Storm Forecast
Valid: Sun 13 Jun 2010 06:00 to Mon 14 Jun 2010 06:00 UTC
Issued: Sun 13 Jun 2010 05:51
Forecaster: SCHLENCZEK
A level 2 was issued for NW Russia mainly for large hail, widespread severe wind gusts and strong tornadoes.
A level 1 was issued for NE Morocco / NW Algeria mainly for large hail.
A level 1 was issued for N-central Italy, SE Austria and Slovenia mainly for large hail and severe wind gusts and to a lesser extent for tornadoes and excessive rainfall.
A level 1 was issued for Croatia, Hungary, N Romania, Slovak Rep. and SW Ukraine for large hail, severe wind gusts and excessive rainfall.
SYNOPSIS
A low pressure system over S Finland associated with a thermal wave over W Russia will move northeastward during the period. Over western / central Europe, a stationary upper trough remains, leading to a quasi-stationary frontal boundary which stretches from the N Adriatic via NE Ukraine towards NW Russia. Hot and moist subtropical air is located ahead of the front with CAPE in order of 1500 - 2000 J/kg forecast by GFS. A region of strong deep layer shear is located near the frontal boundary with still locally 15 - 20 m/s on the warm side. Further west / southwest, a moderately warm airmass overspreads parts of SW / W Europe with only marginal instability created by diurnal heating. An upper shortwave trough will cross the British Isles from NW to SE during the afternoon / evening hours.
DISCUSSION
...W Mediterranean, Iberia, S France, NE Morocco, NW Algeria...
Diurnal heating will create some hundred J/kg CAPE in the vicinity of the upper cold core over France and Iberia with -15°C at 500 hPa. Not much CIN is expected and showers / thunderstorms will start to develop during the morning / afternoon hours. As deep layer shear should be less than 10 - 15 m/s, most storms are expected to be short-lived and only briefly organised. An isolated large hail event is not ruled out but overall threat should not exceed the level 1 threshold.
Further southeast near the southern tip of the trough, a 30 m/s upper jet streak overspreads NW Algeria. Some instability will be created ahead of the trough in an environment with 20 - 25 m/s deep layer shear and strong QG forcing. In the late afternoon / evening, storms will start to develop and some of them may become supercellular, posing a threat of large hail and isolated severe wind gusts.
...British Isles...
A few hundred J/kg CAPE are forecast in the wake of the approaching upper trough. Low level winds are very weak and deep layer shear should be less than 10 m/s in most places but a low cloud base and locally augmented 0-3 km CAPE may allow a brief tornado.
...N Italy, SE Austria, Slovenia, N Balkans, Hungary, N Romania, SW Ukraine...
Ahead of the frontal boundary, instability is forecast to be roughly in a range of 1000 - 2000 J/kg. Strong winds at upper levels lead to a region with 15 - 20 m/s deep layer shear over N Italy and Slovenia and most 00 UTC soundings from Italy indicate some veering with height. Due to the EML which overspreads most parts of SE / E Europe, the airmass is strongly capped and initiation should be expected in the late afternoon / evening hours when an upper vort-max approaches N Italy. Some storms will tend to become isolated supercells with a threat of large hail and severe wind gusts. Even though LLS is not very strong, an isolated tornado is not ruled out. Shear and forcing decrease towards the east but over Hungary, N Romania and SW Ukraine isolated severe storms with large hail and severe wind gusts are still possible. In the late evening / night hours, storms will tend to merge into one or two large MCS with severe wind gusts as main threat. Locally, torrential rainfall may lead to flash floods.
A large high-end level 1 was introduced as convective initiation is questionable in parts of Romania and Ukraine but any storm that manages to develop will pose a threat of (very) large hail and severe wind gusts.
...extreme NW Russia...
The severe thunderstorm outbreak from Saturday is expected to continue as still very unstable air overlaps with 20 - 25 m/s deep layer shear and locally more than 600 m²/s² SRH3 ahead of the front. Almost linear forcing will probably lead to a severe squall line with widespread severe wind gusts due to strong mid level winds and a strong vertical Theta-E gradient. A few supercells may develop either along the line or isolated, posing a threat of (very) large hail and strong (F2 - F3) tornadoes.
