Storm Forecast
Valid: Sat 27 Sep 2008 06:00 to Sun 28 Sep 2008 06:00 UTC
Issued: Fri 26 Sep 2008 22:48
Forecaster: PUCIK

SYNOPSIS

A large ridge of high pressure with its center over the British Isles, affecting Wester/Central Europe and S.Scandinavia will slowly retreat to the west. At the same time, deep trough will rapidly travel ESEwards across Scandinavia. It is accompanied by a 60 m/s jet stream at the upper levels and 40-50 m/s of mid-level SW-ly flow. Ahead of the trough, a deep surface cyclone with a central pressure around 970 hPa will move to the east, bringing widespread damaging wind gusts to the coastal areas of Norway. At the same time, widespread high pressure system affects large portions of Western/Central Europe, whereas, weak low has settled over the S Meditterannean/N.Africa. Around the ridge at 500 hPa level, several cold-core lows are situated, one of them is moving SEwards, above W Russia, another one being placed over the Aegean sea.

DISCUSSION

...Coastal area of Norway...

As mentioned above, surface cyclone will affect
most of the Scandinavia during the forecast period. It should be accompanied by an impressive low level wind field, having windspeeds more than 30 m/s at both 850 and 700 hPa. Cold front is predicted to pass the coastal area by 12Z. CAA behind the front will allow for some degree of destabilization, although MLCAPE values should only be around 200 J/kg. EL levels are predicted in the range -10°C to -20°C so probably few ltg strikes might occur with some showers, but we are not issuing a TSTM area. Very strong forcing will occur just along and behind the front, so strongly-forced convective line is a possibility. Wind gusts should occur mostly due to the PGF, but convection and its downdrafts should enhance their strenght due to the downward mixing of severe winds above. Judging from this, Level 1 is warranted across the region.

...S Spain...

Slight destabilization is expected to occur over the region, as cooler air invading from NE overlays the warmer airmass, modified by the sea temperatures. MLCAPEs should stay around 500 J/kg, but combined with mid-level forcing, deep convection should form and persist for a long time. DLS should be in range of 20 -25 m/s, with 0-3 km layer shear being around 15 m/s, so well organised storms are expected, including supercells. Furthermore, considerable SREH is forecast, with values over 300 J/kg, increasing chances for rotating updrafts and right-movers. In case that such well organised storms develop, severe hail can be expected as well as isolated severe wind gust. Limiting factor is a weak instability, but thanks to the favourable wind profiles, Level 1 seems to be warranted.


...Ionian Sea/Greece...

Small values of MLCAPE will be available across the region, around 500 J/kg mostly. Strong mid-level flow will be present, yielding DLS values around 25 m/s. However, shear in 0-3 km layer will be under 10 m/s over most of the places. This leads us to the point, that shear will be mainly confined to the upper levels. As storms will stay low-topped, strong flow only in their anvil-level might not provide good organisation for TSTMs.Therefore, no severe weather threat is anticipated, except for possibility of marginally severe hail within stronger multicells. We will be however, monitoring closely this region and if instability release will be greater than expected , a Level 1 might become necessary....