Storm Forecast
Valid: Tue 18 Dec 2007 06:00 to Wed 19 Dec 2007 06:00 UTC
Issued: Mon 17 Dec 2007 20:37
Forecaster: TUSCHY

SYNOPSIS

Confidence is growing that the very disturbed upper-air streamline pattern normalizes during the next few days as ensemble forecast of the NAO index indicate a quite consistent positive trend.
For this forecast period, the most prominent feature still is a stout high pressure area, centered over the North Sea. A ring of depressions surrounds this feature, also supporting the convective activity over parts of the Mediterranean and the extreme eastern Atlantic.
One intense depression ( reading yesterday at 12Z was 964hPa at 50°N, 65°W ) southwest of Greenland won't have any effects of our weather, but a developing intense depression (60°N , 40°W ) is forecast to surround the high pressure area, affecting the far northern parts of Europe during the end of the forecast period.
An 120kt upper-level jet and high baroclinity along the edge of the polar vortex assist those intense developments.

A well developed cut-off low stays nearly stationary for the next 24 hours just west of Portugal and Spain, while another cold-core depression approaches the cut-off from the NE and both start to interact . I do not want to use the Fujiwhara effect as an explanation, because both features start to merge quite fast, but it is a quite interesting evolution anyway.

Finally a depression just east of Sardinia has to be monitored, because of a low-end chance of some warm-core development.
Yesterday's QuickSCAT data and synop reports of Sardinia showed 20-30kt winds next to the center ( and the data of QuickSCAT was not contaminated by rain / intence convection ). Latest IR loops showed the center just off the east coast fo Sardinia ( ~18Z ) and sustained 11m/s winds with gusts near 20m/s. GFS still indicates the chance of a developing shallow warm core structure and if convection indeed gets better organized and stronger this could indeed happen. Right now, time-frame for intensification will be very limited as thermodynamic support gradually vanishes during the next 24-36 hours.
This depression has to be monitored for signs of stronger and more organized convective activity.

A strong depression over NW Russia and attendant CAA are another area with some concern, as wind field at all levels is very impressive. Shallow convection is possible with an enhanced wind gust risk, but right now, lightning activity should be too limited for a thunderstorm area.


DISCUSSION

... Sardinia and the Tyrrhenian Sea...

There are no signs for any enhanced severe weather threat...at least what latest model outputs showed. Yesterday's enhanced LL Cape relaxes and the shear in all levels stays weak. The depression should slowly move east/southeastwards and models finally indicate a quite erroneous and diffuse track during the evening and night hours as the feature starts to weaken. Numerous thunderstorms should develop as thermodynamics stay fine ( and so do the lapse rates at mid-levels ) and as the system moves away from Sardinia. Thunderstorm activity and organisation around the depression's center have to be monitored closely.
Stronger storms could contain marginal hail.

... Northern Norway, Sweden and Finland...

A cold front of an intense depression starts to affect the area of interest during the late night hours, but latest model run of GFS backed off a little bit regarding the exact timing and hence we decided not to introduce any highlighted area for the moment. LL shear is quite strong, but instability should start to build mainly in the postfrontal airmass and hence not in our current time-frame.

... An area west of Portugal...

Latest modified IR images already indicated a broad area of cooling cloud tops just west of Portugal as warm conveyor belt gets better organized. For the next 24 hours, best shear and instability are displaced and the only region, where both agee is out of our area of responsibility.
Despite those negative facts, we went with a low-end level-1 area, including the coastal areas of SW Portugal. As mid-levels start to cool down significantly during the night hours, traces of instability are also forecast along the coast, where LL shear is signficantly enhanced. Right now the main risk will be an isolated waterspout report, as 0-3km instability further offshore is pronounced . Otherwise strong to severe wind gusts are possible along the coast as winds at 850hPa increase to 25-30m/s.
Latest thoughts are that a few thunderstorms are embedded in a more stratiform rain shield.
The area has to be monitored for more robust thunderstorm development than currently anticipated, but weak lapse rates as a result of the strengthening WAA should keep thunderstorm activity in the level-1 area quite limited.