What's the deal with this storm?

July 25, 2014 6:05 AM

Currently there is a lightning storm on the France / Spain border which is oddly circular. It looks like this as seen on lightningmaps. Why is it this shape?
posted by Just this guy, y'know to Science & Nature (7 answers total)

It's a whole cluster of storms that happen to be forming a circle. Check out the satellite loop and radar loop. My thoughts (IANAM, TINWA, CTSIYJ)*: looks like a cluster of storms came in across the Bay of Biscay this morning, leaving a bunch of moisture and boundries in it's wake. As that storm moved east, the skies cleared up. Solar heating then caused second groups of storms to kick off in a line across central France and again along the Atlantic coast. What's intriguing is that, if I squint hard enough, I can see a circulation in the radar loop, like the cluster of storms is setting up a wake low in south-central France.

*I am not a meteorologist, this is not weather advice, consult the skies in your jurisdiction
posted by penguinicity at 6:29 AM on July 25, 2014 [5 favorites]

Also, half of that circle is following the mountains, where lightning is more likely. I think.
posted by SLC Mom at 7:37 AM on July 25, 2014

I don't know -- but maybe it's to do with how the pressure systems that give rise to storms and similar remarkable weather, are often round (or round-ish)?
posted by Drexen at 8:00 AM on July 25, 2014

Storms tend to be created when one front bumps into another front, often creating circular-flowing systems.
posted by Thorzdad at 9:28 AM on July 25, 2014

I have no answer, but thank you for introducing me to lightningmaps! I live in south Florida so this is super fun and useful.
posted by PorcineWithMe at 12:18 PM on July 25, 2014

Late to the party as usual, but I think your theory isn't too far off. I wasn't able to see the links you provided, but managed to find some archived radar loops of that day. From what I could tell, the circle of storms seemed to be set off by a complex of storms over southern France earlier that day. At first it seemed to me that the circle of storms were developing along an outflow boundary from the earlier complex. However, the earlier complex of storms could have caused a Mesoscale Convective Vortex (MCV) which in turn set off the later storms. That would be similar to the wake low you mentioned. It is difficult to tell if it was outflow or an MCV without some more data such as surface observations.
posted by weathergal at 6:42 AM on July 26, 2014 [1 favorite]

I don't know enough to speculate on the specifics of the circular pattern, but it is quite striking. The N / S sides seem to outline a course around the highest parts of the Massif central, so I wonder whether topographic effects have something to do with it...

Recent weather over France has been described in forecasts here as a "marais barometrique" or "pressure marsh", with little synoptic scale organisation and weak circulation. This leads to quite static air masses which favour the build up of afternoon / evening convective storms which can be quite sudden and large, but rarely become very organised.

A few resources which provide a bit of info on this, and you may find interesting in any case:

You can find a brief discussion of this storm situation in the forecast archive on here

You can also currently go back to Friday 25th 1200 forecasts for a wide range of measurements here (dropdown is in German, but each map does have an English label).

Also, more lightning map goodness here.
posted by protorp at 7:17 AM on July 26, 2014