Storm Forecast
Valid: Wed 19 Jun 2013 06:00 to Thu 20 Jun 2013 06:00 UTC
Issued: Tue 18 Jun 2013 22:33
Forecaster: PUCIK

A level 2 was issued for Central to Northeastern France mainly for large to very large hail, damaging wind gusts, tornadoes and to the lesser degree for excessive precipitation.

A level 2 was issued for BENELUX to Northern Germany mainly for large to very large hail, damaging wind gusts and excessive precipitation.

A level 2 was issued for extreme Northwestern Switzerland and Southwestern Germany mainly for large to very large hail and severe wind gusts.

A level 1 was issued for Southeastern France mainly for large hail and severe wind gusts.

A level 1 was issued for Denmark mainly for large hail, severe wind gusts and excessive precipitation.

A level 1 was issued for the Alpine, Carpathian regions, Bohemia and Eastern Germany mainly for large hail, severe wind gusts and excessive precipitation.

SYNOPSIS

Volatile setup regarding severe DMC is expected to persist also during this forecast period with Tuesday 12 UTC soundings showing very high CAPE values over majority of Central Europe. This is due to the overlap of steep lapse rates along with high boundary layer moisture (dew points exceeding 20°C). What should change in contrast to the previous two forecast days is that the upper level low centered over Iberia should finally start moving towards northeast, as well as the surface frontal system. Southerly flow will likely induce lee cyclogenesis behind the Alpine range with trough extending towards BENELUX with significant low-level convergence zone running from BENELUX towards Northern Germany. Another surface low will be situated over the Bay of Biscay.

As upper low and frontal system approach an area with pronounced overlap of high CAPE, steep mid-level lapse rates, along with moderate to strong vertical wind shear, scattered to widespread initiation of DMC over parts of France, BENELUX and Germany might result in a regional outbreak of severe weather.

DISCUSSION

... Central to Northeastern France ...

As the upper low approaches the region, mid to upper tropospheric flow over France will reach 20 - 40 m/s while synoptic scale ascent could help in the reduction of CIN (aided by 850 to 700 hPa temperatures drop). While steeper mid level lapse rates, as well as higher CAPE values will be displaced more towards northeastern part of the country (with MLCAPE values possibly exceeding 3000 J/kg), stronger vertical wind shear will be observed over its central part (DLS over 25 m/s). Nevertheless, a significant overlap of these two factor is expected and thus, very strong and organised DMC might be expected. Furthermore, a belt of enhanced SREH (locally over 200 m2/s2 in the 0-3 km layer) is simulated on the back side of the surface trough building from the Alpine region towards north. Low level convergence zones or a slowly translating frontal boundary may serve as the foci for thunderstorm initiation.

At first, isolated multi/supercells may form, posing risks of large to very large hail (thanks to very steep lapse rates) and severe wind gusts. Southwestern part of the level 2 area will likely see a belt of enhanced low level flow with LLS values exceeding 10 m/s and 0-3 km shear values exceeding 20 m/s. Thus, in case that isolated supercells manage to form, tornadoes might occur as well with LCLs around 1000 m. Towards the evening hours, storm clustering might eventually result in one or more MCS (possibly including a bow-echo) capable of wider swaths of severe wind gusts or also isolated excessive precipitation event.

... Southeastern France / extreme Northeastern Spain ...

Poor mid-level lapse rates will likely limit CAPE values over this area, but presence of very strong vertical wind shear could well compensate for this factor with possibility of several supercells capable of large hail and severe wind gusts.

... BENELUX to Northern Germany and Denmark ...

A well defined low level convergence zone is simulated over the area with backing low-level flow near the surface trough. Topped by around 15 to 20 m/s of mid to upper tropospheric flow, moderate deep layer shear values are expected with SREH values between 150 and 300 m2/s2 in the 0-3 km layer. Combining such conditions with steep lapse rates and moderate to high values of MLCAPE, conditions seem to be prime for strong multi or supercells, capable of large to very large hail and severe wind gusts. Later on, storm clustering might favour an MCS paralleling the low level convergence zone and posing a risk of damaging wind gusts and/or excessive precipitation.

... Extreme NW Switzerland and SW Germany ...

Explosive environment with MLCAPE values between 2000 and 3000 J/kg is forecast over the region. Despite the degree of wind shear values being rather modest, strong multicells capable of large to very large hail and/or severe wind gusts are possible in the late afternoon and evening hours as CIN weakens with the approach of the upper level low. Moreover, this region might see an MCS approaching from France in the night to early morning hours with threats of severe wind gusts or large hail.

... Alpine region, E. Germany, Czech Republic, Southern Poland, Carpathian region ...

Especially tied to the local orography, CIN might be overcome with isolated development of DMC. Thanks to the very steep lapse rates and moderate to high CAPE values in presence of the high boundary layer moisture, even in weak wind shear conditions, storms could be capable of large hail, severe wind gusts and especially in the montainous areas, due to the slow storm motion, also of excessive precipitation. A rather broad Level 1 is introduced due to the high uncertainities regarding convective initiation.