Storm Forecast

Storm Forecast
Valid: Sat 17 May 2014 06:00 to Sun 18 May 2014 06:00 UTC
Issued: Fri 16 May 2014 23:04
Forecaster: PISTOTNIK

A level 1 and level 2 were issued for large parts of the Ukraine, SW Russia, Belarus and NE Poland for large hail, severe wind gusts, excessive precipitation and tornadoes.

A level 1 was issued for Latvia, Lithuania and larger parts of Poland mainly for excessive precipitation.

A level 1 was issued for the W Ukraine, Slovakia, Hungary and NW Romania mainly for excessive precipitation and to a lesser extent for spout-type tornadoes.

SYNOPSIS

A large cut-off low, whose center slowly shifts from Serbia across Hungary to the Czech Republic, continues to govern the weather over most of the continent. It steers rather cool air into Central and Southeastern Europe, while a pronounced wrap-around warm air advection regime overspreads the Ukraine, Belarus, the Baltic States and Poland.
Two small satellite upper-level lows over the Belearic Islands and Northern Morocco get picked up by the circulation and slowly drift southeastward. The latter can spark scattered thunderstorms over the Maghreb states, but mainly outside of our forecast domain.
High pressure is present over the Eastern Mediterranean Sea and in a belt from the Bay of Biscay all the way into Northwestern Russia. To the North, a fairly strong Southwesterly flow steers two upper-level troughs and their attendant frontal systems over the Northern British Isles and Northern Scandinavia, but deep convection is not expected.

DISCUSSION

... SW Russia, Ukraine, Belarus, Latvia, Lithuania, Poland ...

A pronounced warm front slowly lifts from the Ukraine northward into Russia and Belarus and northwestward into Poland. In the following warm sector, an elevated mixed layer, originating from the Asian prairies, overspreads very moist low-levels with dewpoints up to 18C. This combination yielded CAPE values up to 1600 J/kg on Friday (refer to the 12 UTC Saratov sounding), and similar or even higher instability can be expected on Saturday.
Deep-layer shear increases from 10-15 m/s over Russia to more than 25 m/s near the bordering triangle between the Ukraine, Belarus and Poland, where the trailing cold front gets overspread by a Southeasterly mid-level jet. Forcing is rather weak over most of the warm sector, though weak warm air advection continues and the background flow is distorted by various subtle vorticity maxima (partly originating from the widespread convection itself). The only better pronounced short-wave trough exits the level 2 area to the West in the morning. Remnants of a nocturnal MCS may affect parts of Eastern Poland in the morning hours with heavy precipitation, which gradually turns stratiform as it outruns the nose of the warm air advection regime and moves over much cooler near-surface air.
Otherwise, isolated to scattered leftover storms may be active in the morning across the warm sector, followed by more numerous initiations from the early afternoon onwards. A large number of multi- and supercells is expected which may bring the full spectrum of severe weather. In their initial stage, large or even extremely large hail is the dominant threat. Towards the evening, upscale growth into several MCSes is expected, and the main threat will shift to severe wind gusts and excessive precipitation. Especially along the northeastward moving cold front, linear organization is favoured and a larger swath of severe wind gusts is possible from the Central Ukraine into Southern Belarus in the evening and overnight. It is not ruled out that even the criteria of a derecho will be met.
Last but not least, enhanced low-level shear and storm-relative helicity (with maxima up to 300 m^2/s^2 in the lowest 3 km) is present in particular in the strongest shear regime near the cold front but also near the warm front, where weaker shear magnitude is compensated by excellent veering profiles. Hence also tornadoes can be expected with discrete updrafts or storms that interact with each other, and even a strong tornado is well possible. This risk seems to be maximized over the Western part of the level 2 area in Belarus and Eastern Poland.
Though remaining uncertainties about areas with the highest coverage of severe weather preclude a level 3, it needs to be emphasized that a large region can experience a remarkable severe weather outbreak!
Clusters of thunderstorms will move west- and northwestward into larger parts of Poland, the Russian exclave of Kaliningrad, Lithuania and Latvia overnight while they turn elevated. Excessive precipitation will be the dominant risk that was covered by a level 1.

... East-Central Europe, SE Europe, Italy ...

Scattered diurnally-driven thunderstorms are expected in an environment of limited CAPE and weak vertical wind shear. A level 1 was introduced for Slovakia, Hungary and adjacent areas of the Western Ukraine and Romania for excessive precipitation and to a lesser extent for spout-type tornadoes. Precipitable water in excess of 20 mm, slow storm motions, low cloud bases and a very convergent and cyclonic near-surface wind field, respectively, hint at these threats.
Otherwise, stronger pulse storms may bring localized heavy precipitation or (mostly small) hail, but coverage of severe weather events should remain too low for a level 1.

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