Extended Forecast
Valid: Wed 13 Jul 2011 06:00 to Thu 14 Jul 2011 06:00 UTC
Issued: Mon 11 Jul 2011 19:24
Forecaster: TUSCHY

A level 2 was issued for N-Italy mainly for significant hail, severe wind gusts and an isolated tornado event.

A level 2 was issued for far SE-Germany, parts of the Czech Republic and N-Austria mainly for large to significant hail reports, severe to damaging wind gusts and an isolated tornado event.

A level 1 was issued for Switzerland, Austria, most parts of the Czech Republic and S-Germany mainly for large to significant hail and severe wind gusts.

A level 1 was issued for the rest of Germany and E-France mainly for large hail,excessive rainfall, severe wind gusts and an isolated tornado event.

SYNOPSIS

A neutrally tilted upper trough moves in from the west and will be situated over France and UK during the forecast period. A sharp short wave lifts out of this trough towards the northeast and assists in rapid surface pressure fall over central Europe. As a result a strengthening surface depression evolves over Germany, which moves to the north/northeast. Also, a second depression/baroclinic wave possibly consolidates south of Switzerland and over N-Italy during the night hours with a much weaker deepening rate compared to the German depression. A weakening upper trough over E-Europe also sparks numerous thunderstorms, whereas high pressure dominates the Mediterranean with hot and stable conditions.


DISCUSSION

A few uncertainties remain although not grave ones, including ongoing deep convection clusters from the night before mainly for Switzerland and parts of Germany, potential foehn effects along the N-Alps and a possible second cyclogenesis south of the Alps. However, ingredients for an outbreak of severe thunderstorms are present for at least a few areas. The main risk will be significant hail events, severe to damaging wind gusts and a few tornado events.

... Switzerland and W-Austria ..

It still remains a bit unclear how far to the NE/E the convective clusters from the night before affect the area of interest during the morning hours but at least areas from N-Italy all the way to the Czech Republic should not see any hampering effects. Current indications are that an active baroclinic zone gradually moves in from the west and strengthens over far NW-Italy and Switzerland. Scattered to widespread showers and thunderstorms are forecast from the start off. Persistent orographic lifting and a sharp vorticity lobe, which draws near from the SW during the afternoon hours, all assist in a large and possible messy cluster of showers/embedded thunderstorms, which finally moves off the northeast during the night hours, as PVA max moves away. However, we can't ignore the 500 - 1000 J/kg MLCAPE in both models, ECMWF and GFS, which could easily be realized with a few hours of insolation. In that case, up to 35 m/s 0-6 km bulk shear and intense directional shear may foster supercell development with significant hail and severe wind gusts. If confidence in the more robust CAPE scenario is higher, an upgrade may be needed. The thunderstorms also move to the E/NE, affecting W-Austria with excessive rainfall, large hail and strong to severe wind gusts.

...N-Italy....

ECMWF and GFS both indicate good thermodynamic support over N-Italy with ECMWF showing up to 1500 J/kg and GFS up to 2500 J/kg MLCAPE. Given latest synop data with widespread 18-20 °C dewpoints and no frontal passage ahead until Wednesday, we agree with optimistic CAPE forecast due to good BL moisture and therefore scattered thunderstorm development as strong vorticity lobe passes to the NW. A potential second cyclogenesis/surface wave may evolve south of Switzerland, which may also keep N-Italy in a moist and unstable air mass for a prolonged period of time. Similar to Switzerland, shear is highly supportive for intense supercells with significant hail and severe wind gusts. Also, dependent on the final surface wind field, an isolated tornado event seems possible, if wind field backs somewhat ahead of the approaching wave.

... Germany, N/NE Austria and the Czech Republic ...

Both ECMWF and GFS agree well in the build up of high-end MLCAPE over far SE Germany with up to 2500 J/kg forecast in both model data. We're not yet sure if this solution completely verifies given potential foehn effects along the N-Alps but "foehny conditions" should only have an effect regarding timing of initation, but none regarding the strength of expected thunderstorms. Initiation along the N-Alps is forecast from both model runs during the afternoon hours and this seems likely as PVA max approaches from the SW. 20 m/s DLS, SRH-3 above 200 m^2/s^2 and moderately steep lapse rates indicate an augmented risk of significant hail with each more discrete supercell next to severe wind gusts.

Instability fans out to the NE with up to 1000 J/kg MLCAPE all the way to E-Germany, NE/E Austria and the Czech Republic. However, kinematic and thermodynamic conditions remain supportive for large to isolated very large hail and strong to severe wind gusts. A significant hail/damaging wind gust risk may materialize over most parts of the Czech Republic and N/NE-Austria, as supercells over SE Bavaria move to the northeast and east. A level 2 was added to reflect the risk of both significant hail events and also the potential development of a NE-ward racing bow echo. Also, an isolated tornado event can't be ruled out.

Further to the west, including SW/central/ N-Germany and E-France, the exact position of the active baroclinic zone becomes more unclear that far out. The same for the evolving surface depression somewhere over NW-Germany. However, the sharpening cold front will be another focus for bowing line segments as 25 m/s 700 hPa jet max moves along that boundary to the NE. Severe wind gusts will be the primary risk with that activity next to isolated tornadoes and excessive rainfall amounts.