Storm Forecast
Valid: Sat 01 Jun 2013 06:00 to Sun 02 Jun 2013 06:00 UTC
Issued: Fri 31 May 2013 23:51
Forecaster: PISTOTNIK

A level 1 was issued for parts of the Ukraine, for Belarus and Lithuania mainly for large hail and excessive precipitation.

A level 1 was issued for Poland for large hail, excessive precipitation and to a lesser extent for tornadoes and severe wind gusts.

A level 1 was issued for parts of Germany for excessive precipitation.

A level 1 was issued for a section of the Croatian coast for waterspouts.

SYNOPSIS

A large cut-off low rests in place over East-Central Europe. Its circulation covers most of the continent, with extensive cold air advection into Western and Southern Europe and pronounced warm air advection into Western Russia and Scandinavia. Numerous vorticity maxima round this low and mainly affect the Ukraine, Southern Poland, the Czech Republic, Austria and Italy.
The upper-level low will also spread over the Balkan Peninsula, where cooler air and weak vertical wind shear finally terminate the recent episode of severe convection. As a consequence, the main jet streak gets pushed further Southeast towards the Ionian Sea, Greece and Turkey, where limited CAPE, a strong cap and a lack of forcing will however (seasonably) preclude deep convection.
Otherwise, Saturday's situation along the periphery of the cut-off low will be strikingly similar to Friday's, and so will be the foci for thunderstorms: a stalled cold front with a tendency for wave developments from the Ukraine to Poland, and ahead of it pronounced wrap-around warm air advection from the Northeast into Germany.
To the West, an aged and non-thundery Atlantic frontal system will graze the British Isles, whereas a subtropical high starts to spread into Southwestern Europe.

DISCUSSION

... Ukraine, Belarus, Poland ...

Rich low-level moisture and fairly steep lapse rates nicely coincide along the cold front, which barely makes any more progress to the Northeast. A few hours of insolation should again result in moderate CAPE (500-1500 J/kg), and another round of scattered to widespread thunderstorms can be expected by afternoon. While an isolated supercell is not ruled out, deep-layer shear of 15-20 m/s and straight hodographs should keep multicells as the dominant convective mode. Development of several large clusters is well possible again. A level 1 is issued for the possibility of large hail and excessive rainfall.
Latest model runs have significantly delayed and sharpened a new frontal wave develoment over Poland. They now largely agree on the formation of another <1000 hPa depression, though discrepancies remain with respect to its timing and exact track. Ahead of it, particularly rich low-level moisture from the Baltics with surface dew points around 16°C gets ingested into the Northeasterly flow, which should yield CAPE values around 1500 J/kg (ref. to Fri 12 UTC Kaliningrad sounding) and precipitable water in excess of 30 mm. Besides, a stronger backing of the near-surface wind field could also bring noteworthy vertical wind shear from the mid-troposphere down to lower levels and improve near-surface veering.
Independent from details, scattered to widespread and well-organized thunderstorms are expected to form over Northern and Central Poland in the noon to afternoon hours and to move west- to southwestward. Large hail and excessive precipitation are the dominant risk, but severe wind gusts and - in case of an indeed stronger cyclogenesis - tornadoes are possible as well. A cyclone track more displaced to the South (as shown by the Fri 12z ECMWF) would further enhance the severe weather risk due to a better phasing with stronger deep-layer shear and QG lift, both of which increase markedly towards the South.
For the moment it was decided to stick with a high-end level 1 for this multiple severe weather threat, but it is emphasized that the difficulty to narrow down the wave's exact track and timing is the only reason which precludes a level 2.

... Germany ...

Ahead of the Polish wave, a new surge of warm air advection enters Germany from the Northeast. A similar scene to Friday is expected, albeit with lower CAPE values and less conspicuous hodographs.
Scattered afternoon storms are expected to form again over Northeastern Germany in the afternoon or to enter from Poland towards evening. They will move to the Southwest, get incorporated into a large-scale rain field and will decouple from the cooler surface layers as soon as they approach the nose of the warm air advection regime over Central and Southern Germany. Due to very low cloud bases, precipitable water in excess of 30 mm and the very rainy history, any storm may cause a flash flood, which is reflected by a level 1 despite a probably lower coverage than on Friday. A tornado is not ruled out in case a storm's updraft can keep the ground contact longer than anticipated into the region of better wind profiles.
However, the most severe weather threat arises from a shield of abundant large-scale rain further South, which wraps around the steering center of the cut-off low and largely overlaps with areas which already face a lot of flooding problems in and around Southern Germany. This situation will further worsen on Saturday, but is not factored in ESTOFEX due to the absence of convective involvements. Please refer to your national (hydro)meteorological service if you seek more information on this issue!

... Italy, the Balkans ...

Scattered thunderstorms are forecast beneath cool upper levels and weak vertical wind shear. An isolated case of large hail is possible, but otherwise severe weather is unlikely.
The only exception is the Croatian coast between Rijeka and Zadar, where limited area models agree on the passage of a mesovortex with associated convective precipitation in the morning hours. Strong low-level buoyancy above the ~18°C Adriatic Sea may be converted into one or two waterspouts. This localized threat diminishes by noon, when the parent vortex fills up.

... Scandinavia, Russia ...

To start with a side note, a maximum temperature of 30.5°C in Kevo (Northern Finland) on Friday meant the highest temperature ever recorded in Finland in May. Besides, even coastal stations of Northern Norway experienced rare daytime temperatures in excess of 25°C with offshore winds. Chances are that on Saturday again the highest temperatures of entire Europe (!) will be measured to the North of the Arctic Circle. A history of sunshine (literally) around the clock has created a very deep and dry boundary layer. Many stations reported dew points around or below freezing.
So far, air mass thunderstorms have been confined to (moister) Southern Scandinavia, but forecast models point out that on Saturday finally the convective temperature may be reached further North as well. Even in the absence of noteworthy deep-layer shear, evaporative cooling might create marginally severe wind gusts with any precipitating convection that occurs. However, confidence in sustained updrafts is too low to issue a categorical risk.
A similar argumentation, albeit in a slightly eased form, also goes for Western Russia.