​​​​​​​​​​​​​​​​​First UK storm chase - 2nd June 2017

On the June 2nd, I went on my first storm chase for over two years, my first ever chase in the UK, and my first solo chase. It turned out to be quite epic! Here I will try to give a coherent account of the chase.

I had been following the risk for severe thunderstorms several days prior to the event, and kept a watchful eye on the various NWP models, to see how the situation might unfold. A combination of a warm and moist airmass, high wind shear, and an approaching cold front were forecast to come together to produce a potent atmosphere capable of producing strong multicell and potentially a few supercell thunderstorms across south-east England and East Anglia. Temperatures were forecast to reach the mid-20s ˚C during the afternoon, along with dew points around 18˚C, locally up to 20˚C. 1000 - 500hPa bulk shear was forecast to be 30 - 40kts, although this was predominantly unidirectional. Surface-based CAPE of around 1000 J/kg was expected fairly widely, with isolated pockets of 1500J/kg. Although storm relative helicity was relatively low, at less than 100m2/s2, thunderstorms that could develop on convergence zones, sea breezes or outflow boundaries were likely to ingest low-level vorticity and had a greater chance of becoming supercellular. 

I issued a Convective Forecast on the morning of the 2nd, in anticipation of the strong thunderstorms, with a SLIGHT risk of severe thunderstorms. My target area was somewhere in north-east Surrey or Kent - the reasons for this were to intercept any surface-based storms that developed on the sea breeze front that moved inland during the afternoon, as well as the ease of access from my home in south London. Although nasty storms were forecast to develop over London, chasing such a large urban area is completely pointless, since speed limits are so slow, and the roads are too full of hazards and other road users. Here are the forecast temperatures for the day, as well as a couple of forecast soundings: 

AROME forecast 2m temperature for 16:00 UTC 02/06/2017. Credit: wxcharts.eu.

ARPEGE forecast skew-t for 13:00 UTC 02/06/2017. Credit: wxcharts.eu.

GFS forecast skew-t for 18:00 UTC 02/06/2017. Credit: wxcharts.eu.

​​The Chase
After checking out the weather during the late morning and early afternoon from some local vantage points, I decided to drive to Botley Hill in northern Surrey, which is located on a high ridge looking south over the M25. I had earlier noticed cumulus congestus clouds developing around 10:30am - storm initiation was forecast to be relatively early in the afternoon. On the drive down, I stopped to get some nice pictures of further cumulus congestus clouds going up, but ultimately either getting sheared apart or losing their low-level buoyancy and dissipating. 

I eventually found a lay-by close to Botley Hill, which had a commanding view to the south, with open views to the north and west as well. Here I sat for several hours, watching cumulus clouds grow and fade, with storms eventually initiating to the north-west by around 2 pm. These storms were tracking over Greater London itself, and I was not prepared to even think about chasing them. Instead, I just waited and managed to get some lovely pics of the growing updrafts and mature thunderstorms from behind. I saw lots of pelius clouds, along with a nice example of a multicell storm developing along a convergence zone to my north-west. Here, the mature updrafts can be seen to the right of the image, with new developing storms on the left. A separate updraft can be seen above the tower. 

Pelius clouds developing above a strong updraft associated with an intense thunderstorm over central London: 

It was nice to be able to see the thunderstorm updrafts so clearly, as these storms tend to be high precipitation, and are often masked by low-level cloud in the UK. The views I had of the storms from afar were by far the best I've seen in the UK - although I've never bothered to chase here before, so this wasn't that difficult! 

Some of the radar images from the day looked very suspicious, with what appears to be classic supercell thunderstorm features, including  v-wing signatures and potential hook-echos. Although these storms formed in an environment with mainly unidirectional shear, they may have ingested low-level vorticity into their updrafts from local cold pools or convergence zones. Take this radar image from 15:10 as an example: 

Credit: netweather.tv.

By around 4pm, I had been closely following a group of showers over central Surrey, as they migrated towards my location. They remained relatively unimpressive for a couple of hours, before one cell suddenly exploded into a large thunderstorm. It started to produce lightning, and moved north-east to the west of my location. This was my chance for an interception, and some proper close-up views of the storm. After a brief drive of about 20 minutes, I stopped close to a rural bridge over the M25, and took a short video of what I think was a high-precipitation supercell with heavy rain and gusty winds in its core. I witnessed several horizontal vorticity columns develop, as the cool outflow generated turbulence ahead of the storm. This storm cell would continue to track north-east, dropping 2.5 cm hail over Orpington, Kent, about 40 minutes later. 

I initially thought the event was over once the cool outflow had passed my location and it started to rain. I briefly went back to the car, but returned to the bridge when I realised the rain wasn't that heavy - this gave me the most spectacular view of the whole chase. I had a view of the updraft of what I think was another supercell thunderstorm. I could see the twisting updraft with a developing wall cloud at its base, with the precipitation and main downdraft in the downstream direction, on the right of the image below:

Suspected updraft of supercell thunderstorm, with a developing wall cloud at its base. Taken at 16:36 UTC. 
Here is the associated radar image, at the time the above image was taken: 

Credit: netweather.tv.

Analysis of subsequent radar images suggests that these storms may have been splitting supercells. Or perhaps the eastern storm developed from outflow from the western storm? Or perhaps it just initiated on its own? I'll leave this one open for debate... Whatever the case, this was an awesome first chase in the UK, and one that I'll never forget. It just goes to show that if you know when and where to look, you can still see some awesome convective weather in this country!