Update: pressure building to North over UK Weds and highest risk thundery activity to very SE with highest euro temps. We expected MUSE but only Justin Bieber turned up!!
OK, so thundery warm weather is on the way for SE and Southern England first half of this week (a “heat spike from Europe with a cool down by next weekend, sadly). What’s the cause? Without mentioning CAPE (convective available potential energy) or LI (lifted index) or instability… here goes a simple and quick version of what is causing the potentially explosive weather early this week… and if you get caught in one of the multi-cell thunderstorms it could deliver a potentially whopping amount of hail and rain in a short space of time. The atmosphere is, at times, going to be unusually energetic. Watch out for spectacular cumulonimbus clouds and anvil heads and possible mamatus clouds.
To get big thunderstorms you need moist warm air which wants to RISE into the atmosphere (called unstable: once it starts rising, it won’t stop… until it hits the tropopause which is about 10 km up and the top of our weather / ultimate lid on clouds). These moist rising thermals naturally occur most freely in LOW pressure where air is liable to rise when the surface heats up in the strong June sunshine. A slack moist air flow and strong sunshine over the UK ticks this box.
As the thermals rise it cools down and any water vapour will condense and form droplets of rain and ice which are visible as clouds. When water vapour condenses it releases more heat (latent heat) that, in turn, gives thermals more lift and drives them higher. This causes the explosive effect of rapidly rising bubbling cumulonimbus clouds. Watch out for these rapidly growing cumulus clouds as precursors to big thunderclouds this week (cumulonimbus). The updrafts of rising air can rise vertically pretty quickly (updrafts at 45km/h or more), like a hot air balloon.
All these elements are in place this week: moist slack air over the UK will be met by an overriding warm plume of relatively dry air from Spain and North Africa (Spanish Plume) at a warm front over the English Channel. The final ingredient for really big multi-cell thunderstorms is a difference between wind speed at the surface and aloft, called wind shear. If thunderclouds stay still the cold rain causes downdrafts of cold air which cut off the ririsng thermals killing the thunderstorm after a few hours. Fast moving air at altitude allows warm updrafts to continue being pumped into the moving cloud and continue the growth of and activity of the thunderclouds. The cold downdraft gust fronts pushing ahead of thunderclouds can cause more warm air to lift and spawns more baby-thunderstorms, hence “multi-cell”.
A more technical view can be seen in the tephigram above. This shows a cross section through the atmosphere and the steep reduction in temperature with height (lapse rate). Notice that, at first, there is a “cap” or lid at low levels (below 500feet) which stops thermals from rising. This inversion is necessary for the most explosive thunderstorm development. The rapid warming of thermals at the surface eventually allows them to break through this lower lid (inversion) and to rise uninhibited to the top of the atmosphere giving a potentially very great vertical height to cloud formation and severe weather below. Hail forms in such thunderstorms because water droplets rising and falling with up and down drafts freeze at great height and fall to earth before fully melting.
Finally, tornadoes could even be formed if there is sufficient wind shear (change of wind speed / direction with height) to rotate rising air columns which eventually twist round into the vertical. Check the diagram and video below for a summary of these processes at different levels of complexity!