Sometimes by late April there can be some very warm weather if the conditions are right. Dry ground helps, along with dry air and given the strength of the sun by then (only 7 weeks to the strongest sunshine at the summer solstice) the Temperature can reach into the high twenties. 

By midday on April 30th 2005 some very dry air was aloft having advected north from France during the afternoon but there was some  Instability present however at mid levels. Towards the south west approaches satellite imagery showed developments with Fronts moving north across the Irish Sea and the south west of England by the evening. Tephigrams taken that evening showed a classic 'warm nose' at about 3000ft where moister warm air lay aloft and a very unstable profile developing. Cold advection was also present aloft which was associated with a weak Trough that moved across England. It was this added energy that allowed Convection to break out as the cold advection aloft allowed Cb tops to readily shoot upwards and reach towards and into the tropopause, where reported tops were as cold as -60C. This is the surface chart for 00Z on May 1st: http://www.wetterzentrale.de/archive/2005/brack/bracka20050501.gif showing the trough moving across South West England and the warm air ahead of it, a warm front lying across the north.

The storms began to develop very impressive mid level Lightning and thunder. However little rain fell at this stage due to the generally dry air aloft still beneath the Cb bases. Over the next few hours the storms grew; a storm across Devon moved into South Wales, another moved from Dorset cell towards the Bristol area and near Oxford and Farnborough new storms developed. By now some heavy rain was being reported at the surface.

The storm over Farnborough grew to produce hail over Berkshire and some very heavy rainfall rates with 11mm in an hour reported from a gauge in Berkshire. There were at least three major storms by 02Z with thunder reported across Wales and up towards Cumbria. The second cell moved into Northern England and out into the North Sea and the third cell moved more NE to be over Essex and then the southern North Sea where it may even have gained super cell characteristics.

The event was a classic example of the breakdown of an early plume event, generating big mid level storms. The storms all had high bases and so the lightning was more impressive than the rainfall totals which were only very heavy in a few places.