Hi!

Eumetsat satellige image from 29 April 2007, 12 UTC shows some features I'm going to explain thereafter:

I took the eumetsat air mass satellite imagery showing air masses of different heights. I inserted a map displaying isentropic potential vorticity for the same date. The greater the IPV values (units of PVU), the more stratospheric air is involved. The borderline between 1 and 2 PVU marks the tropopause. Hence, you can see the run of the tropopause thereby.

The ipv-map indicates a narrow, but intense upper-level trough directed to the south to the west of Ireland. A small-scale trough is situated over the Western british isles as well as over the western mediterranean area. A broad ridge prevails nearby Iceland.

1) The tongue of statrospheric air in the ipv-map coincides with the purple coloured air masses in the eumetsat image. Very dry air masses are descending there. Lightningwizardmaps from Oscar van der Velde yield steep low-level and mid-level lapse rates in the range of the upper-level trough. Due to moisture sources over the Sea, the steepening mid-level lapse rates create some instability released ahead of the trough axis.

Descending tropopause in the range of the dryslot is responsible for the descending cloud tops in the satellite image (depicted brown). However, several convective cell have been developed, partly electrified (see Sferics). Corresponding surface low lieas to the north where a slightly spiralling occlusion front is visible. In contrast, cold front appears with a lot of cirrus and jetfibres. Precipitation signals indicate a narrow stratiform rainband there stabilized by increasing geopotential height respectively increasing PVU values in the IPV map.

2) The next area of interest is Northern France where a narrow low-level convergence helps for several thunderstorms to arise. Respective sounding ascent of Trappes reveals a large area of instability in excess of 800J/kg, considerable values for April. CINH is rather small and lapse rates very steep up to 800mb. Due to weak vertical wind shear, these developing thunderstorms remain quite stationarily, with locally enhanced risk of flashflooding, small hail and sub-severe wind gusts.

3) The third area of interest is situated in the western mediterranean where another maximum of IPV had arrived. Indeed, major area of thunderstorms is located to the east of the IPV-max where strong PVA triggered a lot of multicells. Shear values are weak, too, rather enhanced to the east near Sardinia (> 15m/s). Thus, with eastward propagating vortmax, ongoing thunderstorm activity is forecast, with a slight risk of rotating updrafts and an isolated tornado. Main concern will be, however, large hail and some severe wind gusts there.

12z Caligiari -Sounding (Southern Sardinia) indicates backing low-level winds and weak vertical wind shear. Vertical wind shear is expected to strengthen the next hours. It remains still elusive yet whether the increase will be sufficient for organized storms.