Storm Forecast
Valid: Fri 04 May 2018 06:00 to Sat 05 May 2018 06:00 UTC
Issued: Fri 04 May 2018 01:12
Forecaster: PISTOTNIK

A level 2 is issued for large parts of Slovenia, Croatia, and Bosnia-Herzegovina mainly for excessive convective precipitation and large hail, and to a lesser degree for tornadoes.

A level 1 is issued from NE Italy and Austria to W Greece mainly for excessive convective precipitation and large hail.

A level 1 is issued for N-central Italy mainly for excessive convective precipitation and large hail.

A level 1 is issued for Sardegna mainly for excessive convective precipitation.

SYNOPSIS

A zonal flow with progressive short-wave troughs runs across the British Isles and Scandinavia into Russia. Further south, synoptically quiescent conditions cover the majority of Europe. A large blocking surface high extends from the Azores to Germany and Poland. At upper levels, a cut-off low centered over Sardegna influences the western and central Mediterranean region.
A body of very warm continental air remains in place east of Greece - Austria - the Baltic States. Thursday's 12 UTC soundings characterize it with deeply mixed, dry profiles with little to no CAPE and weak vertical wind shear, allowing only isolated to scattered daytime storms over orographic features with little severe weather potential. More interesting convective situations evolve along its fringes, where better low-level moisture can be maintained and vertical wind shear is a little enhanced. These regions will be discussed below.

DISCUSSION

... NW Russia into Belarus, NW Ukraine, SE Poland, Slovakia ...

A tongue of the warm continental air is ingested into the warm sector of a cyclone travelling over Finland. Forecast models show rich low-level moisture near both the warm front and the cold front, resulting in a CAPE buildup on the order of 1000 - 2000 J/kg with a limited overlap of enhanced vertical wind shear (15 m/s across 0-3 km, 10 m/s across 0-1 km) near the occlusion point. Thunderstorms will likely initiate in the afternoon and organize. They pose a main risk of large hail (especially if a discrete storm mode prevails) or severe wind gusts (especially if a line or line segments develop).
Main limiting factor is the narrowness of the belt of augmented low-level moisture along both fronts, which makes it fragile and therefore prone to being mixed out prematurely. 2m dewpoints were overpredicted by several degrees across the board on Thursday. Most notably, the 12 UTC Kaliningrad sounding showed almost zero CAPE versus predicted values of 700 - 1500 J/kg. The true CAPE magnitude on Friday in the same environment is therefore also expected to be lower, possibly on the order of just a few hundred J/kg, limiting thunderstorm coverage.
A high probability thunderstorm area and a level 1 are issued from central Belarus NE-ward, where confidence in convective initiation and sustenance is best. Although the main short-wave trough lags well behind the cold front and does not interfere, a small forerunning vorticity lobe may facilitate convective initiation in this area. Further downstream along the warm front (Russia) and further upstream along the tail of the cold front (NW Ukraine, SE Poland, Slovakia), limited low-level moisture, a lack of lift and vertical wind shear will probably keep storms isolated, poorly organized and short-lived.

... Hungary, Austria, NE Italy, Slovenia, Croatia, Bosnia-Herzegovina, Serbia, Montenegro, Albania, FYROM, W Greece ...

A more convincing severe weather threat develops once more around the N Balkans, where persistent warm air advection from the SE creates large-scale lift and enhances vertical wind shear. Thursday's 12 UTC soundings from Vienna, Graz, Zagreb and Zadar indicated CAPE between 300 and 1200 J/kg, as an elevated mixed layer slid on top of rich low-level moisture near the diffuse surface warm front. A similar CAPE magnitude is expected again on Friday, overlapping with enhanced vertical wind shear and storm-relative helicity (around 15 m/s and 200 m^2/s^2 across the lowest 3 km, respectively).
Due to residual clouds and the ongoing warm air advection, convective initiation will likely occur rather late. However, in the course of the afternoon, a rapid increase in storm coverage and intensity is expected, mostly over Slovenia, Croatia and Bosnia-Herzegovina. Initially discrete storms will likely turn multi- or supercellular and pose a main risk of large hail, possibly reaching 5 cm in size in isolated places.
Towards evening, clustering into one or several MCSs is likely under the help of synoptic and orographic lift. An extraordinary flash flood risk is foreseen due to a persistent, NW-SE-running and flow-parallel convergence zone across Slovenia, inland Croatia and Bosnia-Herzegovina and ceaseless inflow of warm and moist air from the E to NE. Limited area models agree on backbuilding storms with rainfall peaks in the 50 - 100 mm range. Vertical wind profiles continue to improve in the easterly inflow, and 0-3 km storm-relative helicity is predicted to exceed 500 m^2/s^2 in the bordering regions to Austria and Hungary in the evening. Discrete or tail-end storms may still become supercellular into the night with a risk of large hail and one or two tornadoes.
Activity may continue or regenerate repeatedly throughout the night while slowly weakening. All in all, a level 2 is warranted for the N Balkans and a level 1 for S Austria and NE Italy, which may still see remnants of these storm clusters with a main risk of heavy rain in the evening and night. A high-end level 1 is also extended all the way southward into W Greece, where CAPE as well as vertical wind shear and synoptic lift are a little lower/weaker than further north, but widespread afternoon storms still pose a risk of scattered large hail and heavy rain events.

... central Mediterranean ...

Limited CAPE is present thanks to cool mid-levels around the cut-off low. Vertical wind shear varies due to complex wind fields at all levels, but stays mostly below 10 m/s. Scattered thunderstorms are possible throughout the forecast period with a maximum over land in the afternoon. The distribution of clouds and sunny spells, which are hard to predict, will govern the final placement and coverage of storms.
cONVECTION will be mostly disorganized and partly embedded, which limits its severe weather risk. Easterly upslope flow with embedded convection may create heavy rain on the upwind side of the northern Apennines and Piedmont Alps, while the large hail risk is enhanced on the downwind side around Tuscany and Umbria, where sea breezes may enhance vertical wind shear. Another flash flood risk might materialize over Sardegna, where almost stationary convection is expected beneath the center of the cut-off low. Level 1 areas cover each of these risks.