Storm Forecast
Valid: Fri 12 Sep 2008 06:00 to Sat 13 Sep 2008 06:00 UTC
Issued: Thu 11 Sep 2008 19:32
Forecaster: TUSCHY

SYNOPSIS

A negative tilted and elongated upper trough over W-Europe turns into a closed upper feature as another impulse along its upstream side pushes southwards and geopotential heights increase SW of Ireland due to WAA of another upper trough further upstream. This cut-off slowly advances SE-wards, approaching the NW Mediterranean during the end of the forecast period.Finally autumn knocks on N/CNTRL European's door due to a nice collaboration between an intense 1030hPa high over N-Europe and a diffuse/splitted upper trough over W-Russia, resulting in a large-scale advection of appreciable cooler air all the way down to NE Germany /S-Poland until 06Z,SA.

At the surface numerous fronts are analyzed. One is a slowly eastward propagating cold front over the western Mediterranean, finally replacing the muggy and hot conditions by cooler/drier air and this front will advance to near Corsica/Sardinia until Saturday morning. Another boundary gets established over the Alpine region / central Germany and slowly retreats southward during the end of the day as cold airmass finally pushes southwards.

DISCUSSION

... Parts of central /S-Germany ...

Focus for widespread initiation will be a strengthening thermal boundary, running from SE to NW-Germany. Atmosphere is only weakly capped, so diabatic heating / forcing of various mesoscale convergence zones should be adequate for numerous thunderstorms to evolve during the day, become stronger and higher in coverage in a line Bavaria - North-Rhine Westphalia. Shear is very weak and so the main risk will be marginal hail/strong wind gusts. There is a low-end risk for a tornado/funnel report over SE-CNTRL Bavaria as LL CAPE release increases during the day and a pronounced, eastward moving convergence zone and low LCLs exist, but nothing to put out a level area. After sunset, conditions become worse for thunderstorm development over most parts of the highlighted area but strong convergence and at least traces of instability over NW-Germany could result in a few storms well after sunset.

... S- Austria and Slovenia ...

This region was picked out of the overall highlighted area as environment could become supportive for tornado development mainly confined to the best time of the diurnal cycle, the afternoon hours. Initiation will be widespread already during the late morning/midday hours and a cluster of storms should form over E-Tyrol/Salzburg, moving to the E/SE during the afternoon/evening and latest WRF still hints on another cluster of storms, affecting Slovenia during the evening/night hours. Despite the uncertainty regarding place of development/path of those clusters, weak shear should preclude anything severe so main storm mode will be rapidly clustering single cells with marginal hail/strong wind gusts. However there is a reason for issuing a marginal level area, as LL airmass is very moist with surface dewpoints in the mid/upper tens and a hence LCLs will stay low. Robust 0-3km instability should be available and as topography is quite complex, vortices, propagating along any convergence line could aquire adequate strength to tornadic intensity as they become colocated with the updraft of a more rapidly developing thunderstorm. Otherwise, torrential rain due to slow storm motion will be a big hazard. The level area was epanded well to the SE, where shear and helicity become stronger and a few large hail reports can't be ruled out.

... Western Mediterranean ...

The focus, as discussed above, remains the slowly eastward moving cold front, as the upper disturbance constantly approaches from France. Soundings from the past 2 days showed a capped but very unstable environment with MLCAPE readings locally above 3000 J/kg and MUCAPE values even higher. Pronounced EML slowly erodes, but very steep lapse rates and rich BL moisture should still assist in very unstable conditions ahead/along the cold front. As upper feature approaches, wind fields at mid-/upper levels strengthens markedly and DLS increases to 20-25m/s atop of the boundary. The main concern right now is the level-2 area, as enhanced LL helicity/shear and very low LCLs overlap, which should offset the negative point of still quite warm 850hPa temperatures and hence weak LL instability release. Developing storms should rapidly gain supercell status with an enhanced risk for tornadoes and even a strong tornado can't be excluded. Severe wind gusts and large to very large hail will also accompany storms. Betimes, thunderstorms cluster, while building NE-wards. Numerous clusters could evolve along this boundary, covering a vast area and hence a very large level-2 became necessary.

The level-1 was issued far eastward as cold front outruns better shear and the risk for significant events should decrease but still multicells/supercells with large hail/severe wind gusts and an isolated tornado will go on till the morning hours.

There is a significant risk for flash flooding as background flow becomes more parallel to the front during the evening/night hours. Impressive parcel layer depth, a very moist airmass and numerous, mature and ongoing convective clusters could result in locally tremendous and life-threatening rain amounts (e.g. Corsica, Sardinia and NW Italy but also the Balearic Islands during the morning/midday hours)!

... W/NW France and SW-UK ...

As upper-level cold-pool (readings at 500hPa -20 to -24°C) crosses those areas from the NNW and BL stays adequate moist, some low-end instability could be realized, maximized during the afternoon/early evening hours. Weak shear but enhanced LL buoyancy and various wind shift zones should enhance the funnel/tornado risk, but LCLs well above 1km lower confidence in an adequate coverage for a level-1 over France. SW-UK stays more uncertain, but significant downplay of LL instability in latest model runs also preclude a level area for the moment.

200-500 kJ/m^2 ICAPE and 40m/s 1-8km shear overlap in a confined area over S-CNTRL UK, but EL maps hint on low-topped convection where shear decreases markedly so no organized convection will evolve out of this activity. In fact the thunderstorm area was not expanded to the east as updrafts should stay too weak for an enhanced lightning activity.