18 Jul 2014

Severe thunderstorm warning as heatwave breaks

Today is still on course to be the hottest day of the year so far, with temperatures widely reaching 30C across eastern parts of England.

However, severe thunderstorms are expected tonight and through Saturday, as the heatwave breaks, bringing the threat of some severe weather across the UK.

As intense thunderstorms drift northwards, there’ll be frequent lightning, large hail, gusty winds and torrential downpours, with the possibility of flash flooding. 

severe_lightning_g_wp

The Met Office has already upgraded the weather warning it issued yesterday from a yellow ‘be aware’ warning, to an amber ‘be prepared’ warning, meaning that confidence has increased in there being some impacts this weekend.

thunderstorm_warning_MO_wpIn addition to the weather warning, the Environment Agency has highlighted that much of England and Wales is at risk from surface water flooding on Saturday, due to the expected downpours.

Why will the thunderstorms be severe?

Weather is driven by contrasts in temperature, pressure and humidity (moisture levels), with the most active weather taking place when these contrasts are most extreme.

When it comes to thunderstorm formation, it is down to the collision of cooler air with warm, moist air. As the two collide, the warm, moist air is thrown way up into the atmosphere to around 30,000ft – the height at which commercial airliners fly.

It is effectively like taking the lid off a saucepan of cooking popcorn, resulting in the bubbles of air rushing rapidly upwards in the atmosphere, forming cumulonimbus clouds that produce thunder, lightning, heavy rain, hail and gusty winds.

At the moment, some particularly hot and humid air is being drawn up from Spain – something that is known as a Spanish Plume.

It is this hot and humid air that will provide the fuel for severe thunderstorms as the atmosphere destabilises, resulting in the taking the lid off a saucepan of cooking popcorn analogy that I mentioned above.

Where will be worst hit?

Whilst there is a high level of confidence that severe thunderstorms will drift northwards across much of the UK this weekend, the detail of the precise locations that will be worst hit is difficult to pin down.

This is because the atmospheric conditions for thunderstorms to form and thrive are very finely balanced.

Even the highest resolution weather computer models struggle to pin down every nuance of each individual thunderstorm.

However, what they are very good at doing is picking out the highest general risk areas, meaning that we can be aware of places that may potentially be affected.

Is another heatwave expected anytime soon?

At the moment, there is quite a bit of uncertainty about how the weather will evolve next week.

The latest information available has temperatures coming down to sit in the low to mid 20s by the end of the weekend.

However, there are signs that temperatures will increase again next week – especially across England and Wales, where they could be nudging 30C again by Wednesday.

Don’t forget, you can get the latest forecast on the Channel 4 Weather website. If you take any good lightning pictures, you can tweet them to me on Twitter – @liamdutton

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One reader comment

  1. Paul says:

    “high level of confidence” that it will drift north…? Is the use of the positive inflection due to the fact that the storm will be moving away from the precious south? Perhaps you could have used the term “high level of certainty”…

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