Storm Forecast
Valid: Sat 19 Apr 2008 06:00 to Sun 20 Apr 2008 06:00 UTC
Issued: Fri 18 Apr 2008 21:50
Forecaster: TUSCHY
SYNOPSIS
High over low blocking pattern persists. A strong low pressure complex centered over the southern Bay of Biscay helps to keep weather conditions unsettled over most parts of western Europe as numerous more or less organized disturbances rotate around this feature. Downstream of this vortex, hot desert air advects northeastwards and affects parts of the eastern Mediterranean.
Cold but dry conditions prevail over N - Sweden / Norway.
DISCUSSION
... Slovenia, Slovakia and Hungary ...
A surface depression forms somewhere over Austria / Slovenia during the morning hours. Model pool is consistent in developing this feature and models also agree with the strength as forecast dynamics won't support any rapid intensity fluctuations. The main difference between all those models is the exact path of this feature with UKMO being the northern most outlier.
While forcing at upper-levels is easy to localize it becomes more diffuse at the surface as a broad and elongated surface depression evolves over Austria during the morning hours, moving / developing slowly east / northeastwards. The main focus for thunderstorm initiation will be a northeastward rotating vorticity maximum and attendant compact UVV field. This crosses Slovenia and S-Austria during the morning hours, Hungary and Slovakia from 12Z onwards.
Therefore, showers and thunderstorms should go on / re-develop over Slovenia during the morning hours and scattered thunderstorms are forecast to spread east / northeastwards . Instability will be the most questionable parameter as only 100 - 300 J/kg MLCAPE are forecast. In addition, convective debris ( e.g. more stratiform rain shield from yesterday's convection over N-Italy ) should minimize insolation. Shear at all levels is strong ( 0-1km shear 12-15 m/s and 20-30 m/s DLS ) and any stronger / more discrete thunderstorm could pose a large hail and isolated tornado risk.
A more robust severe thunderstorm event evolves over western Hungary and western Slovakia between 10Z and 12Z. Mid-levels cool down and mid-level lapse rates reach about 7 K / km. Rich boundary layer moisture and aforementioned steepening lapse rates result in a broad sector with 400 - 800 J/kg SBCAPE / locally up to 500 J/kg MLCAPE. Current thinking is that the main activity will develop along the the cold front and at the northern tip of the 850hPa temperature ridge which means NW Hungary and western Slovakia. Thunderstorms will then rapidly develop eastwards expanding also towards the south over central and southern Hungary as diabatic heating and forcing should both overcome capping. Strong storm relative helicity at lowest levels ( 150 - 200 m^2/s^2 ) and LCLs below 600m indicate an augmented tornado risk over western Slovakia passing into a large hail risk over NW / W - Hungary. When thunderstorms reach eastern Slovakia and Hungary between 15Z and 18Z, the risk for tornadoes should even increase and even a strong tornado will become possible as 1km storm relative helicity values top 200m^2/s^2 and LL shear reaches 15m/s.
DLS increases from 15m/s over eastern Slovakia to 30m/s over SE Hungary which implies also the risk for large hail. In addition, a few severe wind gusts could occur.
Further towards the south, including SW Romania, Serbia and western Bulgaria, GFS hints on initiation despite a stronger cap and weaker instability. DLS would be strong enough for an isolated severe thunderstorm with large hail but uncertainties regarding initiation and the degree of instability release preclude any level / general thunderstorm areas for the moment.
... Portugal and Spain ...
Approaching trough axis from the west sparks numerous thunderstorms over Portugal during the morning and midday hours, which move northeastwards. BL moisture mixes out very fast and we think that strongest shear and instability won't overlap. Strong wind gusts and marginal hail will be possible with strongest thunderstorms. Thunderstorm coverage and intensity will diminish during the evening hours.
... Parts of France, Switzerland and western Austria / N-Italy ...
Facing the parameters over those areas, they are not too supportive for widespread thunderstorm initiation as mid-levels warm up slightly and BL moisture and instability release are meager at best . Some insolation and topography ( e.g. orographically enhanced upslope flow ) could offset at least locally all those negative points and we went with a general thunderstorm area. DLS is somewhat stronger over N-Italy but there are no indications for an enhanced hail risk. Stronger pulsating thunderstorms could produce strong wind gusts and marginal hail with maximum intensity during the afternoon / evening hours
... NE Spain, SW - France after about 03 Z ...
Approaching short-wave trough from the SW will be the focus for scattered thunderstorm development. GFS hints on modest instability release as very moist air overspreads the area from the south. Shear is very strong with DLS reaching 30m/s and intense LL shear just to the east of the main instability axis. We want to evaluate more model informations before we include the potential for tornadoes. For now we think that the main risk will be a severe wind gust / large hail risk.
