Warm & Humid = Thunderstorms

There seems to have been a distinct lack of 'warm humid plumes' or 'Spanish Plumes' over the UK in recent years with their associated often lively storms that accompany them. Most of our summer storms over the last few years tending to occur in cool polar airmasses. But, with a warm, humid and increasingly unstable airmass arriving from the near continent over the next few days, it does look like we could perhaps see some lively storms in places this week, before cooler and fresher Atlantic air returns across all areas on Friday.

This change in weather pattern is coming about because an upper low has cut off (or 'disrupted') towards the Bay of Biscay and Iberia from the main belt of upper westerlies across the North Atlantic.  This area of low pressure to the SW of the UK over the next few days will pull a 'plume' of warm and humid air from the near continent north across England and Wales by mid-week. A warm front marks the northern extent of this warm and moist airmass and although there is a little uncertainty over how far north the warm front will advance over the next few days, it seems likely the northern and western extent will be north Wales and northern England, with cooler and fresher conditions remaining to the north over Scotland and Northern Ireland.

Hot and humid air is in place on the near continent, with temperatures soaring to 31-33C across central France this afternoon. An upper warm front marking the northern extent of warm moist air aloft has drifted north across southern and central areas today, bringing an increase in mid-level cloud with the odd drop of rain falling from it reaching the ground, but much of it evaporating. However, the air at the surface is still fairly dry with the wind blowing from the NE. Over northern France is a surface warm front, marking more moist/humid air at the surface and this will push in across the south of Britain overnight.

As well as the increase in humidity from the south over the next few days, warm moist air aloft will destabilise to form storms. Warm moist layers spreading north aloft are likely to be forced to rise by lowering pressure on the near continent and also as cooler air overruns it higher up - steepening lapse rates (fall in temperature with height) and creating warm air to rise into cumulonimbus clouds to a great height. So there will be an increasing risk of heavy showers and thunderstorms over the next few days. Already we are seeing these elevated storms across northern France, affecting the Channel Islands this afternoon. Showers and storms will initially be from elevated/high based cumulonimbus clouds above a stable surface/boundary layer, but as increasingly warm and moist air advects in across England and Wales, there is scope for 'surface based' storms to develop in places inland with help of warm sunshine and surface wind convergence. These surface based storms more likely on Wednesday, when temperatures are forecast to reach the mid to high 20s where the sun breaks out.

The warm and humid air is likely to hang on across southern and southeastern England on Thursday, continuing the risk of surface based storms developing, especially when the sun comes out. But cooler and fresher conditions across the north and west will gradually spread further south and east, so storms will likely merge with a band of rain developing across central and western areas before clearing east on Friday morning to fresher and cooler conditions everywhere.