May 19 thundery showers was not a “severe” weather day but 10mm of rain from 7 showers in 7 hours, several with some moderately intense rainfall rate (30-40mm/hr), small hail and episodes of thunder, was of interest locally and deserves a review of some charts leading up to the event.
Often our area of the SE seems to miss convective action but this time showers perked up as they converged across East Surrey and some developed into reasonably impressive cumulonimbus clouds (cb). As a bonus mammatus clouds were spotted at the end of the day (see photo above). Below is a summary of some key ingredients for the day.
Low pressure: This synoptic set-up saw an unusually cold plunge of upper air flood across the country during May18-19. The synoptic chart above shows the invasion of cold polar maritime air behind a cold front, itself not especially dynamic across the SE. The LOW over the North Sea dragged this unseasonably cool air from the NW: the basic ingredient for instability on this occasion.
Note the u-bend wiggle in the jetstream around the LOW pressure in the chart above. The northward limb of the jet on the inside of the U-bend can become the location for instability, though this was not a fast jetstreak occurrence. The base of the trough also crossed the SE of the UK during the day encouraging lift. Showers were forecast on the charts days before.
Steep lapse rates: Very cold upper air temperatures fell to below -30C at 500hPa (5500m) across the UK. In Spring a cold airmass like this can become unstable over land especially if the sun warms the surface to create thermals of warm air able to lift through the cold environmental air. The 850hPa temperatures show comparatively warm air at 1500m which increases the lapse rate. The morning of May 19 was sunny and the surface heated with 5 hours of May sunshine to a modest 14C, just enough to release thermals and rising parcels of air. Convection requires heat as a key ingredient to steepen lapse rates and create thermals, rising parcels of air.
Low lifted index: LI forecast for Heathrow (below) showed a LI of -3. The lifted index is a measure of instability in the atmosphere and shows how readily bubbles of warmed air will rise from the surface to a great height (convection). It is the difference in temperature between the environmental air at 500mb level (around 5500m) and the temperature of the theoretically lifted bubble of air (parcel).
Negative LI numbers are good for thunderstorms because they show that the parcel is significantly warmer than the air around it at altitude, which means that at 5km the air is still bouyant. http://www.weatheronline.co.uk/reports/wxfacts/Thunderstorm-Probability.htm
Moderate CAPE: convective available potential energy is the energy available to push air vertically up and create those surging bubbles of cumulonimbus clouds. The higher the CAPE the greater uplift potential (above). Note it is potential energy and a high CAPE often confounds convective activity if other ingredients are lacking, such as heat or moisture. 19 May had marginally unstable values for the UK of 800j/kg.
— Richard Aust (@richardaust) May 20, 2015
— Rob Peyton (@robinpeyton) May 19, 2015
In the USA Tornado Alley forecast CAPE values exceed 2,000 j/kg in super cell outbreaks. CAPE on a skew-t chart is the area between the parcel trajectory and the environment temperature. In the SE of the UK we often just get “skinny cape”, where rising parcels are only just warmer than the surrounding environmental air, whilst in Tornado Alley “fat cape” is frequent which indicate parcels much warmer than the surrounding air, increasing bouyancy. http://www.weatheronline.co.uk/reports/wxfacts/CAPE—Convective-Available-Potential-Energy.htm
The colourful charts above from lightningwizard.com show streamlines of wind and air mass equivalent potential temperature. Streamlines show where the airmass is coming from and, if they converge at the surface, can indicate potential for lift. Theta e charts show moisture and temperature characteristics of airmasses. Where there is a contrast of colours between high theta e and convergence at the surface and lower values and divergence of air at altitude this can bring on convective weather. The moisture chart shows that the airmass supplied sufficient moisture to the UK to provide for cloud formation and release latent heat. Dew points stayed relatively low and this showed moisture (and heat) locally was lacking for bigger storms. Compare these forecast charts with the actual occurrence of lightning on the day.
Forecasts for the day (Euro4) showed typical lines of rainfall accumulation which indicate showers. The actual radar showed “popcorn convection” (@metmanjames) over a wide area drifting east.
The weather today will therefore be dictated by ‘popcorn convection’ (been waiting to use that phrase) (Netweather). pic.twitter.com/WZT7i9LFSE
— James Warner (@MetmanJames) May 19, 2015
Recommend following @convectivewx for UK convective forecasts. Their forecast from 3 days ahead was spot on.