Storm Forecast
Valid: Fri 05 Oct 2007 06:00 to Sat 06 Oct 2007 06:00 UTC
Issued: Thu 04 Oct 2007 20:03
Forecaster: TUSCHY

SYNOPSIS

One has to be very careful not to get mesmerized by cyclonic rotation when
appraising the current satellite loop.
At least six cyclonic swirls are present over Europe in different development stages.
This activity is the result of a dipole-like structure of the polar vortex, with an ill defined part over Europe and Asia and a better structured part over NW Canada /USA and the eastern Siberian Sea.
Placement of the different low pressure areas was done by yesterday's afternoon IR-loop and the following results were received:
A strong low pressure area just west of Iceland will cross the island during the next 24 hours and will continue northeastwards thereafter.
A broad but quite well defined low-pressure area north of the Azores should be stationary during the forecast period. Convective activity next to the center shows no sign of better organisation, but conditions during the next 1-2 days become somewhat better ( as GFS indicates a possible warm-core evolution with this system ).
A cold-core upper-level low pressure enters the western Mediterranean during the day, while another, much weaker defined circulation exits the forecast area to the east over the eastern Mediterranean.
A broad circulation somewhere north of Belarus stays stationary and will be the steering mechanism for a well developed short wave / open trough , which is forecast to rapidly cross Poland during the day from NW towards SE.

DISCUSSION

...Western Mediterranean...

The upper-level feature will enter the western Mediterranean somewhere NW of the Balearic Islands during the early morning hours. By this time, the system should have finished its lifting process, as strongest geopotential height gradients and attendant strongest jet stream stay on its southern part. This east-west alignment continues during the forecast period and will start to evolve into a more positive tilted feature during the end of the forecast period. This configuration is very supportive for an area of robust instability release around Sardinia and Corsica as an expanding pool of quite cold mid-/upper-level air spreads eastward. This again strengthens the already steep mid-level lapse rates of the impressive EML around Sardinia and Corsica and given the fact models like GFS were already a few degrees too warm yesterday at 500HPa, lapse rates of 8-9°C/km look reasonable. In combination with dewpoints around 20°C, capped MLCAPE values of 1.5 kJ/kg and more are likely ( and should be present in tomorrow's 12Z sounding reports of 16560 and 07761 ).
Further towards the west, instability values should relax somewhat although rapidly cooling mid-levels and SSTs of 20°C and more still support moderate to high SBCAPE release.

The more easy to forecast and most active part of this outlook will be north of the Balearic Islands and eastwards.
Although the lower troposphere stays quite warm, steepening mid-level lapse rates should support deep convection . A belt of strong PVA crosses the area from the west and scattered thunderstorm activity can be anticipated.
DLS and lapse rates especially at lower levels stay weak and should limit the threat for severe weather. Although not issuing a level-1 there still appears to be a marginal risk for strong wind gusts, marginal hail and even an isolated tornado, as environment provides an increasing amount of vorticity next to the core of the cyclonic vortex. Too many uncertainties / much nowcast to reflect those possibilities in a level-1!

The more interesting part will be found over Sardinia, Corsica and surrounding areas.
As stout cap with 850hPa temperature of 20°C erodes during the day and modified soundings indicate that the cap should break as early as late afternoon / early evening over Corsica and during the night over Sardinia. In addition, GFS shows abundant lift, as a compact UVV field crosses the area from the west during the afternoon / evening hours.
DLS of 20m/s and expected high instability values should support a few vigorous updrafts. Although SRH3 values relax and WBZ stays high, a combination of very steep lapse rates and expected strong updraft strength could support large hail/ strong wind gust reports.

Storms should also evolve east of Corsica in similar conditons and hence the level-1 area was expanded that far towards the east.

...NE Spain...

Yesterday's sounding of Madrid (12Z) showed quite favorable conditions for landspout development and those conditions should continue today over NE Spain although LL lapse rates relax somewhat compared to yesterday. This negative point was the main reason for not issuing any level-area.

...The eastern Czech Republic, southern Poland, Slovakia, eastern Austria, NW /NE Hungary and extreme SW Ukraine/ NW Romania...

An overlap of a quite moist and eastward streaming airmas below 600hPa ( regarding investigation of yesterday's sounding reports over southern Germany ) and an intense, southeastward shifting thermal trough over Poland result in a broad region of weakly capped 100-500 J/kg over the area of interest.
A compact vort max will cross Slovakia during the afternoon / evening hours from the NW while at lower levels a southward moving cold front will serve as a focus point for enhanced convective activity.
Latest thinking is that the highest risk for a local severe weather threat will be in the level-1 area, where EL values hint on possible deep convection, DLS stays strong with about 20m/s and modest steepened lapse rates will be present, which would be the eastern Czech Republic, Slovakia, southern Ukraine and NW Romania. The main threat will be an isolated large hail risk.
However we can't ignore persistent signals in the GFS outputs, which show low-end enhanced SRH-3 values over far eastern Austria and western Hungary in 25-30m/s DLS and we therefore expanded the 1 quite far westwards mainly due to a potential isolated large hail event.
The risk for severe thunderstorms will continue southeastwards and should start to weaken betimes, as instability signals vanish although thunderstorms could develop as far east as western Romania.

... NE Italy, Slovenia and Croatia...

Aforementioned surface front will continue its southward movement and it will cross the eastern Alpine region during the evening hours. As it is not unusual for such situations, the low-level baroclinic zone will be blocked by the complex topography . Hence moderate instability release can be expected over Slovenia, as LL airmass stays warm/humid, but mid-levels cool down. As front arrives from the NE during the early night hours , scattered thunderstorms will develop. DLS is quite strong and so is the shear at lowest 3km, which could support an isolated severe wind gust report in more vigorous storms.
A marginal level-1 was issued for those areas.

The severe weather threat should diminish after sunset, but strong forcing makes thunderstorm development possible as far east as Serbia.

...Eastern Baltic Sea...

A few SFLOC reports can't be excluded mainly over the eastern Baltic Sea, as models show some low-end instability values, but expected coverage will be too low for highlighting any specific area.