26 Apr 2012

Tornadoes in Warwickshire and Essex?

The weather proved to be lively across the UK yesterday, with active showers and thunderstorms rotating around an area of low pressure, bringing torrential downpours and very gusty winds.

As a meteorologist, yesterday I saw signs that the atmospheric conditions were favourable for the possible formation of funnel clouds, which if reached the ground would be classified as tornadoes.

What you may be surprised to know is that tornadoes are not that uncommon here in the UK, with 33 reported each year on average.

Weather computer models were suggesting that there would be notable wind shear yesterday, which means that the wind changes direction and speed with increasing altitude.

Yesterday the wind direction veered with height from south easterly at the surface, to southerly to south westerly higher up whilst also increasing in speed. This had the effect of creating a rotating motion in thunderstorms, which would have given the potential for the more active ones to spawn funnel clouds.

The Tornado and Storm Research Organisation (TORRO) which records and investigates tornado cases in the UK had two suspected tornadoes reported to them yesterday – one in Rugby, Warwickshire and another in Halstead, Essex.

In both cases, sudden, violent winds were reported as well as damage to property and falling trees. However, as of yet, no photographic evidence of any actual tornado or funnel cloud has been seen.

So what evidence is there to suggest that these suspected tornadoes were actually tornadoes? Well using radar imagery, it is possible to pick out certain characteristics to assess whether or not a tornado may have occurred.

Thunderstorms that spawn funnel clouds or tornadoes tend to have a hook shape at the base of the storm on the radar. This is a sign of intense rainfall and that there is some rotation present.

A radar image from the Rugby area at 6pm, the time the suspected tornado hit, shows a slight hook shape, as well as some heavy rainfall, but there was no lightning present.

It’s not impossible that a funnel cloud or tornado was present, however, in active storms violent gusts of 50-60mph can occur suddenly and sometimes be mistaken for a tornado.

A second radar image from Halstead at 4pm is more supportive of the case for a tornado. This thunderstorm was very active with lots of lightning present. Also, the rainfall is very intense, supporting the idea of rapid movement of air upwards and downwards. Combine this with the rotating effect of the wind shear I mentioned earlier and it’s quite possible that a tornado may have occurred.

I asked Paul Knightley, Head of TORRO what he thought about yesterday’s suspected tornadoes. He said: “As part of my role in TORRO, I issued a tornado watch for yesterday as it was fairly apparent in the morning that conditions would be favourable for organised convection, perhaps with a few tornadoes. However, I like to be sure on these things so at this time I would say they’re probably tornadoes.”

He added: “One of our TORRO colleagues is heading to the Essex location today (hopefully) to try to ascertain whether there is a damage ‘path’, which is usually the clearest indicator that it was a tornado.”

Don’t forget, you can stay up to date with the latest weather forecast on the Channel 4 Weather website, or if you ever spot a funnel cloud or tornado, contact me on Twitter – @liamdutton

Radar images courtesy of MeteoGroup.

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