Extended Forecast
Valid: Sun 13 Jul 2008 06:00 to Mon 14 Jul 2008 06:00 UTC
Issued: Fri 11 Jul 2008 21:43
Forecaster: TUSCHY

SYNOPSIS

An unseasonably far southward extending upper trough starts to weaken while moving east / northeastwards. This system separates the forecast area in a cooler and unstable part, covering NW / W and central Europe, while intense WAA on its downstream side results in hot and stable conditions for most parts of the Mediterranean and E / NE Europe. Along its eastern flank, numerous disturbances rotate around the main feature with an enhanced risk for quite widespread thunderstorm initiation.

DISCUSSION

... SE Spain and the extreme western part of the Mediterranean ...

The huge longwave trough splits in two pieces, a more active one over France shifting eastwards and another, more diffuse one over far SW Europe. This constellation should result in a lull of thunderstorm activity during most of the day due to lack of any forcing mechanisms. No question, outflow boundaries of storms of the previous day, but also the topography could result in a few thunderstorms during the day if stout cap can be broken. Initiation will be widespread and thunderstorm intensity more vigorous during the night hours as mid - level cold pool arrives from the west with a strong UVV maximum.

DLS is very strong with 20 - 30 m/s over SE Spain, increasing to well above 35 m/s offshore with strongest wind field at 600 hPa upwards. At the moment we think that storms will be more of an elevated nature despite a weakening cap which could be offset by nocturnal cooling. However, steepening mid - level lapse rates and such a strong wind field in a layer with moderate instability release point to a large hail and strong to severe wind gust risk with strongest thunderstorms. We did not yet issue any level areas as uncertainties about exact timing of the arrival of the upper impulse and the placement of strongest thunderstorm activity are still high. An upgrade may be needed later - on.

... N - Italy, parts of Austria, Slovenia, Hungary, Slovakia, Poland, Lithuania, Latvia, Estonia and western Belarus and Ukraine ...

Downstream of the major trough, numerous disturbances come out of the base in a strong SW - erly upper - level flow. A combination of a slowly eastward moving surface cold front and aforementioned disturbances atop of that front result in a broad area with enhanced thunderstorm chances. Strong moisture influx just ahead of the front will help to boost instability values well above 1000 J/kg MLCAPE as mid - level lapse rates stay very steep. The same for NE Italy and Slovenia.

The final degree of severity depends both on the upper disturbances and possible numerous surface depressions, which both support a significantly veered / backed wind field at mid / lower levels. The past few GFS runs already had an environment favorable for supercells with a tornado, very large hail and severe wind gust risk, running from Poland northeastwards. Confidence is high enough to go with a level -2 area although this is yet a coarse one that far out. Some modifications may be needed in the Day - 1 outlook, e.g. a NE - ward expansion due to an augmented tornado risk.

Another level - 2 was issued for parts of NE - Italy and W - Slovenia, where strong instability release, 20 m/s DLS and enhanced SRH values overlap with significant hail being a distinct possibility. This level - 2 may have to be expanded well to the E / NE due to an enhanced very large hail risk, but we want to wait for more informations ( e.g. new model runs ) before doing so, especially as the DLS stays quite weak with 10 - 15 m/s DLS.

Betimes, numerous thunderstorm clusters will evolve in the broad level areas with an enhanced flash flood risk.

... Parts of Finland and Sweden ...

A broad area of diurnal driven thunderstorm activity in a weakly sheared environment can be expected. Strongest storms could produce isolated large hail but expected coverage is too low for a level - 1. After sunset, thunderstorms will decrease in intensity and coverage.

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