What is a 'Spanish Plume' and how will it affect us?

Dark clouds over Halifax last year.
Cumulo-nimbus thunder cloud over Halifax Photo: Photo: CARL CROWTHER

All the meteorological ingredients will be in the mixing bowl for some lively weather this weekend, especially on Saturday.

We are only two weeks away from the longest day now, the sun is high in the sky and is pumping a lot of heat energy into the northern hemisphere.

Warm air from the western Med.
Weather chart for Saturday. Credit: Met Office

With low pressure anchored to the southwest on the weather chart, a feed of very warm and humid air will be dragged up to our neck of the woods from North Africa and the western Mediterranean; what we meteorologists call a 'Spanish Plume'

The key ingredient for the 'cake' will be the humidity. High humidity is what we feel when there is a large amount of water vapour in the air. This high amount of water vapour, if conditions are fulfilled, will condense into clouds and precipitate into heavy rain, hail and thunder.

Cumulo-nimbus thunder cloud
Cumulo-nimbus thunder cloud from a distance. Credit: Photo: ANGELA POPPLETON

On Saturday, once the plume of warm and humid air has established itself over the U.K. a trough of colder air will encroach from the west. Dense cold air overlaying the less dense, warm and humid air is very unstable. The warm air rises vigorously, rather like a child letting go of a helium filled balloon. As the bubbles of warm air rise, they cool and the water vapour must condense into liquid water - clouds in other words. This manifests itself as deep, dark, threatening cumulonimbus clouds which results in torrential rain, hail, thunder and possible flash flooding.

At the moment (Thursday lunchtime) there is still significant uncertainty in the timing and position of the most active thunderstorms on Saturday.

Keep up to date with the latest forecast and the most up to date weather warnings from the Met Office here.

JON MITCHELL