A brief, rather marginal, teasing Spanish Plume is developing in the next 48 hours and might cause the first significant thundery activity of the year in the SE Thursday into Friday this week. A Spanish Plume (written up here previously from 2014) is a plume of warm air that rides north, out of Iberia, through France and over the UK. It is often accompanied by a preceding south easterly from a warm or hot continent. This mix typically causes humid weather and thunderstorms, but plumes vary in quality!
This particular episode is marginal for the UK, and taunting forecasters, because the heating episode is very brief and most of any heat and resulting instability is forecast to remain over the continent impacting N France and Benelux with decent thunderstorms before they migrate into the North Sea. The UK might see little of this in comparison.
Whilst it is not certain that thunderstorms will develop over the UK, some action is likely in the S/SE and East Anglia during Friday. A MetOffice warning was issued earlier today but note the very low likelihood of moderate impact.
It’s worth recording here because it is a similar synoptic set up to the first plume event of June 7 2014 which created relatively perky thunderstorms to the SW and elevated rumbles and thundery showers over SE. The charts below show streamlines at different levels associated with the plume on Friday.
The plume on Friday is a heavily cut-down version of the “heat wave” that the charts flirted with last week. The heat wave is not going to happen but a day of warm / hot conditions will brush fleetingly past the SE. Upper air over 15C is set to push in from the continent through Thursday night into Friday morning and will quickly waft over the S/SE and raise temperatures to Tmax 27C before quickly being pushed east by cooler Atlantic air arriving later Friday.
This warm “heat” spike for the SE is caused by HIGH pressure (currently over the UK) slipping NE towards Scandinavia, followed closely by an Atlantic LOW edging in from the west. The result is a humid surface S/SE flow from the continent with an upper airflow directed from Iberia meeting cool polar maritime from the Atlantic.
The convergence of these airmasses can cause steep lapse rates (rapid drop in temperature with height) and instability, in which saturated air rises freely to a great height creating tall thunder clouds. The charts below show some of the parameters involved: note the relatively high (but brief) CAPE values and PWAT (water content). The limiting factor might be low SST temperatures over the Channel which could subdue any imported storms from France.
Instability can be caused locally if the sun comes out and heats the surface sufficiently to erode the “cap” and release parcels of warm air that rise, condense and form cumulonimbus clouds. Upper air is cold enough to cause hail as updrafts keep driving precipitation aloft into freezing cloud top temperatures below -30C. If it stays cloudy we can still get elevated thunderstorms imported in unstable upper air from the continent. Skew-T diagrams show the relatively unstable situation forecast for Friday. A dry slot / hydrolapse (where dew point departs markedly from air temperature in a mid layer) can increase potential instability because evaporative cooling in any dry mid-layers can increase instability by increasing the difference in temperature between rising thermals and environmental air (though 700hPa dry layers I think are more significant than the lower layer shown below).
It is most likely that the plume will be cut off rapidly before heat can establish and cause whopping storms. Nevertheless, heavy thundery showers are likely here and there. The latest GFS charts from lightning wizard suggest a low risk of meso-scale convective systems earlier on Friday being imported from France into the south coast in the LOW pressure moving North from Spain.
Later in the day any action is likely to migrate further east into the SE / East Anglia. Most action will be on the continent where some significant thunderstorms are likely. The charts below show how rapidly the cold air makes progress east. Once this arrives the plume is cut off and convective activity associated with it ends.
Looking ahead, pressure is expected to rise into the weekend in the south so it will be dry and pleasant, but not hot. Into next week things look similarly benign with high pressure mostly in charge in the Atlantic feeding a flow of cooler air keeping us a tad below average in terms of temperature. Into mid-June things become more unsettled.