Storm Forecast
Valid: Thu 27 Sep 2018 06:00 to Fri 28 Sep 2018 06:00 UTC
Issued: Thu 27 Sep 2018 04:08
Forecaster: GATZEN

A level 1 was issued for the Ionian Sea mainly for excessive precipitation and to a lesser extent for tornadoes.

SYNOPSIS

Between a high centred over France and low geopotential from Iceland to NW Russia, a strong westerly mid-level jet affects northern Europe. Embedded in the strong flow is a short-wave trough that migrates over Russia. Upstream, the European high will gradually weaken, associated with an amplification of the N-European trough late in the period. South-eastern Europe is affected by a cut-off low. At low levels, a shallow warm-core cyclone is expected to develop late in the period.

DISCUSSION

Ionian Sea

Latest observations indicate increased deep moist convection close to the center of the Mediterranean cut-off low, where rich low-level moisture overlaps with steep lapse rates. Ongoing advection of drier air masses from the north will push the area of deep moist convection to the south where it can affect the African coast in the afternoon and evening. Farther east, warm air advection has already started and pressure falls affect a broad area, with a closed surface low expected close to NE Libya by Thursday morning. Latest models agree that this low will slowly move north-west while intensifying. Excessive precipitation will not affect land according to these forecasts. However, at the eastern flank of this vortex, unstable air masses are advected into south-western Greece. Late in the period, convective storms could move onshore, capable of producing excessive precipitation. Given strong deep layer vertical wind shear, embedded supercells are not ruled out, capable of large hail. Skinny CAPE profiles limit the hail risk, though. Furthermore, tornadoes are not ruled out, especially near the coasts, where low-level vertical wind shear is strongest (about 7 m/s in the lowest km). Weak low-level buoyancy close to Greece may limit the potential for tornadoes as well.

North-west Russia

Some weak thunderstorms are forecast with the trough moving over this area on Thursday. Marginal CAPE and weak vertical wind shear limit any severe potential.

Baltic States into N Poland

A cold front will cross the area late in the period. A tongue of rich moisture will be present ahead of this cold front. However, lapse rates will remain poor due to latest models. A narrow cold frontal rain band is expected. Severe wind gusts are not completely ruled out due to long hodographs and some marginal instability that may form along the cold front.

Portugal, NW Spain

Thunderstorms are expected in a weakly sheared environment, mostly supported by mountain-valley circulations. Slow moving storms may produce locally excessive precipitation before storms decay in the evening.