Storm Forecast
Valid: Tue 19 Mar 2013 06:00 to Wed 20 Mar 2013 06:00 UTC
Issued: Tue 19 Mar 2013 00:48
Forecaster: PISTOTNIK

A level 1 was issued for Moldova, Romania, Serbia, coastal Croatia, Bosnia, Montenegro and Albania for severe wind gusts.

A level 1 was issued for parts of France for severe wind gusts and tornadoes.

SYNOPSIS

A broad area of low 500 hPa geopotential and cold upper-level air stretches from the British Isles to Central Europe. Near the surface, the primary (formerly Adriatic) low becomes stationary over Poland and gradually fills up during the forecast period. It is connected to a second low pressure center over Southern England by a shallow and zonal surface trough, which also forms the main frontal boundary between moderately mild air to the South and still very cold air to the North. Especially Russia and Scandinavia experience a continuation of extremely frosty temperatures under anticyclonic influence.
To the South of the main frontal boundary, an upper-level jet runs from West to East from the Bay of Biscay via Southern France into the Central Mediterranean region, before it culminates in a pronounced maximum which rapidly pushes across the Balkan states during the first half of the forecast period. The associated belt of enhanced vertical wind shear partly overlaps with patchy CAPE along the main frontal boundary and, together with strong dynamics, sets the stage for a day of strongly forced convection in various European regions.

... Moldova, Romania, Serbia, Bosnia, coastal Croatia, Montenegro, Albania ...

Isolated to scattered thunderstorms may already be at work at the beginning of the forecast period along the cold front of the Polish low, while it moves eastward into Croatia, Bosnia, Montenegro and Albania. CAPE will be confined to a few hundred J/kg at best but deep-layer shear is impressive with maxima in excess of 40 m/s. Storms in this environment may turn into multicells, supercells or bowing segments with a robust threat of severe wind gusts. Limiting factors for storm organization are the complex terrain and the imbalance between CAPE and shear which poses a danger of shearing the storms apart before the updrafts become sustainable enough.
Model forecasts and latest surface observations (Tue 00 UTC) raise the confidence that a thin belt of rich low-level moisture will wrap all the way North into Serbia, Eastern Hungary and Romania immediately ahead of the cold front. The left exit region of the mentioned jet streak and the nose of a stratospheric dry intrusion provide strong lift support which will, together with a few hours of insolation, create a narrow belt of neutral to marginally unstable profiles. Convective initiation is expected along the cold front near the Hungarian/Romanian border in the late morning hours, and at least isolated thunderstorms are forecast to move east across Romania and Moldova in the afternoon. Deep-layer shear around 25 m/s is sufficient for organized storms with a danger of severe wind gusts, in particular if they organize into a line. Activity will rapidly diminish over the Eastern Ukraine and the Black Sea around sunset due to boundary layer cooling and a crumbling of the dynamic lift support.
Further Southeast, Macedonia and Bulgaria will see a brief but intense warm air return ahread of the cold front, but much of the Mediterranean low-level moisture will be filtered by the complex topography. As the pronounced vorticity maximum also spares this region, thunderstorms become increasingly questionable, even though the cold front will likely be accompanied by a well-marked wind shift line and a strong drop of temperature in the afternoon to evening hours.

... inland Croatia, Hungary, Austria, Southern Germany ...

In the wake of the Polish low, a diffuse and inactive frontal boundary becomes stationary across Central Europe. Very dry mid and upper levels should allow several hours of insolation after the breakup of residual low-level clouds, and low-end CAPE is expected to build.
With the Northward retreat of the jet axis, a thread of enhanced vorticity will cross the area around noon, which looks rather benign in global models but quite "aggressive" in limited area models. Isolated to scattered showers and low-topped thunderstorms are forecast to form in the noon and early afternoon hours. The background wind field is strong enough for moderate storm organization with a chance of marginally severe wind gusts and isolated sub-severe hail. Wind shear will turn much stronger late in the afternoon, but a transition to pronounced negative vorticity advection and decreasing insolation will likely suppress further convection early enough to go without a level 1 here.
Thunderstorm activity will likely spare the Alpine region itself due to the day before's heavy dump of fresh snow which first needs to be sapped by the sunrays, and also Northern Italy and Slovenia due to a quick progression of dry downslope (North Foehn) winds from the Alps into the lowlands.

... France ...

Further West, the boundary will become warm-active again across France. Limited area models show a pronounced convergence line between strong Southwesterly surface winds in the warm sector and weak Westerlies on the cool side of the boundary. Diurnal heating is expected to allow the build-up of a few hundred J/kg of weakly capped CAPE in the warm air advection regime. Scattered thunderstorms are forecast to form around noon and to persist into the early evening. The Southern part of the unstable belt will be overlapped by deep-layer shear between 20 to 25 m/s and at least slightly enhanced storm-relative helicity. Isolated severe wind gusts are expected, and a tornado is not ruled out if a storm manages to ride the boundary for a longer time and ingest more helicity.

... Spain, Portugal ...

Somwhat detached from the hullabaloo across the rest of Europe, a subtropical wave depression will move onshore from the West in Southern Portugal and Southern Spain around noon. Lapse rates are limited but rich low-level moisture will likely drive CAPE into the 200 to 500 J/kg range in the warm sector. Convective initiation is not expected before deep-layer shear substantially weakens in the course of the day, hence severe weather is unlikely. An isolated heavy rain event can't be ruled out, though.