Storm Forecast
Valid: Sat 29 Jun 2013 06:00 to Sun 30 Jun 2013 06:00 UTC
Issued: Sat 29 Jun 2013 00:33
Forecaster: PISTOTNIK

A level 1 was issued for S Italy, Albania and W Greece for large hail and severe wind gusts.

A level 2 was issued for Macedonia, N Greece and parts of Bulgaria for large hail, severe wind gusts and excessive precipitation.

A level 1 was issued for the rest of Bulgaria and NW Turkey for large hail, severe wind gusts and excessive precipitation.

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

SYNOPSIS

An aged long-wave trough extends from Norway to Italy and slowly moves eastward. Two jet streaks dig from the Northern Atlantic into Denmark and from France into the Central Mediterranean, respectively. Geopotential falls in their left exit regions keep the long-wave trough still in shape during the forecast period, though it otherwise starts to disintegrate due to increasing QG subsidence ahead of pronounced ridging from Iberia to the British Isles.
At lower levels, cyclogeneses occur under the mentioned left exit regions over Southern Sweden and (less defined) over Macedonia. A Northwesterly flow advects cool and mostly stably stratified maritime air into Central Europe and the Central Mediterranean, before conditions start to warm and to clear over Southwestern Europe.
Further downstream, a rather diffuse cold front continues its slow eastward motion over Eastern Europe. Very warm and relatively moist air still advects North ahead of it over the Eastern Ukraine, Russia and Finnland.

DISCUSSION

... E Romania, Moldova, E Ukraine, E Belarus, W Russia ...

Fairly steep lapse rates and rich low-level moisture in the prefrontal warm air mass should again result in CAPE values around 1000 J/kg in many places and locally up to 2000 J/kg. Another round of scattered to widespread thunderstorms is expected, which will peak in the afternoon hours but can regionally last well into the night hours again while growing upscale into large clusters. Compared to Friday, deep-layer shear decreases to only 10-15 m/s, which limits the severe weather risk. However, large hail with strong pulse storms as well as excessive precipitation are locally possible. Severe wind gusts pose a minor threat in case of a stronger cold pool development with clustering storms.
The weakening dynamics, the lack of obvious hot spots for severe weather and possible aftermaths of overnight's convection (at Fri 23 UTC, a large MCS was moving from Ukraine into Belarus) preclude another level 2, though a concentrated region of a higher severe weather density cannot be ruled out in case of favorable mesoscale developments.

... Southern Italy, Albania, Greece, Macedonia, Bulgaria ...

Powerful vorticity advection in the left exit region of the approaching French jet streak spreads eastward and will be sufficient to kindle thunderstorm development also in the so far calm and capped region further South. First thunderstorms can move eastward over Southern Italy and surrounding seas quite early in the forecast period. Additional storms will develop over topographic features in the noon to afternoon hours.
Main concern is the amount of low-level moisture in inland areas: Under the influence of deep daytime mixing, dew points dropped to ~10°C or even below on Friday. This makes the forecast CAPE in the 500 to 1500 J/kg range a bit doubtful. Nonetheless, deep-layer shear around 20 m/s and inverted-V profiles can allow severe wind gusts and large hail with any better organized storm, but coverage should be limited to scattered activity.
In contrast, Friday's observations showed uniform dew points in the upper 10s along the Ionian and Aegean coasts. While this rich low-level moisture was mainly kept offshore with a background of (Northerly) Meltemi winds, the forecasted weak cyclogenesis over Macedonia on Saturday may be sufficient to reverse the wind regime and allow the sea breezes to protrude further inland. Hence, confidence in a more robust CAPE release and a higher storm coverage in coastal areas increases. The model pool agrees on remarkable precipitation signals in the late afternoon and evening hours over Macedonia, Northern Greece and Southern Bulgaria, where the development of one or two large MCSes seems possible. A small level 2 area was drawn for a combined threat of large hail, severe wind gusts and excessive precipitation.
With the further track of the vorticity lobe, convection will likely spread east- to northeastward into Northwestern Turkey, the rest of Bulgaria and Southern Romania overnight, fuelled by plentiful moisture advection from the Black Sea with low-level Northeasterly winds. The severe weather threat gradually diminishes but remains enhanced throughout the night.

... Southern Scandinavia and East-Central Europe ...

Scattered and non-severe afternoon thunderstorms are forecast in a fairly cool air mass immediately ahead of the long-wave trough axis. Limited instability and weak vertical wind shear should preclude any severe weather threat. An isolated "cool air funnel" is not ruled out.