Storm Forecast
Valid: Wed 22 Aug 2007 06:00 to Thu 23 Aug 2007 06:00 UTC
Issued: Tue 21 Aug 2007 23:50
Forecaster: GATZEN

SYNOPSIS

An intense cut-off low is centered over France. A strong upper jet curves around it from southern Scandinavia to southern British Isles, around the trough axis over Iberian Peninsula, western Mediterranean, Alps, Poland, and southern Baltic Sea. A very strong jet streak is expected from southwestern Mediterranean to Austria/Poland during the period, where 300 hPa winds are forecast to reach more than 60 m/s. To the north of the cut-off, a sharp ridge connects an Atlantic high with high geopotential over east Europe.

While very warm air mass spreads into east Europe ahead of the cut-off, maritime air mass over France advects into western Mediterranean. In the range of the broad frontal zone ahead of the cut-off low, and along the northern boundary of east European warm air mass, as well as in the range of the cut-off center, rich moisture is present especially in low levels. This leads to widespread convective potential. Especially in the range of the strong upper jet, potential for a party dangerous situation is forecast during the period.

DISCUSSION

Central/northern Mediterranean, Balkans

Ahead of the cut-off low, a deep southwesterly flow advects very warm air originating from northern Africa into southern Italy, and southern Balkans. Latest soundings indicate 850 hPa temperatures of 25°C and a well-developed EML. Actually, soundings over northern Mediterranean and Balkans indicate steep low-level lapse rates and weak capping. On Wednesday, rather strong capping inversion is expected, but given very moist low-level air, instability will likely increase. Latest GFS indicates 1000 J/kg CAPE as well as LI below -5K.

East of a quasi-stationary frontal boundary from Sardinia to central Italy and northern Balkans, a rather heterogeneous moisture distribution is expected in the WAA regime, and embedded low-level boundaries like outflow boundaries of old convection, sea-breeze fronts, and orographic convergences will likely be the focus of enhanced low-level buoyancy. Given the strong upper jet streak from southwestern Mediterranean to northern Italy, and northern Balkans, QG forcing in the range of short-wave troughs and speed maxima is also likely. All together, initiation of storms is not excluded. Thunderstorms that form will likely become supercells near the frontal boundary, where vertical wind shear is very favorable given 30 m/s DLS, 10 m/s LLS, and 100-300 J/kg 0-3km SRH. Supercells are expected to produce the whole range of severe weather, including large hail, severe wind gusts, and tornadoes. Even strong tornadoes are not ruled out, although there is limited 0-1 km vertical wind shear. Further east, vertical wind shear and veering profiles are weaker, and chance for initiation seems also be weaker. Storms that may form will likely be isolated and will profit from quite high instability. Very large hail, severe wind gusts and maybe isolated tornadoes are not ruled out. Limiting factor is the capping inversion that may be quite strong, suppressing any convection in the southern regions. Thunderstorms are forecast to merge into mesoscale convective systems in the evening hours, moving to the northeast into northern/central Balkans. During the night, latest GFS shows a weak ridge spreading across the region, and thunderstorms may weaken in the southern and western portions of the risk area.

Southeastern Germany, Austria, western Alps, Czech, Slovenia, Poland

Rich low-level moisture is present that will likely remain on Wednesday given warm air advection in the range of the main frontal boundary. Latest soundings indicate steep mid-level lapse rates, and instability is likely to develop. Uncertainty exists about the capping inversion. However, current thinking is that QG forcing at the cyclonic flank of the jet streak as well as low-level forcing in the range of frontal boundaries will likely lead to initiation during the day. First storms are forecast to move across Poland in the range of a short-wave trough ahead of the jet streak, while additional thunderstorms are forecast to develop in the afternoon/evening hours further south. Favorable veering profiles and strong vertical wind shear will likely lead to rotating updrafts, and supercells are forecast, capable of producing large or very large hail. Limiting factor for high winds and tornadoes is weak low-level vertical wind shear, but a few events are not ruled out given 20 m/s 0-3 km vertical wind shear. Will exclude parts of northern Italy, and central/western Alps, where latest model output indicates cool air mass that will likely be stable. Convective activity is expected to go on during the night hours given persistent forcing. Severe potential will only slightly weaken due to slowly decreasing low-level temperature.

Northern Balkans/southern Finland to western Russia

At the northern boundary of east European warm air mass, rich low-level moisture has developed as indicated by latest ascends. Additionally, steep low- to mid-level lapse rates are present in the warm air mass. GFS model shows that CAPE will develop in the range of the frontal boundary. Current thinking is that CIN won't be too strong. Thunderstorms are expected to develop in the range and south of the frontal boundary during the day. Given a short-wave trough that separates from the cut-off low and accelerates eastward, QG forcing and increasing vertical wind shear is expected. In the afternoon/evening hours, GFS calculates up to 20 m/s 0-3 km vertical wind shear and 150 J/kg 0-3 km SRH. Given weak CIN, high boundary-layer moisture, and favorable veering profiles, thunderstorms that form are forecast to evolve mesocyclones with the whole range of severe weather. With 20 m/s 0-3 km vertical wind shear, severe wind gusts and tornadoes are expected, while large hail can also be possible with every rotating updraft that forms. Convective activity is forecast to spread eastward during the evening/night, as upper short-wave trough continues eastward, too. Persisting QG forcing is expected, and a well-developed MCS may develop, capable of producing high winds in association with bowing lines. Rather widespread severe winds may be possible if this scenario comes true.

Southwestern Iberian Peninsula, northern Africa

Although low-level moisture is limited and capping inversion is rather strong, QG forcing in the range of the propagating strong upper jet as well as low-level forcing may be strong enough to initiate thunderstorms that will benefit from strong DLS and will rapidly become supercells. Large hail and severe wind gusts seem to be possible. Limiting factor is that instability and forcing will be relatively weak. Thunderstorms are forecast to weaken after sunset.

Northern Germany, southern Denmark

At the northern flank of the cut-off low, a very moist boundary-layer and steep low-level lapse rates are present, and continue to spread westward into northern Germany, and southern Denmark. Strong low-level buoyancy is expected to develop during the day. Given almost no CIN, thunderstorms are likely. Due to northeasterly surface winds and increasing easterly winds aloft, quite favorable veering profiles are expected to develop. GFS indicates 100 0-1 km SRH values. Mesocyclones, capable of producing tornadoes, are forecast. Isolated hail and strong winds are also possible. However, there exist some uncertainty about low-level temperature given sea-breeze fronts that may destroy low-level instability.