Storm Forecast
Valid: Thu 03 Jun 2010 06:00 to Fri 04 Jun 2010 06:00 UTC
Issued: Wed 02 Jun 2010 17:13
Forecaster: GATZEN

A level 1 was issued for eastern Poland and westen Belarus and surroundings mainly for tornadoes and excessive rain.

A level 1 was issued for Belarus, southern Baltic States, western Russia, and northern/central Ukraine mainly for large hail.

A level 1 was issued for northern Italy minly for large hail.

SYNOPSIS

A cut-off low leads to an unseasonably cool period from the Alpine region to the Balkans. At its northern flank, a tongue of warm subtropical air spreads into Belarus und Poland, while cool air is advected into most of the Mediterranean. A ridge over western Europe leads to strong diurnal heating across Iberia.

DISCUSSION

Western Ukraine, southern Poland, and surroundings

A tongue of humid air is situated to the north of the cut-off low, associated with low-level mixing ratios of about 10 to 12 g/kg. As the axis of a short-wave trough situated over the Black Sea turns north-ward, strong DCVA is forecast to lead to QG lift across the area. Models agree on a wave that travels eastward along the main frontal boundary that separates the moist air to the north from cooler and mostly stable air in the range of the cut-off low. This is associated with the formation of a low-level jet streak reaching about 15 to 20 m/s at the 850 hPa level that spreads from the Ukraine to southern Poland during the day. While warm air advection is expected to the west of the wave, a cold front will cross the Ukraine in the wake of it.

The development of instability seems to be quite likely in the warm sector air mass as the low-level moisture will remain and even increase compared to Wednesday. Additionally, the lapse rates above the boundary-layer may even increase due to the lift as indicated by latest GFS and ECMWF model output. Given some diurnal heating in the morning hours over Belarus and north-eastern Poland and about 24/17°C surface temperature/dewpoint values, 1000 J/kg MLCAPE may be possible.

Given increasing north-easterly low-level winds ahead of the low, this moist and unstable air will likely spread into central Poland during the day. Initiation is most likely in the range of the warm front, where low-level convergence is expected. Given a 0-3km vertical wind shear of about 10 to 15 m/s and 200 m²/s² 0-3km SRH, overlapping with the instability to the north along the frontal boundary, well-organized storms are possible. Especially isolated cells may evolve into supercells that move westwards and will be capable of producing tornadoes. The chance of tornadoes is forecast to increase in the evening hours over central and southern Poland as low-level vertical wind shear will likely increase to more than 10 m/s, while models predict quite strong low-level buoyancy and low LCL heights.

Limiting factors will be the widespread nature of convection and stratiform precipitation forecast by latest numerical output. Low-level buoyancy may be weaker as expected due to rain and weak diurnal heating. Additionally, storms may merge shortly after initiation along the warm front. However, current thinking is that at least some isolated storms will develop to the south of the frontal boundary posing a threat of tornadoes. Large hail and locally severe wind gusts are not ruled out with every supercell that forms. Later in the period, storms may merge into an MCS with an enhanced threat of excessive rain that continues during the evening and night hours over southern Poland and portions of the Czech Republic and Slovakia. Dependent on the development of vertical wind shear, an upgrade to level 2 may be warranted later on.

Further east, some storms will also develop along of the cold front over the Ukraine and Belarus. While the vertical wind shear decreases during the day and cold air advection becomes dominant, severe potential is limited. However, quite impressive thermodynamic profiles are forecast and at least a few large hail events and maybe a brief tornado are not ruled out.

Central Ukraine

An elevated mixed layer is present over the central Ukraine, and given rich boundary-layer moisture, diurnal heating will likely lead to MLCAPE in the order of 1000 J/kg. In the wake of a trough axis spreading northward, increasing mid-level southerly winds will lead to a DLS of about 15 m/s in the afternoon hours. Along of a cold front that travels northward, storms are likely, and with increasing shear, the potential of large hail also increases. Storms will decay in the evening hours.

Russia/Baltic States

The axis of the warm and moist air will be a focus of convective activity. Although a mid-level ridge and warm air will lead to weak QG forcing and inversions that may suppress convection over most places, low-level convergence is expected to increase as two cold fronts move in from the north and from the south. In the afternoon and evening hours, initiation seems to be likely along of one of maybe a couple of convergence lines or outflow boundaries. While the vertical wind shear is weak, substantial instability due to rich boundary-layer moisture and strong diurnal heating as well as elevated missed layers may support pulse storms, capable of producing large hail locally. The main threat will be local excessive rain due to the slow moving storms. Low-level stabilization and weakening of the storms is expected during the night hours.

Adriatic, central Balkans, northern Aegean, Bulgaria and Romania region

Underneath the trough, a cool air mass will likely become unstable due to diurnal heating especially over the Adriatic region and Bulgaria, where models predict instability. Given some low-level convergence and weak capping, showers and thunderstorms are likely during the day. Rather weak vertical wind shear is forecast and most thunderstorms will be not associated with severe weather. An isolated gust or hail event is not ruled out. Storms will likely decay in the evening hours.

Italy

A northerly jet streak is forecast to spread into Italy at the western flank of the cut-off low. Cold air advection is forecast especially at low levels that will lead to stabilization in the morning hours, but diurnal heating may lead to sufficient instability during the daytime, especially at the cyclonic flank of the mid-level jet streak where DCVA will lead to some forcing. Initiation is most likely along of sea breeze convergences, and a few multicells are expected due to deep layer vertical wind shear of about 20 m/s. These storms will have a potential of large hail with the main focus over the northern regions where models do predict CAPE underneath a trough axis. In the afternoon and evening hours, storms are expected to decay as DCVA is forecast to weaken.

Eastern Iberia

Models predict showers across eastern Iberia in the afternoon hours. These may develop in the wake of a mid-level trough axis in rather cool mid-level air, while the low-level will see strong diurnal heating. Weak forcing and instability will be limiting factors, but isolated storms may develop in the range of sea-breeze fronts and over the mountains. Storms that form may organize given about 15 m/s DLS, but low-level convergence may be too weak for permanent updrafts. An isolated hail report cannot be ruled out completely, as well as a local wind event due to the dry and well-mixed air, but current thinking is that the chance of severe weather is too weak for a categorical risk level.