Storm Forecast
Valid: Fri 15 Mar 2013 06:00 to Sat 16 Mar 2013 06:00 UTC
Issued: Thu 14 Mar 2013 17:54
Forecaster: TUSCHY

A level 1 was issued for parts of the Aegean Sea and W-Turkey mainly for a few waterspout events (an isolated strong event possible) and isolated large hail.

A level 1 was issued for NE Romania, parts of Moldova and parts of the Ukraine mainly for an isolated tornado event and gusty winds.

SYNOPSIS

The most active and strongest part of the polar vortex is still parked over the Barents Sea with a deep trough extending from that vortex far south, covering most of Europe. In fact this trough is also visible in the lower stratospheric maps which is a good hint at the strength / thermal structure of that feature. Quite common with such highly meridional troughs, the southern most tip becomes the focus for agile cyclogenesis, which is also the case during this forecast period in terms of a sub-985 hPa depression in the left exit region of a 40 m/s mid-level streak, which moves north from Romania (06Z) towards far W-Russia. A tightening baroclinic zone (< -10 °C at 850 hPa on the cold side with > 5 °C along the warm side) guides this depression to the north and W-Ukraine, Belarus into far W Russia will see blizzard, freezing rain and gusty wind conditions. Please refer to your local national weather services for warnings and local forecasts.
This depression sends a cold front to the S/SE along its backside, which approaches the Aegean Sea during the end of the forecast. At the same time, sub -30 °C temperatures at 500 hpa move in from the west and overspread Greece and the Aegean Sea, which once again helps to increase shower and thunderstorm activity.

Further west, the Tyrrhenian and the Ionian Sea serve as the northern boundary for adequate BL moisture supply with enhanced shower/thunderstorm activity. Further north, meager moisture keeps DMC probabilities very limited.

A deep vortex just west of Ireland sends somewhat less cold temperatures at mid-levels towards W/SW Europe. This advection along with marginal BL moisture suppresses deep convection. An isolated and short-lived thunderstorm event may be found beneath the main trough axis, where 500 hPa temperatures have trouble to exceed -35°C and where local moisture pooling may assist in local places with marginal SBCAPE. This area encompasses SE/E/NE Germany, W-Poland and offshore areas just off the coast of NW Norway. Probabilities remain too low for a lightning area. Also spotty thunderstorm development may be found N/W of Ireland.

DISCUSSION

... Tyrrhenian and Ionian Sea ...

A SE-ward shifting surface trough increases LL convergence from the Tyrrhenian Sea to the SE. That's the focus for enhanced activity, which was highlighted by a 50-% lightning area. Weak background flow/shear keeps thunderstorms disorganized, although we would not be surprised about numerous waterspout reports with overlap of modest LL convergence signals and LL CAPE (e.g. W coast of Greece until noon).

... Greece, Aegean Sea, W-Turkey and parts of Bulgaria ...

It's pretty obvious that initiation of scattered to widespread thunderstorms won't be questionable as -25 to -30 °C at 500 hPa overspread the 12-17 °C SSTs (from north to south) of the Aegean Sea. A plume of modest SB/MLCAPE in the range of 500 - 1000 J/kg is forecast to hover over the Aegean Sea before decreasing from north to south during the night as drier air filters in from the north. The main attention turns to a waterspout risk mainly along the N coast of Greece and W coast of Turkey and given amount of CAPE, an isolated strong spout event can't be ruled out. LL convergence signals are not impressive neither in global model fields nor in SKIRON, but that's enough to foster deep updrafts along healthy mesoscale convergence zones. Beside that risk, a few large hail events can't be ruled out with more intense storms, given pretty decent mid-level lapse rates and a lowering WBZ. The heavy rainfall risk, although present on an local scale, should not be a big troublemaker, given only modest LL mixing ratios forecast.

Numerous transient LL depressions, which evolve over N-Macedonia and Greece assist in a persistent onshore flow of aforementioned unstable air towards NE Greece and S-Bulgaria. Despite modest LL mixing ratios, moisture pooling along a slowly southward moving cold front should be enough for scattered CI along that boundary. Shear at all levels remains weak, but mesoscale models hint at a few spots with enhanced helicity either due to ageostrophic deflection of the BL wind in the NE quadrant of those surface depressions and/or along the frontal boundary itself. Therefore, an isolated better organized thunderstorm event is possible with an isolated large hail and tornado event (LCLs at or below 800 m). Probably the most inhibiting factor for tornadogenesis will be the cold air mass just north of the boundary, which might assist in pretty cold downdrafts for 'frontal boundary riders', which undercut any updraft quite soon. Hence the final magnitude of the tornado rik remains questionable and very low for the moment. This and the mixed model signals preclude an onshore level 1.

... NE Romania, Moldova into the Ukraine ...

A slight risk of isolated thunderstorms exists along the interface of a northward moving triple point and a northward surging dry slot. Overlapping of aforementioned features and strong lift should result in some potential instability build-up and a few thunderstorms. Despite somewhat marginal CAPE, favorable shear at all levels should support a few better organized thunderstorms with an isolated severe wind gust risk and an isolated tornado risk (very low LCLs, high BL helicity and 10-20 m/s 0-1 km shear). A level 1 with a low lightning probability area was issued. The risk decreases during the afternoon hours as the merging fronts occlude. Also BL moisture decreases, as onshore fetch for trajectories increases, which emerge from the W-Black Sea.