Storm Forecast
Valid: Fri 08 Sep 2006 06:00 to Sat 09 Sep 2006 06:00 UTC
Issued: Thu 07 Sep 2006 21:53
Forecaster: TUSCHY

SYNOPSIS

The main weather features for the next 24 hours will be a strengthening high pressure area over the United Kingdom and a strong depression over Sweden and Finland ( creating a pressure difference of about 30hPa )..... This constellation will cause the advection of a cool and stable airmass over parts of NW-Europe....An airmass boundary, running from Spain towards W-Russia, will be the focus for TSTM development.

DISCUSSION

...Parts of Belarus and extreme W - Russia...

The synoptic configuration seems favorable for a few severe TSTMs over the broad level-1 area with numerous areas of enhanced TSTM development !

Replication of 18Z WV image and 12Z GFS model run indicates that GFS has a good handling with the trough over Sweden and Norway.... Upper-level jet configuration will result in a continual amplification and hence no significant eastward shift is expected....
The first concern for an isolated TSTM will be in the warm sector ( covering western Ukraine, parts of Belarus and western Russia during the afternoon hours )....The near neutral aligned trough axis should avoid any significant overlap of cool mid-levels and warm / humid low-levels and therefore no major instability release can be expected....However, modified soundings from this region indicate the possibility of at least low-end instability release, if diabatic heating and increase in LL moisture can be realized....
Current thinking is that strong WAA at LL and mid-levels should suppress any prefrontal convective activity over W-Ukraine.... WAA sector further towards the north ( E-Belarus and extreme W-Russia) will benefit of cooler mid-levels and accompanied insolation should help an isolated storm to penetrate the cap.... The warm sector is highly sheared ( up to 200 m**2/s**2 SRH at 0-1km and 15-20m/s DLS ) and steep lapse rates are also present...each developing storm would mainly pose a risk for large hail / severe wind gusts but also an isolated tornado development is not beyond question.

The next area of interest will be the cold front and its postfrontal region.... Models agree in convective precipitation along the cold front....
GFS showes a very tight geopotential height gradient, crossing Estonia and extreme W-Russia from the west....An interesting point is, that the American model has the most active convective area west of the strongest surface theta-E gradient under a NE-ward moving dry slot....Forecast vertical scans show a still pretty humid boundary layer and accompanying insolation should be enough for TSTM development in a nearly uncapped environment.
DLS would be extreme ( ~40m/s ) and any developing storm would pose a severe wind gust and isolated large hail risk.... Conditions have to be monitored if small lines of storms can develop with a more concentrated and augmented severe wind gust risk.

Further towards the east, along the eastward moving cold front, strongly veering profilers and low LCLs show the possibility of an enhanced tornado threat ( affecting the NE-part of the broad level-1 area)... Right now, only isolated storms are expected due to very slim instability values.

... The Baltic Sea and most parts of Estonia and Latvia...

Main concern for enhanced TSTM development exists during the late afternoon and evening hours over the western part of the level-1 area.... A well established upper-level disturbance will leave E-Sweden during the afternoon hours, reaching Estonia and Latvia during the evening hours.... Models like GFS indicate 100-300 J/kg SBCAPE ( mainly offshore ) in an environment with very high helicity values and very low LCLs ( mainly over Estonia )... There would be an enhanced tornado/severe wind gust threat if enough instability can be realized.... Evolution of this scenario has to be closely monitored.

... N-Spain, S-France,N-Italy, Slovenia and Croatia...

Airmass boundary will reach the coastal areas of the N-Mediterranean, where scattered TSTM evolution can be expected....Shear will be weak, but up to 1000 J/kg instability should be enough for a few storms, reaching the severe criteria ( isolated large hail ).