Storm Forecast
Valid: Sat 25 Aug 2007 06:00 to Sun 26 Aug 2007 06:00 UTC
Issued: Fri 24 Aug 2007 22:42
Forecaster: TUSCHY

SYNOPSIS

A disturbed weather pattern will persist over most parts of Europe with a strong upper-level low pressure just west of Portugal and a strengthening high pressure area west of Ireland. An active polar front jet will continue to steer depressions towards Norway and Sweden.
Hot and dry conditions prevail over the central and eastern Mediterranean.

DISCUSSION

....Portugal and Spain....

A strengthening, closed upper-level low ( centered just west of Portugal ) will be the main player for the next 24 hours and widespread thunderstorm activity can be expected.
We split the broad forecast area into 2 separate discussions to alleviate the overview.

... SW / NW Spain and Portugal ...

The general model pool coincides in the general evolution of this thunderstorm event. A well defined cyclonic vortex just west of Portugal will be nearly stationary during the next 24 hours. Attendant mid-/upper-level wind field favors a region of intense upper-level divergence over Portugal, which is forecast to shift slowly towards the N / NNW.
In addition, GFS allows small pieces of energy to cross the region from the south, which should also enhance the forcing. As mid-level lapse rates steepen somewhat during the forecast period and mixing ratio increases to at or above 10g/kg, a weakly capped atmosphere will be present with up to 1000 J/kg MLCAPE.

Widespread thunderstorm development over SW Spain and S / central Portugal will be underway during the morning hours and a cluster of storms should evolve with a NNW- movement.

Shear is not that strong with DLS of 15-20m/s but should support multicell storms and isolated supercells ( mid-level rotation could be supported by intense cyclonic vorticity ). LCLs are quite low and an isolated tornado is possible although the main risk will be a hail risk ( possible large ).
A better environment for tornadoes could evolve over N-Portugal and NW Spain during the evening / night hours, if latest model trends of GFS area right as LL shear increases to more than 20m/s and 400 J/kg SRH1will be present ! This looks reasonable regarding the intense veered flow .

...Central / E Spain...

The main focus for thunderstorm development will be short waves, rotating northward on the eastern flank of the strengthening upper-level low. Again, GFS is quite consistent in developing thunderstorms over the area of interest during the daytime hours with the main activity starting during the evening and night hours.
The combination of steep lapse rates and impressive PBL moisture ( especially over eastern Spain ) should yield moderate instability values with isolated pockets of more than 1kJ/kg MLCAPE.
Each developing storm will be placed in a strongly sheared environment with nice veering throughout the whole layer. DLS will be range between 20m/s and 30m/s with a broad belt of 10m/s to 20m/s LL shear and significantly enhanced SRH values. The main uncertainty is if storms will be able to root into the boundary layer but degree of PBL moisture should support at least a few surface based storms.

Those storms would pose an enhanced tornado and severe / damaging wind gust and large hail risk and confidence is high enough to highlight those regions with a level-2.
Otherwise either shear or instability are somewhat worse outside of the level-2. Still severe storms could produce all kind of severe weather.

.... Strait of Gibraltar, coastal areas of Morocco and NW Alegeria...

Models are quite inconsistent regarding the southward extent of the thunderstorm activity with GFS being the most reluctant one. The main reason should be the intense cap at 800hPa, as true African air ( at or above 25°C at 850hPa ) advects towards the north. An impressive elevated mixed layer will be present ( like yesterday in this area ) with mid-level lapse rates of 8-9°C/km !
The cap has to be overcome, but dewpoints of 20-23°C and some upper-level forcing should support this. Wind field and strong instability release would support an enhanced large hail risk, as storms tend to be elevated.
If storm coverage is higher than expected higher probabilities will become necessary later on.

...Romania, Moldova and Ukraine...

Dewpoints of 17 - 22°C and quite steep mid-level lapse rates are present over those areas. A weak frontal boundary / convergence zone is forecast to slowly shift towards the SE during the forecast period. GFS indicates strong streamline convergence signals with high mixing ratios. Moderate instability release should be realized and DLS will increase to 15-20m/s over NE Romania / Moldova and central Ukraine. Latest thinking is that the main risk should be a large hail threat, as storms will be mostly elevated and some enhanced SRH 3 values will be present ( and even a significant hail report over NE Romania and N-Moldova will be possible, as strongest instability and shear overlap during the afternoon hours ). Will stick with a level-1 for the moment as degree of storm coverage is still uncertain and overall set-up looks to be too marginal for a level-2..


... Parts of southern Austria and Slovenia...

Partially diurnal driven thunderstorm activity ( also enhanced by an approaching upper-level trough ) should take place over the area of interest but right now only a few isolated storms are expected to develop . Moisture was on the increase during the past 12 hours and should lead to dewpoints in the higher tens. Moderate instability release and lapse rates will support a few strong, pulsating thunderstorms. DLS of 20-25m/s is quite strong and updraft/downdraft separation could support a few longer-lived storms with a hail threat ( to a lesser extent large hail, but rather an intense small hail event ).
This is a marginal level-1 situation and time frame is quite short ( between 14Z and 17Z ).