Storm Forecast
Valid: Tue 28 Aug 2018 06:00 to Wed 29 Aug 2018 06:00 UTC
Issued: Mon 27 Aug 2018 20:21
Forecaster: DAFIS

A level 2 was issued for N Spain and SW France mainly for severe wind gusts and large hail.

A level 1 covers a large part of France, Spain and extreme N Portugal for severe wind gusts, large hail, and excessive precipitation.

A level 1 was issued for Greece mainly for excessive precipitation. Waterspouts are expected near the coasts.

A level 1 was issued for SE Belarus, Ukraine, Romania, W Russia and extreme N Moldova mainly for excessive precipitation and severe wind gusts.

SYNOPSIS

A ridge has formed over the W Mediterranean basin with two cut-off lows at its both sides: one weakening over Greece but still it creates a very unstable environment and the second one is approaching the Iberian Peninsula and later France, from the Atlantic.
A shortwave trough is also crossing the Baltic Sea providing lift to unstable air masses and brings a cold air mass from NW which meets warm and moist air masses over eastern Europe. This results in an elongated frontal boundary extending from NW Russia towards Romania.
The main feature on Tuesday night will be a low-level vortex in the Gulf of Biscay which is expected to form in response to the approaching cut-off low.


DISCUSSION

.... N Iberian Peninsula and France ....

Ongoing thunderstorm activity in Portugal will migrate north in the early morning of Tuesday. Diurnal heating will then drive several storms mostly in N Spain, locally producing large hail and severe wind gusts. Later, as the Spanish plume propagates north and the cold air mass approaches from the west, strong instability and DLS will lead to severe thunderstorms. Probably one MCS will form in S France able to become a supercell given the strongly sheared environment and veering winds. Severe wind gusts will be the main threat, mostly where a level-2 is placed, but locally large hail cannot be ruled out.
Tornado threat is limited but not zero, especially near the coasts given the "S" signatures in the forecast hodographs and strong LL shear.


.... Greece .....

Warm and very moist air masses are found over Greece where many storms formed on Monday 27/08. The weakening cut-off low will still be able to create a very unstable environment on Tuesday, especially in the morning hours where vorticity maxima will translate south towards Crete. Locally high amounts of rainfall in a very short period of time are expected where a level-1 is issued. Thunderstorm activity should stop in the late afternoon.

.... East Europe ....

Thunderstorms along the almost stationary front in eastern Europe can take advantage of some 10-15 m/s DLS and 1000-1500 J/Kg CAPE to produce locally severe events. Given the vertically saturated profiles, excessive precipitation will be the main threat. Thunderstorm activity will weaken during the night but it will not stop.