A Severe Thunderstorm in Reading - 15/09/2016
At the end of the extreme September heatwave that I mentioned in my last blog post, a particularly intense thunderstorm developed over south-central England on the 15th September. Here I will just touch upon some of the features and impacts of a storm that evolved in a particularly interesting and dynamic environment.

After several very hot days across southern and central parts of Britain, with a maximum temperature of 34.4˚C at Gravesend on the 13th, thunderstorms were inevitable. "Spanish Plume" events like this tend to bring some of the hottest weather to the UK in the summer months, but they also bring an enhanced risk of thunderstorms, especially towards the end of the hot weather - when a thundery breakdown is triggered as Atlantic depressions and their fronts provide extra lift and instability. 

Spanish Plume events in the UK are characterised by a warm southerly flow of air from the near continent. In the summer, the high Iberian Plateau heats up significantly and this hot air is drawn over the UK. What is most interesting is that this hot air that has been heated at elevation provides a "cap" (that is a temperature inversion) over the warm airmass below, since it is much warmer than the air would otherwise be at this elevation. This cap acts to inhibit conviction, often leading to only fair-weather cumulus developing, or even clear blue skies. However, as temperatures build during the day, a very unstable environment develops at the surface, that is just waiting to explode into thunderstorms. The cap acts to "bottle up" instability and convection until the end of the afternoon, when the air becomes warm enough to ascend through it, leading to the vigorous development of cumulonimbus towers. Of course, sometimes the cap is too strong and no thunderstorms can form, which is why we don't see thunderstorms everywhere and every day during these events! 

An extra added ingredient is wind shear, which can be increased by an Atlantic trough or front approaching from the west. This rare combination of high instability and high wind shear can lead to intense supercell thunderstorms, with the risk of flash flooding, large hail, strong winds and tornadoes. 

The afternoon of the 15th September was another hot and sultry day across southern Britain, with a maximum temperature of 30.1˚C. Low-level winds were light, at about 10 knots from the south, while upper-level winds were also slack, at about 10-20 knots from the east. 
The sea breeze was rather strong as a result of strong heating inland and the light southerly airflow. WRF predicted the sea breeze would move as far inland as London and Reading by the end of the afternoon. Isolated thunderstorms began to kick off in west London by about 4:00 pm and slowly drifted westwards through the evening. One cell in particular became very intense and moved over Reading during the evening. The capping inversion was very strong and there was very little cloud throughout the day. I believe the storms developed on the sea breeze front, which provided the extra lift needed to break through the cap. 

This is the GFS modelled sounding at 5pm for Fairford, about 60 km to the north-west of Reading. By this time, heating at the surface has eroded away the capping inversion that was at about 850 hPa earlier in the day. Note the steep lapse rates in the upper atmosphere, but lack of significant wind shear. 

By about 6:00pm, the storm was situated over Reading and had a very intense precipitation core containing torrential rain and marginally large hail. (Hail about an inch in diameter was reported in Wokingham). It also took on the characteristic radar reflectivity echo of a high-precipitation supercell for a short while, with a well-defined inflow notch. High dew points and precipitable water values increased the risk of torrential rain and flash flooding, especially for urban environments. 

Note that the annotated meso-warm and cold fronts on the storm are my own opinion, and the nature of the storm cannot be verified properly without high-resolution radar velocity data, which I don't have access too. Although atmospheric wind shear was very weak, I suspect that ingestion of local-scale vorticity from the sea breeze front into the storm allowed it to become a high precipitation supercell. 

In any case, the Reading University Atmospheric Observatory took a direct hit from the storm, and the observation data is quite remarkable:

​These observations are the classic signatures of the passage of a severe thunderstorm - the temperature drop and pressure rise from the cold outflow are both particularly impressive. The storm caused localised flash flooding in parts of Reading and Newbury, along with numerous lightning strikes on properties in the area.  The storm later took on more multi-cell characteristics, with several storms forming on its southern flank later in the evening. Multiple thunderstorms developed in southern England overnight, eventually merging to produce heavy stratiform rain in the south-east by the next morning. A cold front later moved across the country from the west, bringing an end to the storms and the heatwave. 

A radar loop of the storms over southern England:

​​ Dewpoint, drybulb and relative humidity plots from the Reading University Atmospheric Observatory. Credit: University of Reading Meteorology Department. 
Accumulated rainfall and rainfall rate. Credit: University of Reading Meteorology Department.
Barograph. Credit: University of Reading Meteorology Department.