracking across Holland and merely clip Kent with thundery showers and miss us entirely. Models also generally exaggerate these early on and then things flatten out nearer the time considerably. Nevertheless, it is worth watching this develop as our first potential “warm plume” of the year. If we take a direct hit the SE could have some heavy rain.
The GEFS summary below clearly shows the two main rainfall spikes tonight and Monday night.
Later mid-week the LOW is expected to drift east across the UK bringing in a more westerly pattern so unsettled showery weather is likely for a while. Thereafter, a rise in pressure from a developing Euro high pressure may then take place from the south and settle things down for us in the SE, though this might only make faltering progress.
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