Storm Forecast
Valid: Thu 17 Sep 2020 06:00 to Fri 18 Sep 2020 06:00 UTC
Issued: Wed 16 Sep 2020 23:39
Forecaster: GATZEN

A level 2 was issued across south-western Greece for extreme rainfall.

A level 1 was issued across the Ionian Sea region and southern Greece mainly for extreme rainfall and tornadoes.

A level 1 was issued across Portugal and Spain mainly for severe wind gusts, tornadoes, extreme rainfall, and large hail.

A level 1 was issued across south-eastern Belarus, northern Ukraine, and western Russia mainly for severe wind gusts.

SYNOPSIS

West European blocking intensifies with a ridge across the North Sea and troughs to the west of the Iberian Peninsula and across the Ionian Sea. Eastern Europe is affected by an intense trough spreading south-east. It is associated with a cold front moving into central and eastern Europe, pushing unstable air masses to the south-west. The Ionian Sea is affected by a subtropical cyclone that enters western Greece during the period.

DISCUSSION

Ionian Sea towards Greece

Models still indicate intensification of the subtropical cyclone during the period, with winds reaching Hurricane intensity. The vortex reveals very moist profiles, with a deep layer of rich moisture, skinny and marginal CAPE profiles and moderate to strong shear especially in the lowest 3 km in response to the warm-core low. Despite marginal CAPE, embedded deep moist convection is capable of producing excessive rain due to an effective warm rain process with a large warm cloud layer, rich moisture and favorable low-level inflow for training storms. These are forecast to contribute to the flash flood risk, especially where upslope flow is present. Elongated and curved hodographs in the northern quadrant indicate an augmented tornado risk in the northern part of the level 2 area.

Iberian Peninsula

Ahead of the mid-level trough, a south-westerly mid-level jet progresses across south-western Europe. It provides DCVA that increases in the afternoon and evening across Portugal and western Spain. At lower levels, a frontal boundary slowly moves east with some cold air advection affecting the area. Rich low-level moisture will spread north that overlaps with modest lapse rates. Given moist profiles, convection initiation is expected despite the strong shear. These storms will be paertly elevated due to the rather cool low level air, but some surface based storms are also forecast to develop. These will likely organize given strong 0-3 km shear, and bowing segments are expected, along with some supercells due to strong 0-6 km shear. Most storms are forecast to cluster duec to storm motion along the frontal boundary. Allover severe risk is limited by the tendency of the storms to be elevated, but still, all kinds of severe weather are possible, including sevrre wind gusts and excessive rain with clusters of storms and tornadoes and large hail with more isolated storms. Clusters of storms are expected to spread northward until the end of the period.

Belarus into Ukraine and western Russia

A cold front rapidly moves eastward on Thursday. The prefrontal air mass is characterized by rather steep lapse rates that overlap with a narrowing zone of enhanced low-level moisture. MLCAPE in the order of some 100 J/kg is forecast and storms are expected ahead and along the cold front. Strong 0-3 km shear will support storm organization along the cold pools that form along the frontal boundary. Storm motion will be parallel to the front with a tendency to move towards the cool side of the boundary, and isolated storms with a risk of large hail and tornadoes are less likely. Therefore, main threat will be severe wind gusts where CAPE and shear overlap along the cold front. Late in the periond, CAPE becomes marginal, with convection decaying along the cold front.

Tyrrhenian Sea

In a weakly sheared, unstable environment with numerous storms expected along the land-sea-breeze convergence, enhanced potential of waterspouts is forecast through-out the period.