During Saturday 30th April a plume of High Theta W moved NNE having originated from the Spanish Plateau over the previous 48hrs. The plume moved north behind a warm front which was lying across Scotland late in the day.

Some very dry air aloft was present through much of the day on the 30th having advected north from France during the afternoon. Mid level instability was present however on the 12Z Brest ascent though with dry layers beneath.

To the SW of the UK WV imagery was showing a very complicated upper forcing pattern with minor short waves ahead of a much more vigorous long wave trough to the N of the Azores. During the evening and night one such shortwave moved north across The Irish Sea with a filament running SE from it across England. High resolution models had been forecasting some mid level destabilisation across these Irish Sea Regions, but in the end the main events occurred more to the SE than had been predicted.

The 00z ascent from Brize Norton - (see link here) shows the instability aloft. The boundary layer is out of the equation and the warm nose at 850mbs classically shows where the spanish plume lay aloft. There is however already instability in the mid layers with only a small decrease required to send tops well up to the tropopause at just under 250-200mbs. The fairly steep lapse rate on the profile showed how the atmosphere had become more unstable aloft since the 12Z ascents. The warm nose at 850mbs had increased whilst cold advection above 500mbs had decreased the temps there. The net effect was to create a more unstable profile - and one that became more unstable with time.  Analysis of the 300mb and 500mb temperature advection charts show that cold advection was present aloft associated with a weak trough moving SW to NE across England.  This trough allowed the convection to break out and release the reservoir of mid level CAPE which was present. The cold advection aloft helped allow the tops to readily shoot upwards and reach the tropopause. Tops recorded by MSG Met-8 were near -60C which extrapolated into the 00Z Brize Ascent would indicate tops to the tropopause. 

Cells were initiated over Devon and Dorset at around 22.30Z on the 30th, these storms began to develop with impressive mid level lightning and thunder. Little precipitation was recorded at this stage due to the underlying dry zone beneath the storms initiation level. Over the next few hours the storms grew from three sources. The storm over Devon moved N to lie over Wales by 00.45Z, the Dorset cell moved north to lie over Bristol and Oxford as an expanded cell and a new cell started to develop nr Farnborough. As the storms continued to grow they moistened the low level atmosphere with some heavy rates being reported at the surface by this time.

The cell over Farnborough grew to produce hail over Berkshire and some very heavy rainfall rates (not entire rainfall rate reached the surface due to high cloud base) 11mm in the hr was reported from a gauge in Berkshire. A detailed report of this storm can be found here on the UK Weatherworld website forum by Nick Verge.  

By 01.45 there were at least three major cells occurring, the cells moving in slightly different directions as the upper flow over N England and S Scotland was bifurcating. This is to say that clouds at steering height were moving east to west over N Ireland but west to east over NE England. It appears this divergence aloft created an enhanced environment aloft for the storms to grow in. The Welsh storm continued to affect Wales and parts of NW England (now rumbling over Cumbria). The second cells moved into N England and into the N Sea and the third cell moved more NE to be over Essex and then the S North Sea where it may have gained mid level super cell characteristics (tops were over -60C) and had a good appearance on imagery.

The event was a classic example of the destabilisation aloft of a spanish plume by a relatively inert upper trough which created enough differential thermal advection to generate big mid level storms. The storms all had high bases and as such the electrical activity was far more pronounced than the rainfall (which was very heavy in a few places where the atmosphere allowed the moisture to reach the surface.

Figures illustrating the above can be accessed below (by kind courtesy of Avbrief, Meteosat and 'UKWeatherworld')

Fig 1 - NMM High Resolution Theta W( (WBPT) for 03Z - The storms can easily be seen to be focused in the area of High Theta W (Greater than 14C) This providing the storms with good mid level ingredients and allowing instabilty to be released)

Fig 2, the MSG Satellite for 00.30Z and Fig 3 the MSG satellite for 0145Z;

Fig 4, the regional radar for central and southern UK at 0045Z and Fig 5 at 0130Z showing the growth of the three cells discussed above.

The tops reached nr -60C in places and this equates to the the top of the tropopause on the Brize Ascent above.

(C) P Blight 2005