THUNDERSTORM T0106
Date: Saturday 27th May 2017   Time: 01:00 Local
Location:  Exeter, Devon UK.
Path: Few miles west, about 20 miles for Dartmoor cell, and developing cells overhead

Synoptics: Plume strongly destabilised by upper trough (low out to the west)
Duration: about 35 minutes
Type: Large Multicells (MCS) sharing same Anvil
Average lightning type: IC
Footage Quality: Full HD

 

This was one of the most significant lightshows the southwest has seen in recent years. I haven't seen this frequency of lightning since being in the US. The initial MCS developed off the coast of Plymouth and swept north and spread in an arc across Cornwall, Devon, then eventually Dorset, Somerset and into South Wales during the early hours of the 27th.

Most of the lightning was high-level intra-cloud, often in call-and-response with other towers, and the environment was relatively dry enabling clear view of the towers which are quite often obscured in these continental plume situations. The upper-level nature of the lightning with low-CG count meant that the thunder was relatively quiet and ambient. There were a few positive CGs however, one of which reached 170kA (see scientific paper below). Not only this, but 23 sprites were observed from the UK Meteor Network. Plymouth saw nearly continuous 1-4 second discharge rates, mostly up in the cloud with the odd ground strike, and occasionally less than 0.5s discharge rates (gleaned from Youtube footage).

[Click to enlarge] CG flash rate by polarity (blue stacked bar for -CG, red stacked bar for +CG), +CG proportion (black dotted line) and 5-min average peak current of +CG (red dashed line). The black circles correspond to the time of observed sprites. Source: Paper from Uni of Bath & Met Office, studying this storm's lightning and sprite activity (Atmospheric Research Vol 249 (2021) #105357).

Here in Exeter, I arrived late to the scene just in time for overhead passage. I shot video looking west-northwest where the bulk of the MCS was developing. In the video, it looks time-lapsed, but it isn't (apart from the montage at the beginning). Mature cells weren't quite overhead, but were developing with some early tower flashes and sparse drops of large rain. The lightning remained mostly upper-level, often in "call and response" between different towers, almost giving the impression there was intelligent life up there and casual chatter.

Environmental conditions were quite unusual for southwest England, yet perfect for severe elevated storm development. However, these "severe" environmental conditions were not immediately obvious in model data. It all looked very good in terms of upper-forcing (very strong cold upper trough), a mid-level jet was present, feeding pre-heated well-mixed air from a prior cloudless warm day across France, and a reasonable (but not excessive) amount of CAPE. But something else was leading to high lightning rates.

There was no Saharan dust present either, where Saharan dust is thought to enhance lightning rates through giving more cloud condensation nuclei to the atmosphere and thus offering more ice particles to charge. One environmental aspect that may have been favourable was the low vertical wind shear through the column. Normally this would not be a favourable severe storm condition and can kill cells as they form, but as the forcing was so strong below and seemingly independent of downdraughts, the continual pumping of mass into the core anvil aloft perhaps allowed for very efficient dipole charging.

CHARTS (Credits)

SCIENTIFIC PAPER ON THIS STORM BY UNIVERSITY OF BATH
Signatures of large peak current lightning strokes during an unusually intense sprite-producing thunderstorm in southern England or PDF Download

UK METEOR NETWORK SPRITES
UFlikr Video by Richard Fleet of UK Meteor Network's Sprites over the thunderstorm

PUBLIC FOOTAGE/NEWS
UK Weather Chase (Sam Whitfield) footage from Plymouth
Will Woodgate's insane footage from Plymouth
Footage from StreetZips

Video of live lightning strike sferic reports from Iain Meteorscan

Student's Snapchat catches huge lightning bolt striking just metres away from him