Storm Forecast
Valid: Tue 03 Aug 2010 06:00 to Wed 04 Aug 2010 06:00 UTC
Issued: Mon 02 Aug 2010 23:05
Forecaster: KOROSEC

A level 2 was issued for NE Poland and across western Baltic States including the eastern half of Baltic sea mainly for severe wind gusts, excessive convective rainfalls, tornadoes and large hail.

A level 1 was issued for surrounding areas of level 2 including much of northern and eastern Poland, parts of eastern Sweden, SW Finland, much of Baltic States and extreme NW Belarus mainly for strong wind gusts, large hail and excessive convective rainfall.

A level 1 was issued for southern Alpine region including parts of north-east Italy, southern Austria, Slovenia, Croatia and northern Bosnia and Herzegovina mainly for strong wind gusts, excessive convective rainfall and (marginally) large hail.

SYNOPSIS

A large long-wave trough over central Europe continues slowly moving east. Associated with this trough, a well defined DCVA region moves from central Poland towards central Scandinavia with a deepening surface low during the forecast period. A sharp frontal system will be the focus for robust severe weather period. Another short-wave trough on the southern edge of this large trough moves across the Alps. Its attendant cold front moves SE-wards onto the Balkans.

DISCUSSION

... north-east Poland, Baltic States and areas surrounding the Baltic Sea ...

Different models are in quite good agreement that development of severe weather is likely today along/ahead of the sharp surface cold front. Starting in the early morning hours, elevated convection could be already on going from the nighttime activity while a new development will spread NE-wards into central Poland along the front. As the trough comes closer and surface low deepens, increasing low/mid-level flow advects hot/moist airmass northwards towards the Baltic States. Moderate to strong MLCAPE builds up and overlaps with around 15-20 m/s of deep-layer and 0-3km shear. Strong QG forcing together with backed flow across the warm sector suggest that a rapid development of surface based severe storms becomes likely along or just ahead of the surface front. A couple of strong multicells and a few supercells will be possible ahead of the front, bringing the threat for large hail, strong winds and intense rainfalls. A tornado threat is well enhanced as well, given the backed flow, near 10 m/s of shear in the lowest 1km and at least 200 m^2/s^2 SR helicity in place.

Later on, rapid clustering seems likely once the LL jet increases with continuous deepening of the surface low. Towards the midday/early afternoon, a large MCS is likely to form over north-central Poland moving NNE-wards. Given the robust conditions in place, quite high possibilities for a severe squall line with bowing segments exist. Therefore, a severe wind gusts and excessive convective rainfall will be the primary threat, while tornadoes cannot be ruled out either, given the still strong LL shear/SREH in place well into the evening hours. This large cluster/MCS is likely to continue through the evening/nighttime hours across eastern Baltic sea and Baltic States. For this reason a level 1 was extended northwards into SW Finland as GFS is pushing the cluster into these areas by Wednesday morning.

... southern Alpine region into NW Balkans ...

With a well recovered LL moisture south of the Alps, environment becomes unstable ahead of an approaching short-wave trough. A surface front will probably be the focus for convective intiation around the midday hours, starting along the southern Alpine flank. As it usually happens with such trough over the Alps, NE-early flow around the eastern Alps then push the surface front SSW-wards where a deep convergence occurs along the Dynaric Alps. The convective development will likely continue with the moving front, where moderate MLCAPE around 1000 J/kg releases ahead of it. Overlaping with weak to moderate shear (15 m/s in 0-6 km layer) suggests that organized storms are possible, mainly acting as multicells or multicell clusters. Those (partly elevated storms) could bring threat for some strong winds and intense rainfalls while large hail threat is limited to areas more to the west across western Slovenia and NE Italy where instability will be maximized. However, models have an enhanced SREH ahead of the front towards Croatia and northern Adriatic where rotating updrafts will also be possible. Those could bring higher threat for large hail and stronger winds, as well as intense rainfalls given the high PWAT values.

A couple of organized storms will also be possible over central Italy, given the moderate instability and some shear in place. However, limited coverage precludes a threat level 1 there attm. But a couple of large hail/strong wind gusts events could be possible.