Storm Forecast
Valid: Sun 23 Sep 2007 06:00 to Mon 24 Sep 2007 06:00 UTC
Issued: Sat 22 Sep 2007 19:09
Forecaster: TUSCHY

SYNOPSIS

A glance on the weather maps reveals that autumn approaches.
An intense cyclonic vortex, centered at about 58°N / 27°W at 22nd September,12Z with a central pressure of 976hPa, is forecast to slowly shift northeastwards, while slowly weakening. Of main interest for this forecast will be a developing depression ( latest NCEP forecast brings it down to 994hPa ) on its southern flank, affecting Ireland,Scotland and United-Kingdom during the night hours.
Strong warm air advection downstream of this huge low pressure system will influence most parts of southern and central Europe and as a result, quiet and warm conditions prevail under a strengthening high pressure system.
Numerous disturbances over the Mediterranean and the Black Sea region will support unsettled but still warm conditions.

At lower levels, a well defined cold front will cross NW Europe during the evening and night hours from the W / NW and a warm front over Sweden and Finland will rapidly move northwards.

DISCUSSION

....Coastal areas of Algeria, Tunisia, Sardinia and parts of the Tyrrhenian Sea ....

Latest WV loops show a nice cyclonic vortex over the Alboran Channel, moving slowly eastwards. Satellite presentation and model forecast hint on a gradually weakening system, as lower troposphere gets warmer. That's why we think that thunderstorm coverage will be more shivered and numerous small storm clusters are forecast to develop south of the Balearic Islands and Sardinia. DLS will be enhanced ( 15-25m/s south of Sardinia ) and as 0-3km shear is also quite strong a few strong to severe wind gusts can be expected south of Sardinia and over NE Algeria / N-Tunisia. The large hail threat also increases onshore as steeper lape rates and increasing SRH-3 values overlap.

Shear relaxes towards the west and so does the severe weather threat.

As the upper-level feature approaches Sardinia from the west during the night hours, thunderstorm coverage will increase significantly and there are persistent signs of a developing thunderstorm cluster just south of Sardinia during the latter part of our forecast period. Shear is still enhanced with 15-20m/s DLS and the severe wind gust threat gets especially enhanced if storms line-up.
Shear and increasing storm relative helicity values hint on a mixture of multicells and a few supercells but the coverage and threat should not match a level-2.

Only as a side note: Yesterday's IR /WV loops showed a slow but constant increase in convection next to the center of the upper-level low-pressure system although there were no signs of any persistent deep convection next to the center. The latest QUICKSCAT ( ascending pass at 22nd Sept., 1623UTC ) had no hints on a closed LL circulation under this upper-level feature ( more like a west-east aligned shear zone just south of the Balearic Islands). In addition, a broad SW-erly wind field of probably non contaminated 20-30kt was detected just south of the Balearic Islands (north of Algeria ) but latest thinking is that this is more of baroclinic nature ( as strong jet at mid-levels is still placed over this area). Despite a slow eastward shift over 24°C SSTs, a warming atmosphere and the aforementioned weakening trend should preclude any subtropical development for the moment.

....E-Spain, S-France, parts of Switzerland, western Austria and SW Germany....

A typical diurnal driven convective regime should occur over those regions. Strong diabatic heating, at least moderately steepened mid-level lapse rates and dewpoints at or above 15°C will allow numerous thunderstorms to develop. Topography and local convergence zones should be the main foci for development. Weak instability signals and negligible enhanced shear will preclude any storm organisation although stronger storms could contain marginal hail and produce gusty winds.

...Ireland, United Kingdom and Scotland....

An intense 140kt jet will curve around the upper-level trough and should keep the main trough axis in a neutral position. In its left exit region, strong pressure fall will help to sustain / strengthen a short wave, which will cross those regions between 21 and 06Z from the west. This short wave will be placed on the warm side of this trough axis and hence in an environment with weak lapse rates / instability release.
Strongest LL shear / storm relative helcitiy values and winds at 850hPa can be found in the warm and quite stable airmass ahead of the cold front. However there are signs that a line of strongly forced storms could develop along the eastward racing cold front, affecting mainly the (south-)western parts of United Kingdom and Scotland between 00-06Z.
LCLs stay low, LL shear is at least marginal enhanced ( mainly along the coastal areas ) and as CAA progresses and LL lapse rates steepen, 0-3km CAPE release increases. Latest thinking is that SW United Kingdom and W / SW Scotland should see the best conditions for an isolated tornado report.
0-6km DLS of 35-50m/s and 1-8km shear of more than 60m/s are very impressive values and each thunderstorm, penetrating deeper in the atmosphere could produce a few severe wind gusts.
The main reasons for not issuing a broad level-1 right now are the quite weak shear at 0-3km ( about 10-20m/s ) and the uncertainty about the final thunderstorm coverage. As EL temperatures stay warm, convection is mostly expected to stay shallow, not possible to incorporate the stronger winds from above.
Probabilities will be augmented if there are signs for a more robust thunderstorm event.

Ireland was partially excluded of the thunderstorm area, because the short-wave just develops while passing over this area and won't be yet in contact with a tongue of higher Theta-E's, which should foster the possibly enhanced convective activity. Thereafter, weak ridging should suppress any deep convection until 06Z.