Storm Forecast
Valid: Tue 13 Jul 2010 06:00 to Wed 14 Jul 2010 06:00 UTC
Issued: Tue 13 Jul 2010 05:51
Forecaster: KOROSEC

A level 2 was issued for most of Sweden mainly for severe wind gusts, large hail, tornadoes and excessive convective rainfall.

A level 1 was issued for surrounding areas of level 2 towards the north and western Finland, and to the south across NE Germany and western Poland mainly for severe wind gusts, excessive convective precipitation and lesser extent for large hail.

A level 1 was issued for the Baltic states, much of Belarus, Ukraine and southwards across the eastern Balkans mainly for excessive convective precipitation and locally marginally large hail.

A level 1 was issued for south-central Alpine region including Slovenia and mountainous N Italy, parts of south-central Italy and SE France mainly for excessive convective rainfall, strong winds and marginally large hail.

SYNOPSIS

An intense trough which resulted in widespread severe storms on Monday continues tracking NE-wards across Scandinavia, while another deep trough/upper low enters western Europe and will result in another outbreak sequence on Wednesday afternoon. A weakening mid-level low over Ukraine moves slightly towards SE while slowly building omega-like ridge pushes into central Europe again.

At surface, a strong frontal system associated with the afforementioned robust trough serves as a focus for an outbreak of severe storms across Sweden. The cold front extend southwards into western Poland by the afternoon, while strong WAA ahead of the trough pushes warm front into the northern Scandinavia.

DISCUSSION

... Sweden and areas NE-wards towards north-western Finland ...

Starting early in the day, storms activity will soon continue from the past nocturnal cluster entering Sweden from the SW. A rather strong forcing along the deep frontal convergence will be the focus for the main activity. Rich BL moisture across the warm sector (0-1km mixing ratios up to 13 g/kg) will result in around 1000-1500 J/kg of MLCAPE, overlaping with 20-25 m/s of 0-3km and deep layer shear. Therefore, a well organized deep convection is quite probable. With backing LL flow, quite strong at places especially over central Sweden, around 200-300 m^2/s^2 of SREH3 will result as well as around 15 m/s of low level shear. This all and including low LCLs should there support a couple of supercells with tornado threat, especially if stronger cell could form ahead of the main cold frontal boundary activity. Strong forcing and shear vector mostly parallel to the N-S front orientation seem likely to support a large MCS with a strong squall-line with bowing segments along the front, quite rapidly racing ENE-wards. As such, a threat for severe wind gusts, very intense convective rainfalls and locally large hail will exist.

... NE Germany and western Poland ...

Along the cold front extending from the surface low over Sweden, storm initiation is expected in the early afternoon hours with peak surface heating. Though only marginal instability with MLCAPE between 500 and 800 J/kg builds up within 10-15m/s of deep layer shear. This far south, intiation will be more isolated than further north where frontal boundary is better defined. However, a still relatively strong frontal convergence and some mid-level forcing with backing surface flow ahead of the boundary, will root deep convection. A locally well organized storms will be possible, mostly acting as multicells. Some rotating updrafts could form as well. Strong winds and intense rainfalls will be the primary threat given the more linear mode and training effect of the storms along the front. While a large hail will be limited to the more isolated and intense cells.

... SE France, north and central Italy, southern Austria and Slovenia ...

Placed in the hot and very moist boundary layer, high instability with MLCAPE exceeding 1500-2000 J/kg will result. These areas will again see daytime driven activity in slightly better sheared environment around 10 m/s than previous days. Even the threat for severe storms is rather limited, a low-end level 1 was issued to cover the most intense storms. Given the pulsating nature of the storms and high PWAT values over these areas, excessive rainfall and locally small hail will be the main threat. High delta theta-e culd also support locally strong wind gusts. A small level 1 area was also issued for parts of south-central Italy where similar conditions could support a couple of flash flood events as well. A bit better shear overlaps with instability over western part of level 1 area across SE France and western Alps. There, enhanced risk for intense convective rainfalls, strong winds and large hail exists. With an approaching trough towards the Wednesday morning, strong warm advection takes place from Spain NNE-wards into southern France. An elevated convection seems likely to form along the northwards advecting warm front over SW France during the second half of the night.

... A large area from Baltic states towards eastern Balkans ...

While a large area with weak geopetential heights finally shifts away from Ukraine towards SE, another day of mostly pulsating storms is expected. In the late morning/early afternoon hours widespread convective activity will first take place along the mountainous terrain as well as along the numerous old surface outflow boundaries. With very high MLCAPE and PWAT, but limited deep-layer shear staying below 10m/s, only pulse storms with intense rainfalls and small hail are expected. Per GFS, slightly better shear around 10-15m/s seems over the Belarus and surroundings which could enhance a threat for large hail there as well.