Storm Forecast
Valid: Fri 24 Aug 2007 06:00 to Sat 25 Aug 2007 06:00 UTC
Issued: Thu 23 Aug 2007 21:07
Forecaster: TUSCHY

SYNOPSIS

An unseasonably strong upper-level jet covers most parts of SW and central Europe, framed by higher pressure towards the east and west. A quite active polar front jet over northern Europe directs strong depressions towards Norway and Sweden while hot conditions prevail over SE Europe.

DISCUSSION

....Strait of Gibraltar, SE / E Spain and the Balearic Islands ...

Latest loop of WV images and sounding ascents ( yesterday, 12Z ) provide enough confidence that global models like GFS handle current strength and placement of weakly positive tilted upper-level trough over Spain / Portugal. Intense 110kt streak curving southwestward on its upstream side ( refer to Brest 12Z ), supporting a constantly SW - NE elongation of this upper-level feature with no notable movement. Degree of amplification and strength of developing LL depression over Morocco are the main uncertainty regarding coverage of expected thunderstorms.

Cold front ( penetrating well inland during the past 36 hours and being placed along the Atlas mountains ) will transform into a warm front during the next 24 hours as pressure gradually falls over Morocco and WAA picks up during the afternoon and evening hours over NW Algeria. This frontal boundary is forecast to reach a line NE Morocco - Balearic Islands during the evening hours and will be the focus for the strongest thunderstorm threat.
Areas east of this front will be mostly capped as 850hPa temperatures soar at or above 20°C.

Numerous short waves, ejecting out of the base of the upper-level trough will likely enhance the lift over SE / E Spain and the Balearic Islands although the strongest one is forecast to cross the area from the SW druing the evening and night hours. Although GFS tries to develop a coupled upper-level jet structure ( also indicative as models boost divergence values ) the spread in the model pool is still too large to buy one solution.
Latest thoughts are that numerous thunderstorms develop in the level-1 area during the forecast period, which could easily accrete in small clusters of rapidly forward propagating lines of storms.
During the evening and night hours, scattered to widespread storms should evolve mainly west / southwest of the Balearic Island and eastern Spain, moving rapidly towards the NE. NOGAPS and GEM indicate a possible cluster of storms during the latter part of the forecast period just west / northwest of the Balearic Islands with an attendant severe weather threat.

During the whole forecast period, parameters for rapid storm organisation are fine as DLS increases to 30m/s and lower wind field responses to the evolving LL depression over Morocco with backing and hence a broad area of intense LL shear. SSTs of about 25°C and a humid PBL all indicate the chance that storms over the Mediterranean will be surface based and parcels of the highly helical environment ( ~ 400 J/kg SRH3 and 150-300 J/kg SRH1 ) could easily be incorporated. Mid-level lapse rates will steepen during the period and weakly capped ICAPE values of above 2000 J/m°2 should be realized. The main risk will be a severe to possible damaging wind gust threat, especially if storms line up ( with embedded bowing line segments or supercells ). The tornado threat is also enhanced with mesoscale features like outflow boundaries playing an important role.

The hail threat should increase over eastern / southeastern Spain, as instability rapidly ease further inland and storms tend to be more elevated in nature. Shear and helicity are still impressive and an isolated large hail report will be likely.

A broad 1 was issued because there are still too many uncertainties left. Higher probabilities will be issued if model trends finally point to an area of significantly enhanced severe weather.

The same set-up, but somewhat weaker shear will be present over the Strait of Gibraltar and a few severe thunderstorms with an attendant severe wind gust risk can be expected.

...Sardinia, Corsica and surrounding areas...

The same game like above. Numerous short wave troughs translate rapidly towards the NE, supporting a locally enhanced thunderstorm threat and we would not be surprised to see one or two storm clusters around and north of Sardinia during the morning hours. Shear will be supportive for a severe wind gust and tornado risk, especially over the islands,where ageostrophic wind component could locally enhance LL shear.
Exact forecast track is impossible to tell that far out and model pool is still way too inconsistent to commit on any solution and hence a broad area was highlighted.
Storms will slowly diminish during the late afternoon hours, as strengthening WAA helps to increase CIN values.

... Slovenia, Croatia, Bosnia and Herzegovina,SE / E Austria, Hungary and parts of Slovakia...

One should take this forecast with a pinch of salt as GFS seems to have some convective feedback problems with storms over NE Morocco / NW Algeria ( yesterday evening ). Significant jumps of run to run and strong discrepancies between models make this a quite doubtful forecast. As peace of energy ( in conjunction with the NE-ward moving short wave around Sardinia ) reaches Croatia and Bosnia and Herzegovina during the late evening / night hours, convective activity should increase, as modest instability will be present over those areas.A cluster of storms should develop and move towards the NE, crossing Hungary and possibly the eastern parts of Austria after 00Z. Instability is on the decrease but moderate DLS ( especially enhanced at lowest 3km ) will be enough for multicell storms with a mostly marginal hail / gusty wind threat.
Cells over southern / eastern Hungary could profit of enhanced SRH with an enhanced hail threat.
No level was introduced right now, but a small level 1 over parts of Hungary could become necessary later on.

....Western Russia...

An eastward shifting upper-level trough and attendant cold front should spark numerous thunderstorms over a broad area and models are pretty consistent in this activity. Strengthening WAA and gradually weakening mid-level lapse rates limit severe weather threat although shear ( especially at lower levels ) is still enhanced. Strong moisture advection will be present, too.
A few severe wind gusts and an isolated tornado will be a distinct possibility. The highest threat should peak out after 18Z, as instability is forecast to decrease rapidly.