Extended Forecast
Valid: Tue 27 May 2008 06:00 to Wed 28 May 2008 06:00 UTC
Issued: Sun 25 May 2008 20:25
Forecaster: GROENEMEIJER

SYNOPSIS

East of the large-scale trough over the Bay of Biscay and Iberia a shortwave trough moves northward from the western Mediterranean Sea to central France. This shortwave is embedded in a strong southerly jet with speeds of 30-35 m/s at 500 hPa. A surface low is projected to move from near the Balearic Islands to Brittany.

DISCUSSION

Level 1 and 2 areas...

Widespread convective precipitation is expected over NE Spain and SE France in an area of upward vertical motion ahead of the aforementioned low pressure centre. Those embedded storms will probably be elevated, but steep lapse rates and strong low-level winds could still produce isolated severe hail and strong to severe gusts.

A warm front is expected to stretch from the Pyrenees to the Benelux countries. On its warm side dry air is advected northwestward out of the Alpine region into France. The evolution of the preciptation and cloudiness across southern France is hard to forecast, but per GFS this should shift northwestward to western France in concert with the frontal zone. This may leave an area in its wake where the sun has opportunity to heat the boundary layer. This could potentially create some surface-based instability in a region with large low level hodographs (300 m2/s2 SRH) and create a threat of strong tornadoes. In case low-level instability remains too limited for surface-based tornadic storms, they will still be likely to produce rather widespread severe gusts, given the already strong 25 m/s wind speeds at 850 hPa. Possibly, enough forcing will be available for a well-defined squall-line to move northward more or less across the area indicated as level 2.

An open question is to what extent storms will form further north along the warm front across NW france and the Benelux countries. It is not unthinkable that, during the late afternoon and evening, some storms develop there also. Given the high storm-relative helicity that is forecast, these may develop into supercells with a threat of hail, damaging gusts and tornadoes as well. Currently, our best estimate is that surface-based storms will mostly be restricted to central and W-central France until early evening, but that NW France the Benelux and possibly SE England will be affected by organized storms overnight.

Low storm coverage is forecast across NE France, Germany, Switzerland and NW Italy. But, as more than 1000 J/kg CAPE, steep mid-level lapse rates and moderate deep-layer shear are expected, storms could easily produce large hail and strong downdrafts, so that a level 1 appears to be warranted for this area also.

# # #