You may have heard the phrase ‘Spanish Plume’ in the news or read in the papers for the brief heatwave starting today, so what exactly is it? Well, to try and put into layman terms as much as I can, there are various ingredients that technically define the Spanish Plume in meteorological terms, which may differ and seem more complex from what you may hear/see from the media.
First of all, like the next few days, an upper trough stalls to the west of western Europe over the east Atlantic, ahead of the trough a strong southerly upper flow develops across western Europe, marked by a strengthening southerly jet stream over the next few days. This will draw very warm and increasingly humid air north from SW Europe. And with the ‘partial’ thickness of this airmass (i.e. in the lowest 5km) becoming rather high across southern Britain over the next few days, it’s going to become hot over the next few days, with 30C possible today in the southeast of England and 32C possible here tomorrow.
So the surface airmass will become increasingly humid from the south later today and through Friday and Saturday, with dew points rising into the high teens and perhaps low 20s across southern Britain, so it will turn increasingly sticky and uncomfortable at night for sleeping.
We do have an increasing risk of thunderstorms too from later today. Now, the Spanish plume is characterised by warm and dry layers of air aloft (say 1000m up) originating from Morocco and Spain, this will advect north across southern Britain ahead of the upper trough to the west. This in itself is not the precursor for thunderstorms, but with the surface or ‘boundary’ layer becoming increasingly hot and moist, the atmosphere will become increasingly unstable, as the relatively warm and dry layers aloft create a ‘lid’ on this instability being released and also will serve to steepen lapse rates.
Therefore, we need a trigger for this energy to be released and later today this will come from large scale ascent created by a well-defined shortwave upper trough moving N/NE over the Bay of Biscay this afternoon/evening ahead of the main long-wave trough to the west. This will force warm and moist layers upwards to create thunderstorms. These storms look to develop initially over the Bay of Biscay/NW France along/ahead of the northward moving trough and with fairly strong vertical shear (i.e. winds strengthening and changing direction with height) beneath the southerly jet, storms look to organise into an MCS (Mesoscale Convective System) which then drifts north across SW England, perhaps central-S England, Wales, W Midlands and Ireland overnight. This storm system is likely to become ‘elevated’ (not rooted to the surface/boundary layer) though, as the boundary layer is still likely to be dry and more stable than further south, with the higher moister not arriving yet. This potential storm system moving north up the western side of UK tonight could, though, produce gusty winds, hail and frequent cloud-to-ground lightning.
The storm system looks to rumble its way north across Ireland, NW England and perhaps southern Scotland on Friday. Behind from the south, the airmass stabilises as weak ridging takes over from the SE, so a generally dry, very warm or hot and humid day across England and Wales, temps hitting 31-32C in the southeast.
But real trouble is brewing later on Friday, as the southerly low-level jet strengthens and a thundery low with frontal system develops over France, so another MCS looks to move north, this time further east, from France across S England in the early hours of Saturday, before moving north across Wales, The Midlands, N England and Scotland during Saturday. The models are showing large amounts of potential convective energy (CAPE), strong vertical shear and jet stream aloft, so there’s potential for some of these storms to be severe, with a risk of strong/damaging wind gusts, frequent cloud-to-ground lightning, hail and torrential rain leading to flooding. If storms can become surface based on Saturday, especially near cold front slowy clearing from the southwest, there’s even potential for a tornado. Some uncertainties from Friday evening, as is usual in these set-ups, over exact track of storms, so it will be a case of nowcasting but also watching model updates.
Further storms appear possible across eastern areas on Sunday, as the remnants of the plume hang on here, but turning somewhat fresher and drier in the west.