Storm Forecast
Valid: Wed 02 May 2007 06:00 to Thu 03 May 2007 06:00 UTC
Issued: Tue 01 May 2007 20:51
Forecaster: TUSCHY

SYNOPSIS

Split flow regime continues over Europe with high pressure in - between, centered over NW Europe.
An upper-level trough over NE Europe will start a weakening trend, while moving towards the NE. A shallow unstable layer and some forcing could be enough for a few thunderstorms over extreme western Russia , but coverage won't justify the issuance of such a large TSTM area.
The same for Estonia, Latvia and southern Finland, where ESE-ward racing cold front could suppot an isolated thunderstorm. Although parameters for an isolated tornado would be present, low confidence in any significant thunderstorm threat preclude any higher probabilities right now.
A southeastward shifting upper-level low will cross the Iberian Peninsula, while another upper-level low will bring unsettled weather conditions for SE-France.

DISCUSSION

...Portugal, Spain and the Strait of Gibraltar...

An upper - level trough continues its SE-ward motion, crossing Portugal and S-Spain during the forecast period. Latest cyclone phase evolution forecast of various models indicate some slow organisation and strengthening of this system.
Yesterday's 17-18Z stallite loop had the well defined system WNW of Portugal with a straight SE-ward motion. Warm front / sector appearance constantly improved with cooling cloud tops over the warm sector, already affecting Portugal.
Satellite based measurements denote a humid airmass ( appraised total PW values of at or above 40mm) streaming in the warm sector .
Succeeding cold front and warm front are embedded in a strong westerly jet and hence rapid eastward translation will bring them over southern Spain during the forecast period.
An accessorily hampering factor will be the predicted occlusion process, which should take place somewhere over southern Spain.
Recapitulating we have to discuss numerous fronts and attendant thunderstorm chances.

The warm front and warm sector should pose the most limited theat for thunderstorm evolution. A well saturated and warm airmass will keep lapse rates low and strong and prolonged forcing should support a more stratiform character in the warm sector although an isolated embedded storm can't be excluded.

The cold front will cross Portugal and SW Spain between 6Z-12Z and a few storms should evolve in a low-end instability and high shear environment. Best LL shear should stay displaced towards the east but DLS of 20-25m/s and at least somewhat steepened mid-level lapse rates should favor an isolated large hail and severe wind gust risk.
Models then show a complete occlusion of this fronts somewhere around the Strait of Gibraltar, while moving slowly towards the east.
Drying at upper-levels but still nice forcing should be enough for a persisting and possibly strengthening thunderstorm activity along this front over extreme southern Spain.
The main threat will be an isolated large hail and severe wind gust risk, but low LCLs and enhanced LL shear along the occlusion also favor an isolated tornado risk as far north as Madrid.

18Z onwards, most robust thunderstorm threat will shift offshore with mainly a severe wind gust risk.

High PWAT values over the level-1 region and repeated thunderstorm evolution along the eastward shifting frontal boundary also indicate a flash flood risk.

... SE - France, extreme NW - Italy...

Another area of scattered thunderstorm evolution will develop under a slowly NE-ward shifting upper-level disturbance.
Rapidly cooling atmosphere and SSTs of 17-18°C south of France are enough for up to 500 J / kg SBCAPE.
Deep layer shear of 15-20m/s and steepening mid-level lapse rates indicate a risk for hail with each stronger storm and an isolated large hail risk can't be excluded.
Although not explicit hinted in the models a low-end tornado risk may be possible due to some veering and low LCLs.

...Rest of France, Switzerland...

No significant changes compared to the past few days.
W / SW-wards shifting moisture convergence zone will start to slowly retreat towards the north as a consequence of a weak LL depression over extreme SW France.
Pulsating storms with mostly subsevere wind gusts and small hail will be the primary threat over west-central France.

Stronger DLS (up to 20m/s ) and steeper lapse rates will be present over eastern France, Switzerland and extreme SW Germany.
Stronger cells may be able to produce locally hail, matching the severe hail criterion.
Slowly northward retreating convergence zone could help to increase PBL moisture as far towards the east as extreme SW Germany which is the main reason for the far eastward expanded level-1 area.

....S-Bulgaria and NW-Turkey...

A cold front from the north will reach S-Bulgaria and some low-end instability release looks reasonable. Limited instability will likely preclude a more significant severe weather event , because DLS of 25-30m/s and still fine lapse rates will be present. Developing storms will mainly produce isolated large hail / severe wind gusts.