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I've just endured a nightmarish drive along the M4 from Bristol to Reading and back again.

It was a journey punctuated by two extremes of weather:Ìýpassing beneath ink-black skies and torrential downpours for a few miles; then suddenly emergingÌýinto calm spells of blue sky and sunshine, only to see the next crop of ominously dark clouds looming-up a few miles furtherÌýahead.

Another line of dark Cumulonimbus clouds loom on the M4 passingÌýthrough Wiltshire (Photo: Doug Fergusson)

The return journey to Bristol was quite simply treacherous. Beneath vast walls of towering clouds, the M4 through Wiltshire seemed as if gripped by the darkness of night. Long stretches of standing water routinely flicked the steering wheel through my grasp as the risk of became ever prevalent. A nasty-looking seemed to prove the point: these were truly perilous conditions on the motorway.

As the spectacular rainfall radar image below shows, we've been subjected to a spiral of heavy showers and thunderstorms today, all circulating around a low pressure centre passing northeastwards across the Westcountry. It's the rotatingÌýlines of heavy showers -Ìýso visible here -Ìýthat created the on-off natureÌýof wet versus dry weather I passed through along the M4.

Met Office rainfall radar at 1030hrs on Sunday, November 29th 2009, clearly shows spectacular clusters of heavy showers spirallingÌýaround a centre of low pressure as it crosses Exmoor, moving east-northeast (Image: via Met Office MBS system)

Around the main vortex of this low, the winds are tending to blow readily in different directions and speeds at low levels in the atmosphere.

This , or , could well create funnel clouds to form in a few spots today. I noticed some impressive-looking lowering of the cloud base in some areas along the M4 corridor, but no obvious funnels. But then a detailedÌýstormspotting effortÌýis hardly easy - nor wise -Ìýwhen trying to avoid aquaplaning into the roadside scenery near Lyneham!