Storm Forecast
Valid: Sun 28 Sep 2008 06:00 to Mon 29 Sep 2008 06:00 UTC
Issued: Sat 27 Sep 2008 17:22
Forecaster: TUSCHY

SYNOPSIS

The European large-scale pattern changes step by step during the next few days. Latest CMC upper atmosphere analysis has a rapidly weakening upper trough at roughly 40°-50°N/40°W and a cut-off process is underway. At the surface at 40°N/40°W, a strong 994hPa surface depression has formed and downstream of both features (surface depression and upper trough) strong WAA is underway, resulting in a thickness increase mainly between 40°-50°N/20°-30°W. This evolution results in a retrograde movement of the pronounced high pressure area, which affected NW Europe during the past few days. A quite intense cyclonic vortex ( analyzed with 971hPa at 06Z) over far N-Europe shifts slowly eastward, while weakening so a shift to a more active weather period ( not necessarily convective wise ) can be expected over parts of NW/N/CNTRL Europe. SW-NE elongated upper trough is still placed over S-Europe although a slow break-down into numerous upper vortices will occur during the forecast period. A few thunderstorms could develop over far W-Russia as diurnal driven marginal instability release is forecast.

***Additional information***

Development of an intense surface depression at ~ 40°N/40°W is on track with latest pressure estimation of 994hPa. QuikSCAT has uncontaminated flags of 30-40kt along its southern quadrant but no additional data available. Latest WV animation has strongest convection in N/W quadrant, but agree with models, which keep this feature extratropical for at least the forecast period. Rising geopotential heights NE of this depression will induce a southwesterly drift over SSTs of 26°C during the next few days. Relaxing shear and some days left over this anomalous warm pool of water should support a transition to a sub-/ or even tropical cyclone before the next trough approaches from the west. Again no direct impact for Europe is expected, so further notification from the appropriate agency (e.g. NHC) if development occurs.

DISCUSSION

... Mediterranean ...

Main thunderstorm activity will be heavily tied to numerous upper troughs and attendant pool of colder mid-levels. Not much instability release expected over the Ionian/Aegean Sea and best shear/instability fields are displaced. There exists some overlap, but right now, strongest wind fields are forecast at 5km and above, so it is questionable if storms take profit of this shear as most of them will be of shallow nature (beside N-Ionian Sea, where deep convection is possible, but shear is very weak). Hence the main concern again will be waterspout development, which seems possible over the N/E Ionian Sea, where LL CAPE release is the strongest, but environment is not that promising to issue a level area.

DLS is stronger over the Alboran Sea and a few storms could gain enough organisation to produce some strong to isolated severe wind gusts and marginal/isolated large hail, especially as SRH-3 increases during the day. West of the Strait of Gibraltar, the DLS relaxes again, but very strong LL CAPE release points to an enhanced waterspout risk, so the level area was expanded well to the west.