Storm Forecast
Valid: Sun 01 Apr 2007 06:00 to Mon 02 Apr 2007 06:00 UTC
Issued: Sat 31 Mar 2007 21:02
Forecaster: TUSCHY

SYNOPSIS

Large-scale upper-level low pressure area will be nearly stationary over SW Europe, while high pressure will dominate the rest of Europe.
A powerful depression will affect Norway and Sweden with an attendant severe wind gust risk ( regarding 25-30m/s at 850hPa ), but there are no indications that convection will evolve during this scenario.

DISCUSSION

...Portugal, Spain and the Balearic Islands...

Despite some minor discrepancies regarding the strength and final placement of the surface depression / low-pressure channel, models agree in the overall synoptic situation over SW Europe.

The broad area of interest has to be split up into two regions.

Portugal and W / NW Spain:

Center of upper-level system will reach the aforementioned regions and will help to steepen lapse rates at the mid-levels.
The main negative factor for scattered storms well inland will be a pretty dry PBL. Expected 8-14°C and dewpoints into the lower singles will create pretty high LCLs of 1-1.5km over Portugal and extreme western Spain.
Support of the upper-levels is weak and there area no real forcing mechanism.

Shear will be too weak for any storm organisation, but weak background flow and rapidly steepening lapse rates could favor an isolated waterspout risk along the coastal areas / offshore.

NE Spain and the Balearic Islands

A channel of marginal lower surface pressure will be present from NE Spain towards the SE with an attendant frontal boundary, which will stretch from SE Spain towards the N / NNE.
This slowly eastward shifting boundary will be the main focus for scattered to widespread thunderstorm initiation.
At upper-levels, strong divergence and DPVA are forecast to affect the region for most parts of the forecast period with numerous UVV maxima crossing the eastern part of Spain from the SSW.
The main negative factor for a more pronounced severe weather threat will be the fact that the environment will stay pretty uncapped during much of the period and hence widespread showers / thunderstorms can evolve for most of the day.
This will have affect on the mid-level lapse rate gradient, which should stay pretty weak.

Nevertheless, GFS indicates a broad area of at least low-end instability release in an environment with strongly veering profilers at lower levels.
SRH-values are enhanced and a humid PBL will keep LCLs very low. GFS also indicates EL values of 6-7km in an environment with 20-25m/s DLS .
Right now NE Spain will be the area with the highest possibility for an isolated tornadic event, given strongest veering at lower-levels, especially if storms stay more discrete. Hail threat should be suppressed by weakening lapse rates, but indicated high EL values suggest that an isolated large hail threat can't be exlcuded, given strong DLS.
Otherwise, strong influx of warm and humid air by a constant SE-erly LLJ along this slowly eastward progressing front will favor storm clustering / training storms along this front . Excessive rain with an attendant falsh flood risk will be a threat along this whole front,too.

Between 18Z-06Z, models indicate an advection of a very warm and dry airmass from N-Algeria towards the north, affecting the Balearic Islands during the latter part of the forecast period. Pretty moist BL, a stout cap at 850hPa and expected EML advection will be present.
Shear ( especially directional ) at lower levels would be favorable for storms to organize, but right now I'll go with the global models, which do not break the cap.
Increasing forcing and the approaching front will be present around 03-06Z and the area has to be monitored for storm initiation.
Did not include the Balearic Islands into the general thunderstorm area until more confidence for initiation will be present.