Storm Forecast
Valid: Thu 05 Jun 2014 06:00 to Fri 06 Jun 2014 06:00 UTC
Issued: Wed 04 Jun 2014 21:52
Forecaster: GATZEN

A level 1 was issued for central Finland mainly for excessive precipitation.

A level 1 was issued for northern central Turkey mainly for large hail.

A level 1 was issued for north-eastern Belarus mainly for large hail and excessive precipitation.

SYNOPSIS

An intense short-wave trough moves across the UK and northern France and cuts off over Denmark today. At its southern flank, a 25 m/s westerly mid-level jet travels into central Germany, while cold and dry air is flooding central Europe. Downstream, the east European trough lifts into Scandinavia, whereas the southern vorticity center cuts off over Turkey. Russia remains under the influence of a ridge.

DISCUSSION

Central Finland

The mid-level ridge axis moves across Finland on Thursday, providing favorable upper ventilation. Together with low-level warm air advection, upward vertical motion becomes likely. Main focus will be a warm front over central Finland that is nearly stationary. Rich low-level moisture has developed in a weakly capped air mass.

Current thinking is that cluster of storms will develop on Thursday morning that may go on until the late afternoon, before spreading northward together with the upper level ridge axis and low-level warm air advection regime. Given weak vertical wind shear and limited lapse rates, storm organization is expected to be weak. Given the high storm coverage, a few pulse storms are not ruled out, capable of producing large hail. Strong to severe wind gusts may also occur but the threat is also very limited due to weak vertical wind shear.

The main threat will be excessive precipitation due to the slow storm motion. This threat will slightly decrease in the evening when storms start to spread northward, before weakening storms may affect the Barents Sea in the morning hours.

Northern Turkey

A cut-off low will be centred over Turkey. Rather cool mid-level air and strong diurnal heating over the Turkish plateau will lead to persisting steep lapse rates. An elevated mixed layer has already spread across parts of the Black Sea. On Thursday, building thermal low over Turkey will allow well-developed sea-breeze convergences to move onshore and upslope over northern Turkey, advecting rich low-level moisture southward. Initiation is expected in the noon hours and numerous thunderstorms are forecast.

Weak vertical wind shear is expected over most of the forecast region. However, southerly mid-level winds over north and north-eastern Turkey will create at least moderate vertical wind shear of 15 m/s, and supercells are not ruled out, capable of producing large hail and local severe wind gusts. Directly along sea-breeze convergence zones, a tornado is not completely ruled out.

Further west, severe potential is only marginal. However, given numerous storms, flash flooding is not ruled out.

Belarus

Along the frontal boundary over Belarus, conditions remain favorable for storms with rich boundary-layer moisture and the advection of steep lapse rates from the east. Diurnal heating will likely result in CAPE above 500 J/kg on Thursday.

Ahead of the low geopotential over Europe, a 15 m/s south-easterly 700 hPa jet streak spreads over the unstable air mass, allowing for better storm organization. Multicells and probably a few mesocyclones may form, capable of producing large hail and excessive precipitation. Storms will move north-westward and decay during the night hours.

Romania into central Ukraine

Thunderstorm activity will go on along the frontal boundary over eastern Europe due to frontogenetical forcing and diurnal heating. However, mid-level ridging and warming may result in increasing inhibition and weaker low-level convergence. Additionally, weak vertical wind shear will limit storm organization. Excessive precipitation and isolated large hail are not ruled out until the storms decay rapidly after sunset.

Northern Italy

A westerly jet extends over northern Italy. At low-levels, a rather moist air mass remains and will slightly destabilize in response to diurnal heating. Although initiation seems to be unlikely, convergence in the Po valley may allow for some isolated storms. Given 15 m/s vertical wind shear in the lowest 6 km, a supercells are not ruled out, with large hail the main threat. Weak indications for initiation are given, and a thunder or threat level was not issued.

North-eastern Spain

Ahead of another Atlantic trough, warm air advection sets in over western Europe. A plume of steep lapse rates will spread across Spain into France late in the period. Whereas low-level moisture will be very limited over France, GFS indicates Mediterranean moisture advected into eastern Spain. CAPE is forecast that will be capped and initiation is unlikely. The best potential of storm initiation is forecast over north-eastern Spain in the evening hours. If a persistent storm manages to develop, mesocyclogenesis is likely due to favorable curved hodographs in the warm air advection regime and low-level easterly onshore winds. Large to very large hail is not ruled out if this scenario comes true, but current indications are too marginal to issue a thunder or threat level.