Storm Forecast
Valid: Sun 18 Jul 2010 06:00 to Mon 19 Jul 2010 06:00 UTC
Issued: Sat 17 Jul 2010 23:50
Forecaster: SCHLENCZEK

A level 1 was issued for parts of Sweden and Finland mainly for severe wind gusts and tornadoes.

A level 1 was issued for the Baltic States, N Poland and W Belarus mainly for severe wind gusts and excessive precipitation and to a lesser extent for large hail and tornadoes.

A level 1 was issued for SE Poland, W Ukraine, Slovak Rep., Hungary and Balkans mainly for large hail and excessive precipitation and to a lesser extent for severe wind gusts.

A level 1 was issued for Italy and Slovenia for excessive precipitation, large hail and severe wind gusts.

SYNOPSIS

Most convection on Sunday should be associated with a frontal boundary which stretches from central Sweden via W Poland and NE Italy towards S France. Hot and very humid air (mixing ratio between 12 and 16 g/kg) is present in the south / east of this cold front. Diurnal heating will lead to CAPE values between 1000 and 2000 J/kg along the cold front.

Over N Italy, a weak upper low will move to the southeast and will affect central and S Italy. Near the trough axis, some QG forcing is expected and deep layer shear will increase to 15 - 20 m/s. Instability should be somewhat less than in eastern Europe but nevertheless sufficient for some severe thunderstorms.

Another upper shortwave trough should move across the W Baltic Sea towards Estonia but forcing as well as shear will decrease with time. A more intense shortwave should cross the northern parts of Finland during the afternoon / evening hours after affecting northern Sweden. Instability should be rather marginal but with strong (25 - 30 m/s) deep layer shear, severe weather may occur.

DISCUSSION

...Italy...

Both GFS and WRF-NMM show moderate values of CAPE over the central and southern parts of Italy in the afternoon. The approaching upper trough should provide enough lift for initiation and in this environment with 15 - 20 m/s deep layer shear and locally augmented 100 - 150 m²/s² SRH3 some organised multicells and supercells may form. Large hail and severe wind gusts are expected but due to a very moist boundary layer and a very slow storm motion, excessive rainfall will become the primary threat. A significant event (very large hail, damaging gusts) is possible but because of a relatively low expected storm coverage no level 2 was introduced.

...Balkans, Hungary, Slovak Rep., E-central Poland, W Ukraine...

Weak winds at higher levels but CAPE in order of 1000 - 2000 J/kg may allow some organised multicells over this region that may merge into one or two MCSes. Storms should travel very slowly from NW to SE and may lead to excessive precipitation. An isolated large hail / severe wind gust event is possible as well. Thunderstorm activity should be greatest in the evening and is expected to decrease in the late night hours.

...N Poland, W Belarus, Baltic States...

In the northern part of this large level 1 area, kinematic conditons are more favourable for a linear MCS (squall line / bow echo) as 700 hPa winds reach 10 - 15 m/s, the orientation of the storm-relative winds is almost parallel to the front and DLS should be in order of 20 m/s. There is a zone of high to very high (400 - 600 m²/s²) SRH3 on the cold side of the front but even in the warm airmass, some enhanced values of SRH should be present. At the southern tip of a possible linear MCS (with embedded mesocyclones), cells should tend to stay discrete for some time and will have good conditions to become supercellular. Low level wind shear is rather marginal but rich BL moisture may lead to a low cloud base in some parts of the front and an isolated tornado cannot be ruled out. Large hail is expected shortly after initiation but later on the main threat should become excessive rainfall due to rather slow storm motion, and severe wind gusts that may occur more widespread. This region may need an upgrade if a strongly forced bowing line starts to develop.

...N Sweden, N Finland...

Ahead of a strong surface low with intense gradient winds, some low-end instability will be created by advection of warm and moist air to the north. Deep layer shear should be in order of 30 m/s and some QG forcing may allow a bowing line with severe wind gusts as main threat. Enhanced LLS and low LCL heights are expected and a tornado could develop. The severe wind gust / tornado threat is limited to a time-frame from the late morning until the late afternoon,.