A particular favourite of the EOTS team are showers and thunderstorms produced from slow moving, narrow convergence lines. These localised strips of uplift, often resulting from topography or areas where winds are converging from all directions, can create relatively small but potent thunderstorms, often responsible for funnel or weak tornado sightings. Once such convergence line looked like setting up in south Devon on 6th August 2007, as the coast and the land interacted, augmented by the high ground of Dartmoor. It was a post cold front air mass with plenty of CAPE and other ingredients required to produce heavy downpours. As the sea breeze pushed inland during the afternoon, it encountered winds flowing across Devon from the opposite, north-westerly direction. The result was dramatic uplift right across south Devon and the generation of showers and thunderstorms. The following synoptic chart shows the weather situation at the time (provided by http://www.wetterzentrale.de and copyright UK Met Office):

By early afternoon, the magic had begun and the convergence line across south Devon had commenced. The satellite image below, thanks to the Modis Rapid Response System, shows the development of line of heavy showers stretching right across south Devon, and also south Cornwall and into Somerset and Dorset. We positioned ourselves in the South Hams area of Devon to look back northwards at the developing line. The South Hams area is the triangular point stretching southwards out of the cloud, roughly in the centre of the image below.

And so the chase began....

Taken just before 16:00, this was the view looking southwards towards the coast of the South Hams area, with nothing more than blue sky and some patchy cumulus.

And this was the view looking northwards and westwards at the same time, with towering cumulonimbus clouds sprouting high above the Dartmoor countryside.

Even looking around to the north-east, the line was clearly visible and, already at this point, was beginning to release heavy showers in the direction of Exeter.

It was quite clear from the solid black curtain beneath the convergence line that the rain coming out of the bottom of these clouds meant business. No lightning, yet.

By positioning ourselves to the south of the line of showers, we could keep not only dry, but also in the sunny spells that were coming and going.

As mentioned earlier, convergence lines can be prolific producers of tornadoes. With this knowledge, we were keeping a close eye on occasional lowerings from the cloud bases. This one above was rotating quite quickly but never really took hold.

It was at this point, around early evening, that thunder started sounding in the distance and we took this signal to head northwards, punching through the line of heavy showers to get to the north side of the action, hopefully with a better view of any lightning.

We were now perched on a high high to the east of Exeter, looking back westwards at the line of heavy rain which was now blanketing the city. We'd already driven along the M5 motorway, encountering cloud to ground lightning and flooding. The large bridge of the M5, crossing the river Exe, can be seen in the photo above.

Here's a panoramic view of the convergence line, looking from south (left) through east to north (right). Please click on the image above for a larger version.

We've produced a short video clip of the day's convergence line action, which includes predominantly timelapse video of the line of showers and thunderstorms across south Devon:

Format: Windows Media Video (wmv)
Video: 15.8mb, 1:32