An increasingly unstable atmospheric environment is expected to develop in the coming 24 hours and as a result there is an increasing risk of thunderstorms across parts of the British Isles. Two separate periods of instability are expected. The first of those is expected to develop this evening across western and southern parts of Britain. Some storms are likely overnight across Irish Sea Coastal areas – SW England, Wales, Northern Ireland and Western Scotland. Perhaps also Cumbria. Some hail is possible from these storms, as well as the risk of frequent lightning and strong winds in and around the systems. Some further thunderstorms are possible in the central English Channel later in the night, these may affect southern coastal counties of England later in the night with the possibility of frequent lightning and heavy rain, these may make progression inland towards the home counties. As a result – the following risk map has been issued for tonight.
It is important to note that this is just a risk – and some areas (even within the highlighted zones) are bound to avoid the thunderstorm activity.
As temperatures rise through tomorrow, instability will increase yet again, this time more concentrated over central and northern England. There is a risk here of significant thunderstorm development into the afternoon. As a result the following risk map has been issued for this area. As these storms head off into the North Sea late on Friday they could also affect Eastern Scotland. There is also a continued risk of storms in Northern Ireland.
DETAIL
An upper level trough is currently making progress across western Britain and is engaging in relatively high surface and mixed layer CAPE across western England. As a result, scattered yet strong thunderstorm activity is expected to develop in the south west (Irish Sea area between SW England and Ireland – and migration up towards Northern Ireland and western Scotland later in the night). The elevated nature of these storms means some frequent lightning, as well as hail is a possibility. Some high resolution models also have some moderate storm activity across western England and NW England overnight – however this risk is more uncertain and less likely due to a strong atmospheric Cap in these areas.
A second cluster of storms associated with relatively high mixed layer CAPE (roughly 900j/kg) across the central English Channel is expected to develop, after midnight. This development could become organized in nature as it makes landward progression towards southern coastal counties of England – potentially between Brighton and Southampton – towards the early hours. With this development, there is some uncertainty about the likelihood of initiation, which is somewhat less present in the above area further west. This is due to relatively strong atmospheric capping (lack of trough presence in this region).
The above areas of storms could develop significant vertical ascent and thus duration potential early on Friday. As a result, some northwards progression into the western Isles, NW Highlands and Stowaway is possibly early on Friday.
Later on Friday, very intense ground level atmospheric instability is expected to develop across central and eastern England with surface based CAPE values close to 2000j/kg in places, which is rare for these parts. The trigger for such instability seems to be greatest further west, a region from the North Midlands, into much of Northern England currently looks most favored for initiation, however in terms of intensity, any storms which initiate further east hold greater severity potential. As a result, a blanket yellow level covers these regions. The risk remains higher further NW despite the slightly lower apparent atmospheric instability. Some multi-cell activity is possible with surface based rotation also possible. Large hail, strong winds and rainfall totals over 50mm may occur locally. Again, the risk of initiation is somewhat debatable due to the presence of atmospheric cap, and the potential for a mistiming or misplacement of the trough feature. Therefore the risk of initiation is only moderate at this stage. If storms do initiate, limiting factors discussed may prevent these storms from developing to a very severe level – such as seen on 28th June 2012.