Storm Forecast
Valid: Mon 23 Sep 2013 06:00 to Tue 24 Sep 2013 06:00 UTC
Issued: Sun 22 Sep 2013 22:27
Forecaster: PISTOTNIK

A level 1 was issued for parts of the Ukraine for severe wind gusts.

A level 1 was issued for Belarus, Lithuania and Latvia for severe wind gusts and tornadoes.

SYNOPSIS

Pronounced upper-level ridging and high surface pressure continue to bring calm and warm weather over the Southwestern half of the continent. To the Northeast, an intense jet streak (up to 70 m/s at 300 hPa) stretches from Iceland across Southern Scandinavia, the Southern Baltics and Belarus into the Ukraine. An explosive cyclone develops in its left exit region over Westernmost Russia and will likely deepen to 980 hPa late in the forecast period. A strongly forced, almost winter-like convective situation is expected along and behind its cold front.

DISCUSSION

... Northern and Eastern Europe ...

The cold front is forecast to exit Southern Belarus and move into the Ukraine early in the forecast period. A pronounced dry intrusion will overrun this cold front and will likely award it a "split front" appearance. The model pool agrees that strong QG lift will result in a narrow band of neutral to slightly unstable profiles immediately ahead of it. All in all, the signs in the forecasts - including a rapid translation of the front and an accompanying strong pressure signal - hint at a strongly forced and cold pool driven convective line with a risk of severe wind gusts.
The best chances for convection deep enough to produce lightning exist in the bordering region of Belarus, Russia and the Ukraine, where QG lift is maximized in the left exit region of the jet streak but vertical wind shear is limited. Further to the Southwest, vertical wind shear quickly turns quite impressive (0-3 km: 15-20 m/s, 0-6 km: more than 30 m/s), which increases the severe wind threat despite slightly shallower convection. With a background of 25 m/s mean winds at 850 hPa, it is expected that at least some places in the Northern Ukraine will experience severe wind gusts.
The tail of the cold front decelerates over the Southern half of Poland, where precipitation forecasts turn quite meager. Despite equally impressive kinematics and weak CAPE signals, the position of the front in the right exit of the jet streak and an orientation more parallel to the background flow are thought less conducive for an involvement and (linear) organization of convection. Yet it needs to be emphasized that severe wind gusts are not ruled out in the case that (even shallow) convection forms over Poland.

In the wake of the cold front, deep-layer shear decreases on the cyclonic side of the jet, but the flow below 3 km stays strongly sheared (15-20 m/s) and impressively helical (SRH up to 400 m^2/s^2) over Latvia, Lithuania and Belarus. Further upper-level cooling and a little insolation will create low-end CAPE, and several mesoscale voriticity maxima will help to initiate postfrontal convection in the noon and afternoon hours. Due to the somewhat irregularly spaced sources of dynamic lift, the convective mode is difficult to assess. One or two comma-like features with bowing lines and again a severe wind risk are possible, as are more isolated, low-topped multi- and supercells. The latter could even spawn one or two tornadoes, given that much of the shear and helicity will be concentrated in the lowest levels.
The strongest postfrontal development will likely be initiated over the Baltic Sea. The limited area models agree that the Western Baltics will be hit by a southward-moving feature beneath another small but intense vorticity maximum in the afternoon and evening, which can be interpreted either as a comma or as a shallow frontal wave, as strong latent heat release over the Baltic Sea (SST ~14°C) soon creates a "pseudo warm sector" in which 10m mean winds up to 20 m/s are simulated. "Lake effect" convection, though not necessarily electrified, can easily bring severe wind gusts. Besides, tornadoes along the sharp convergence line are also possible, as well as localized heavy precipitation in case one of the convection bands becomes stationary for a longer time. These risks seem to be maximized over the Western coast of Latvia, which fell just short of a mesoscale level 2 area.
Vertical wind shear decreases overnight and the convection moves gradually southward into Lithuania and the Russian exclave of Kaliningrad, while it slowly weakens. However, the risk of (spout-type) tornadoes stays enhanced throughout the night in coastal areas.

Another mesoscale vortex with a warm core at low levels, equally placed on the cyclonic side of the jet, will linger off the coast of Central Norway, where numerous showers and some weakly electrified thunderstorms are expected throughout the day. Severe weather is not expected.

... Southern Black Sea, Aegean Sea, Ionian Sea ...

Scattered showers and isolated thunderstorms may still extend for some hours into the current forecast period, left over by a southeastward departing upper-level trough. Isolated waterspouts are possible, especially near the coasts along nocturnal land breeze fronts.

... Portugal, W Spain ...

A narrow plume of steep lapse rates detaches from Northwestern Africa ahead of an Atlantic cyclone and moves northward into Portugal and later Western Spain. It helps to create low to moderate but strongly capped instability where the maritime boundary layers gets advected inland. Surface-based convective initiation is very unlikely throughout the forecast period. Isolated elevated storms in the warm air advection regime are not ruled out overnight, but are considered too unlikely to issue a thunder area.