THUNDERSTORM T0092
Date: Thursday 17th July 2014   Time: 21:30 Local
Location:  Exeter, Devon UK.
Path: 6-7 miles east/northeast of SW Exeter, right over Rockbeare

Synoptics: Very high level plume strongly destabilised by upper trough
Duration: about 20 minutes (fast movement)
Type: Elevated Multicell
Average lightning type: CG
Footage Quality: Full HD

 

Marking the start of a more widespread Spanish plume event across southern UK, this rogue isolated and very high level thunderstorm dropped some pretty strong negative CG lightning right over Rockbeare, 7 miles ENE of my location in Exeter. There was a thick sheet of altocumulus it grew out of, and took me by surprise since it only initiated when it was in the vicinity.

The cloud base was over 10000ft with dry air underneath, so a very spectacular view of lightning. It was surprising to me how it favoured dropping CG rather than CC or IC like most other mid-level storms do. The CAPE perhaps wasn't spectacular judging by the slow discharge rates, so perhaps negative charge was able to build more strongly instead of getting sheared away by strong updraughts, or perhaps something to do with the dry air underneath. I didn't get chance to look at a tephigram on the day, so not sure what the 500-200hPa altitude profile was like.

The show started with some a flash I didn't see (pointing the other way) then a weak CC. Then the 3rd event (below) was a huge multi-CC and CG strike spanning most of the horizon (best portrayed in the video).

You could see multiple CGs accompanied with small CC crawlers. I wouldn't class them as Anvil Crawlers since they occurred under the main base, appearing to strike through precipitation curtains, but I guess it's a similar setup to a true anvil crawler.

Some time had passed (several minutes) before the next strike happened, which appeared a little closer than the last despite the cell moving away. Another cell was growing just to the right of the field of view, dropping a much closer rain curtain. This was to give rise to a barrage of nothing but CG over Rockbeare.

 

The high base gave a false impression that the lightning was close, so I was half expecting big and impressive thunder, they never came. Even at (only) 7 miles away most were barely audible, which led me to the conclusion these were not positive strikes (much more explosive).

One of the final strikes as it headed into the distance was captured both on video camera and DSLR below, only better on the video camera since it was on a higher tripod. I was considering positioning myself into a better view during the storm, away from my street and the chimney stacks, but I knew by the time I did that I would have missed the action, so persisted with whatever I could get from my room window. Chasing it also wasn't an option since it was being carried rapidly with the upper flow, would have meant speeding was essential.

Rockbeare suffered a power outage as a result of this slow CG barrage.

Other thunderstorms were experienced all the way from the Isles of Scilly to London on this evening and night, forming along the line of the plume and likely upper trough, and with many being more intense than this one. Much CG lightning was present in those as documented by public footage across the country.

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