Storm Forecast
Valid: Sat 30 Jun 2012 06:00 to Sun 01 Jul 2012 06:00 UTC
Issued: Sat 30 Jun 2012 09:45
Forecaster: VAN DER VELDE / PISTOTNIK / PUCIK

A level 3 was issued for NE France, SW to NE Germany mainly for very large hail, widespread severe wind gusts.

A level 2 was issued for NE France, Germany and Poland mainly for (very) large hail, severe wind gusts and some chance of tornadoes.

A level 1 was forecast for S France as well as parts of Poland, Czechia, Slovakia and Austtria mainly for large hail.

SYNOPSIS

A mostly cloudless frontal boundary lies almost stationary across France, Germany and Poland. Over France it is associated with clouds and LCL heights below 1000m and more under the influence of cold air advection, whereas over Germany and Poland warm humid air (14 g/kg near the surface) is advected into the region. Very steep lapse rates coming from Africa and föhn effects of the Alps and Pyrenees cover the area and crreate large MLCAPE values, from 1000 J/kg over S France, to over 2000 J/kg MLCAPE over S Germany, solidly supported by multiple models.
SInce the airmass is initially somewhat capped for mixed 1000m layer parcels, most likely afternoon intiation of storms will be central/northeastern France and Poland as indicated by model precipitation. Vertical deep layer wind shear (15-30 m/s) is supportive of supercells over a large area. Then followed by initiation along the north side of the Alps, moving off northeastward, building into a large MCS or MCC, which keeps being fed by a nocturnal jet tapping from a large body of surface-based CAPE throughout the night.

DISCUSSION

...northeastern France, Germany and western Poland...

Initiation will initially be hesitating, but large rounded hodographs combined with solid CAPE can support long-lived supercells while strong cold pools owing to evaporational cooling (delta-theta-e greater than 20 degrees) can set off rapid upscale growth into mixed multicell/supercell clusters. Storm-relative helicity should reach 200 m2/s2 over a large area, some models produce values of 350 m2/s2 closer to the front (France and more northwest in Germany). Together with relatively high cloud bases (broad updrafts and little warm rain production) and CAPE density well increasing with height, very large hail is likely over southern and central Germany. Supercells and high delta-theta-e support severe/extreme micro/macro downburst potential. The widespread hail and gust potential will organize more into one MCS as the night falls, with Hirlam indicating also 10-15 m/s 0-1 km shear vectors which support bow echoes as well as an isolated tornado. A derecho event (widespread extreme wind gusts) is a possibility as the system organizes over eastern Germany, possibly supported into Poland by rich boundary layer-based instability even through the night. On the other hand, Corfidi MCS motion vectors will be rather parallel to the front and low to mid level wind speed seems to remain below 25 m/s.The hodographs are supportive of a southward-building asymmetric MCS rather than a fast forward moving squall line. However, it builds into excellent instability.
The well beyond marginal, richly overlapping signals, mainly for very large hail, and dramatic simulations by LAMs are supportive of a level 3, but if negative factors appear (e.g. in 12Z soundings) a modification will be issued. We stayed with level 2 over France (lower cloud base, narrower CAPE zone) and Poland.