Storm Forecast
Valid: Wed 14 Jun 2017 06:00 to Thu 15 Jun 2017 06:00 UTC
Issued: Tue 13 Jun 2017 21:40
Forecaster: PUCIK ET AL.

A level 1 was issued for S France mainly for large hail and to the lesser degree for severe wind gusts and excessive precipitation.

A level 1 was issued for N Italy, S Switzerland mainly for large hail, excessive preciptiation and to the lesser degree for severe wind gusts.

A level 1 for W Balkans mainly for large hail and severe wind gusts.

SYNOPSIS

At mid and upper troposphere, a ridge will stretch from the Mediterranean into southern Central and Western Europe by 12 UTC Wednesday. To the north of the ridge, several short-wave troughs dictate the weather pattern. A rather deep trough is forecast to reach the British Isles by Thursday morning. A shallow trough will translate from Bay of Biscay towards France during the day. Last and the deepest trough will reside over much of Eastern Europe. Short-wave troughs are surrounded by strong flow, often exceeding 20 m/s at the 500 hPa level. At the lower troposphere, the best conditions for DMC are simulated over parts of France, with a significant overlap of abundant lower tropospheric moisture (dewpoints locally over 20 deg C) and steep lapse rates advected from Iberia. Over N. Italy, significant lower tropospheric moisture is observed as well, with dewpoints between 15 and 20 deg C. Isolated to scattered DMC is forecast over parts of Russia, in the CAA regime but with modest lower tropospheric moisture.

DISCUSSION

... France ...

As of 21 UTC Tuesday, several convective clusters are ongoing over the region of interest and the activity is forecast to persist overnight given steep lapse rates and abundance of elevated CAPE. This activity and possibly the outflow boundaries laid by these storms may influence the activity during the current forecast period. Both lower tropospheric moisture and steep lapse rates will persist over the region, resulting in moderate to high MLCAPE values, locally exceeding 2000 J/kg, especially to the north of Pyrennees, where MLCAPE values above 3000 J/kg are forecast. Approaching trough, together with slight WAA, will provide rising motion especially over Central to E / SE France by 12 to 15 UTC. Storms will initiate along the local topographic features and outflow boundaries laid out by previous convection. Given abundant CAPE, steep lapse rates and moderate to strong DLS (15 - 25 m/s, decreasing towards north and evening hours), well organized convection, including transient supercells is forecast with threats of large to possibly very large hail, severe wind gusts. Towards the evening, when storms cluster and storm motion decreases, excessive precipitation may occur as well. Uncertainty in the forecast arises due to the early passage and decay of the trough, along with the early morning convection, thus Lvl 2 has been discarded atm.

... Northern Italy ...

Prevailing north-westerly flow may provide for a favourable situation with shear enhancement due to the lee cyclogenesis in the wake of the W Alps. 15 to 20 m/s of DLS along with moderate CAPE will provide favourable conditions for well organised convection, including supercells, capable of large hail and severe wind gusts. Initiation will likely start along the mountain ranges and may spread into the lowlands (Po valley), which is questionable atm with diverging model solutions. Especially along the mountains, high PW values along with relatively slow storm motion (at or below 10 m/s) may result in a few excessive rainfall events.


... Western Balkans ...

In the northwesterly flow aloft, surface heating of local elevated areas may provide for marginally steep lapse rates. These, in combination with abundant lower tropospheric moisture along the coastline may result in moderate CAPE values around 1000 J/kg. At the same time, moderately strong vertical wind shear, around 15 m/s in the 0-6 km layer may result in several well organized multicells initiated on the local topography. These may be capable of large hail and severe wind gusts.


** This forecast was made by Pucik / Beyer / Baart de la Faille / Soljan / Oberhuber **