Storm Forecast
Valid: Fri 16 Jun 2017 06:00 to Sat 17 Jun 2017 06:00 UTC
Issued: Thu 15 Jun 2017 23:02
Forecaster: PISTOTNIK

A level 1 was issued for NE Italy, Slovenia and inland Croatia mainly for large hail and to a lesser degree for severe convective wind gusts and excessive convective precipitation.

A level 1 was issued for parts of FYROM and Greece mainly for excessive convective precipitation and to a lesser degree for large hail.

SYNOPSIS

A broad NW-erly mid-level flow is in place between low 500 hPa geopotential in N Scandinavia and Russia and a subtropical ridge over Iberia. It contains a progressive pattern of short-wave troughs and ridges. The dominant short-wave trough amplifies from the North Sea and cuts off over Poland, accompanied by a surface cyclone whose cold front proceeds SE-ward across central Europe. A mid-level jet streak rounds the trough's rear flank and overspreads BeNeLux, Germany, Austria and Hungary, gradually catching up with the cold front at the surface.
High pressure dominates across the Mediterranean region, where the temperature level is seasonable in its eastern and central part while the heat wave with maximum temperatures well above 40°C continues on the Iberian Peninsula. The upper-level ridge starts amplifying towards the North Sea and a strong, warm surface anticyclone forms over France.

DISCUSSION

... Scandinavia and east-central Europe ...

A few hundred J/kg of CAPE forms in response to diurnal heating of polar air over Scandinavia and in the postfrontal environment over east-central Europe. Scattered thundery showers are expected, but their severe weather risk is low due to limited instability and vertical wind shear.
Along the cold front from Poland towards Hungary, vertical wind shear is slightly enhanced and more plentiful low-level moisture might allow the buildup of more CAPE (up to 1000 J/kg) after some hours of insolation. However, latest model runs accelerated the cold front and make it arrive already before noon. Besides, stratiform rather than convective precipitation is simulated. Confidence in an occurrence of thunderstorms in this confined belt with better kinematics is therefore limited.

... N Italy, Slovenia, Croatia ...

Plentiful low-level moisture is still in place in the very warm air to the south of the Alps, and CAPE will likely exceed 1000 J/kg over much of N Italy. The "back door" cold front will enter Slovenia from the NE around noon and is expected to trigger at least scattered storms. With the approach of the mid-level jet streak, vertical wind shear increases to 20 m/s across the 0-6 km layer while it remains weaker across lower levels. Storms will likely organize into multi- or supercells and pose a main risk of large hail. After some hours, upscale growth into a larger cluster is possible which could affect inland parts of Croatia and even Bosnia-Herzegovina in the evening. In this case, severe wind gusts and excessive precipitation are not ruled out, either.
The potentially best setup exists in N Italy, where the overlap of CAPE and vertical wind shear would be sufficient to allow strong supercells with very large hail (>5 cm). However, convective initiation becomes too uncertain to upgrade this small area to a level 2. In NE Italy, the tail of the cold front or the outflow from the Slovenian afternoon convection may create enough lift to break the cap and trigger an explosive round of evening storms. Further west, the increasing mid-level flow across the Alps provokes strong subsidence. It is likely that dry donwslope winds will just push the moist air southward without any thunderstorm activity in most of the Po Valley.

... from Iberia to the W Balkans and Greece ...

Plentiful low-level moisture over the Mediterranean Sea results in CAPE up to 1000 J/kg over wide areas. However, under anticyclonic conditions this moisture is often shallow and capped by a strong subsidence inversion. It will likely be diluted and mixed out before upslope circulations could transport it to mountains high enough to penetrate the cap.
Afternoon thunderstorms are therefore expected to remain isolated to scattered and strictly tied to orographic features. A small fraction of these storms may become strong enough to pose a risk of localized flash floods and (marginally) large hail. The highest storm coverage it expected over parts of FYROM and Greece, where a vorticity lobe creates additional lift and weakens the cap. These parts were covered with a level 1 mainly for the possibility of a few flash flood events.