The unseasonably high temperatures continued for much of the week beginning the 20th across England and Wales, with temperatures climbing above 25C every day in London for 8 consecutive days, from the 17th until the 24th. An extension of the Azores high ridged NE to keep skies mostly clear and very warm by day – with depressions in the North Atlantic steered North by a displaced jet stream. This once again advected a very warm, moist and unstable plume north across the UK by the 24th, and brought a stormy and wet period to replace the settled hot spell.

 

A shallow and complex low developed over N France on the 23^rd (synoptic analysis). Some very strong surface heating (temps widely into the low-mid 30’s degC) initiated some deep convection (tops nr -60C) and thunderstorms which affected the central part of Northern France during the afternoon and evening. Toussus Le Noble reported 39mm in 1hr. This low was forecast to drift north slightly overnight and bring a dramatic end to the heat wave with the potential for some severe thunderstorms capable of supporting large hail, and tornadoes.

 

The 23rd over England and Wales remained dry, sunny and hot with temperatures once again exceeding 32C at Gravesend in Kent. Though later in the day, signs of a very unstable atmosphere became apparent in the form of Ac castellanus (a precursor to thunderstorms and indicator of instability in the mid-levels in the atmosphere), and also a chaotic evening sky with outflow from the storms in N France causing a thick shield of cirrus over part of the South. Some thunderstorms (mostly medium and high based) developed in the Channel during the late evening and flirted with the Dorset coast in Lyme Bay, with some impressive fork lightning displays lighting up the summer evening sky. (see photos here).

 

Convective activity increased during the early hours of the 24^th with further thunderstorms initiating across S Wales, SW England, S England and the Isle of Wight. A mass of storms continued to move NNE and affect a region stretching from the Midlands SSE across Wiltshire, and Somerset/Devon throughout the morning. This brought with it torrential downpours worst affecting parts S Devon and Wiltshire where an mesocyclone caused some localised flooding (Teignmouth reported 52.3mm – see Table 1 below) and lightning cut off power to over 16,000 homes.

 

Storm damage included the setting alight of a hotel by a lightning bolt in Fareham, Hampshire; lightning also set alight warehouses near Uckfield, East Sussex. A teenage boy was struck by lightning inside his first floor bedroom as a lightning bolt blasted through the roof in Poole, Dorset.

 

In addition thunderstorms brought disruption to some of the summer events across England. Storms brought serious flooding to the Glastonbury Festival in Pilton, Somerset through the morning. The fields were soon transformed into a sea of mud. Several hundred tents and belongings were washed away by the rapidly rising water levels – lightning also struck some of the stages and delayed the start of some headlining acts. The storms also interrupted the play at Wimbledon later in the day, for the first time in this year’s tournament.

 

Table 1 : Selected high 24h rainfall totals to 18Z on the 24th June.

Station	Rainfall (mm)
Teignmouth, Devon	52.3
RAF Lyneham, Wiltshire	30.0
Lake Vyrnwy	27.8
Hereford / Credenhill	27.0
Coleshill, Warwickshire	24.0
Cardiff Weather Centre	21.0

 

After a brief respite on the 25th and 26th brought about by a cold front sweeping south east, it rapidly became stormy again, more especially on the 28^th (synoptic analysis) as a trough moved up from France to affect England and Wales.

 

It was a very warm start to the day in places (29.8C recorded at midday on Jersey - where there had already been some spectacular overnight thunderstorms), and during the afternoon strong surface heating initiated some deep convection across the far south and south west of England. Thunderstorms quickly developed along with some torrential downpours initially worst affecting south Devon. Storms caused considerable flooding here across parts of Torquay and Dartmouth. A house in Plymouth was also struck by lightning and set alight, destroying much of the roof.

 

During the afternoon thunderstorms developed further North to affect the Midlands and parts of West London. At Edgbaston the cricket was washed out, umpires halted the one-day cricket match between England and Australia after several lightning bolts were observed to strike near the pitch. The thunderstorms that developed were fairly violent (for the UK) with extremely frequent cloud to ground strikes, and fed by an incredibly strong low level jet (30 – 40kts) providing plenty of moisture and therefore sustained torrential outbreaks of rain. Several hundred homes across the Midlands were left without power and several were also set ablaze by direct strikes. Some relatively high hourly rainfall totals were reported (see Table 2 below).

 

Table 2 : Selected high hourly rainfall totals on the 28^th.

Station	Rainfall (mm)
Honiton, Devon	15.2
Bournemouth, Dorset	13.6
London Weather Centre	7.6

 

Further thunderstorms developed during the afternoon of the 29th in a line extending from Mid-Wales across the Midlands and through to coastal East Anglia. Heavy rain accompanying a storm over Oxford lead to flash flooding leaving some motorists stranded in the subsequent floodwaters. The storm also caused significant damage to St. Clement’s Church in Oxford after the torrential rains and flash flooding caused its 19^th Century roof to collapse and flood the nave. There were reports of ice cube-sized hailstones across parts of Suffolk near Ipswich, and further flooding at the Luton and Dunstable Hospital accident and emergency department. All in all the events concluded a remarkably thundery period across England and Wales.

 

Many thanks to Philip Eden for rainfall information.

 

© Dave Jameson

 

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