Storm Forecast
Valid: Mon 19 May 2008 06:00 to Tue 20 May 2008 06:00 UTC
Issued: Sun 18 May 2008 22:59
Forecaster: GROENEMEIJER
Grammar corrected 9:10 UTC
SYNOPSIS
On Monday at 06:00 UTC, a cold mid/upper low pressure system is located over the Skagerrak. A shortwave trough extending southwestward from its centre over the North Sea moves eastward and reaches Estonia Tuesday morning. A strong SWly jet found over the Baltic States and NW Russia. Over the Iberian Peninsula, a separate low with a westerly jet on its southern flank moves eastward during the forecast period.
DISCUSSION
...Belarus, the Russian oblasts Smolensk, Bryansk, Kaluga, Moscow and Vladimir and their surroundings...
A near-stationary front is expected near a line from Moscow to Minsk to the central Slovak Republic early on Monday. The warm air-mass is quite moist with around 10 g/kg mean mixing ratios expected in the boundary layer. In response to diurnal heating, that will in some places be hindered by remnant clouds of earlier convective storms, 1000-1500 J/kg of MLCAPE should develop near the front. GFS model runs are consistent in letting convective storms initiate in the early afternoon over central and eastern Belarus as a vorticity maximum, that is initially located over SE Poland, approaches.
Because the warm front is not too far displaced from the axis of the jet over the Baltic States, about 20 m/s of 0-6 km layer bulk shear should be present near the front. Low-level shear should increase on approach of the upper-level system and will be supportive of strong low-level rotation in storms. This is also indicated by high values of storm-relative helicity calculated by the GFS model. Hence, this appears to be a potent setup for severe weather. Initially, some storms will likely develop into supercells. These could well produce a number of tornadoes, some of which may be strong. During the late afternoon and evening the storms will likely cluster into an MCS capable of producing damaging winds as it moves eastward. During the late evening, the storms will generally weaken as the vortcity maximum moves away from the warm air and CAPE decreases.
... SE Poland, E Slovakia, E Hungary, W Ukraine...
This entire area will also be within the warm, and locally moist, air-mass. The triggers for convective development will be diurnal heating and the approach of a second vorticity maximum, that should be located over the Czech Republic early on Monday. Deep-layer wind shear of 15-20 m/s and CAPE values up to 1000 J/kg suggest a distinct severe threat will be present as storms may become well-organized, including a few supercells.
Because low-level shear is expected to be limited, the threat of tornadoes appears to be small in general, but near the frontal boundary, i.e. over E Slovakia, SE Poland and extreme W Ukraine, they cannot be excluded. Elsewhere, large hail will be the main threat while some strong to marginally severe winds will be possible too.
...Central Balkans...
Across this area, diurnal heating will be the main trigger for convective initiation during the afternoon. Moderate deep-layer shear should suffice to support some multicell clusters capable of producing large hail. Some strong, but mostly sub-severe winds are possible as well.
...Central Italy, Sicily...
Scattered storms are expected to develop across the italian mainland during the afternoon as a trough approaches over the Tyrrhenean Sea. Strong, 20-25 m/s deep-layer shear suggests some rotating updrafts may develop and large hail will likely be produced by some of the storms. Additionally, damaging gusts are possible.
... Tell Atlas, Northern Tunesia...
Below a westerly mid/upper-level jet that moves eastward, strong insolation will likely cause convective initiation over the mountain ranges. Given the strong deep-layer shear, these storms may well turn into supercells capable of producing large hail and damaging winds.
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