UK braces for its most extreme thunderstorm risk ever, supercells possible. Know more

Weather models are showing the highest instability ever simulated over the UK. Here is the science behind why Britain may face a historic week of severe storms from June 22.

In Short

• UK weather models show the highest storm instability ever simulated
• Extreme CAPE values could trigger supercells, hail, winds and tornadoes
• Monday looks the likeliest day for widespread storms across England

Britain built its national character on grey skies and gentle drizzle. This week, its weather computers are predicting something that belongs to the American Midwest.

An independent UK weather forecaster has shared model data pointing to the most violent thunderstorm conditions ever simulated over the country. He does not say it lightly. If the right ingredients fall into place, he warns, Britain could face a historic week of severe weather.

The risk begins on Monday, June 22, and may run through to Friday, June 26. Very frequent lightning, large hail, damaging winds, flash flooding and possibly even tornadoes are all on the table.

THE FUEL THAT FEEDS A STORM

At the heart of the warning sits a measurement called CAPE, short for Convective Available Potential Energy. Think of it as the fuel in a storm’s tank.

CAPE describes how much energy is available to push a parcel of warm air upwards through the atmosphere.

When air near the ground is warmer and lighter than the air above it, it climbs on its own, the way a hot air balloon rises. The more energy stored, the more violently it can surge skyward.

CAPE is measured in joules per kilogram. In Britain, a summer value of 1,000 is already notable. The models this week show 2,000 to 5,000, with one reading beyond 7,000. Such figures usually appear over the central United States, not Bristol or Gloucester.

A SWELTERING, SODDEN AIRMASS

Two things are loading this atmospheric gun. Temperatures are forecast to reach the mid to high 30s Celsius. At the same time the dew point, the temperature at which air holds so much moisture that droplets begin to form, may climb into the low to mid 20s.

A high dew point simply means the air is heavy with invisible water vapour. When that vapour condenses into cloud, it releases hidden warmth, making the rising air even more buoyant. Heat plus moisture equals raw energy.

THE MISSING SPARK

Fuel alone does not start a fire. The atmosphere needs a trigger, something to nudge the warm air upward until it rises by itself. A cold front, a range of hills or two colliding breezes can each do the job.

Monday offers clear triggers, which is why widespread storms look likely, particularly along the M4 corridor near Bristol, Swindon and Gloucester, and into North Wales. Later in the week the energy lingers, but the spark grows uncertain.

When winds also change speed and direction with height, a feature called wind shear, storms can begin to rotate. These rotating giants are known as supercells, and they unleash the fiercest hail, winds and tornadoes on Earth.

Britain rarely sees these ingredients gather at once. This week, it just might.

- Ends