Storm Forecast
Valid: Fri 26 Sep 2014 06:00 to Sat 27 Sep 2014 06:00 UTC
Issued: Thu 25 Sep 2014 17:29
Forecaster: TUSCHY

A level 2 was issued for NE Morocco and N-Algeria mainly for excessive rain and large hail.

A level 2 was issued for the S-Aegean Sea and Crete mainly for large hail, severe wind gusts, excessive rain and an isolated tornado.

A level 1 surrounds both level 2s for a lesser severe risk.

A level 1 was issued for CNTRL Sweden, the CNTRL Baltic Sea and S-Finland mainly for severe wind gusts and an isolated tornado risk.

SYNOPSIS

A progressive trough north of Scotland (26th, 06Z) crosses Norway, Sweden and most of Finland from west to east until 27th 06Z. A pronounced high is placed to the south and keeps regions from France to the W-Mediterranean stable. A strong cut-off evolves over SE Europe with another but weaker one over Portugal.

DISCUSSION

... Norway, Sweden, parts of the Baltic Sea and Finland ...

A rapidly eastbound moving occlusion/cold front will be the focus for enhanced convection. Left exit of a powerful 45 m/s mid-layer jet overspreads the front and most models produce MLCAPE of 200-500 J/kg. Combined with intense forcing, an organized line of showers/at least isolated thunderstorms is forecast. It remains questionable, when orographically induced subsidence wanes east of the rough orography of Norway/
Sweden. With LL trajectories from the southwest and extrapolating eastbound moving forcing, we began the level 1 over CNTRL Sweden and expanded it east. 850 hPa winds of 25 m/s or more and LL shear of 15-20 m/s will be enough for severe wind gusts along any line of showers/thunderstorms. Deeper updrafts take profit of even stronger shear. Low LCLs and modest SRH-1 favor a few tornadoes ... especially along the coast, where warmer SSTs enhance LLCAPE. Lightning activity diminishes during the night, but the wind gust risk remains with decaying convection. Hence the level was expanded far east. No upgrade to a level 2 due to uncertainties in magnitude of CAPE and E-displacement of strongest LL winds ahead of anticipated convective line.

... Ionian Sea, Aegean Sea, Greece, Turkey and Crete ...

Leisurely southbound moving cold front and abundant lift of the cut-off ensure scattered to widespread CI. Overall scenario becomes rather messy as cold front, a developing LL depression over Turkey and the upper low interact. Expect organized convection offshore and along the coasts, where CAPE approaches 1000 J/kg. Rapid decay of onshore CAPE is forecast, although models might exaggerate. DLS aoa 20 m/s will support organized multicells/isolated supercells with large hail and severe wind gusts. Despite meager LL shear, CAPE in the lowest 3 km increases substantially. Therefore a few tornadoes are forecast and overlap of 150 J/kg LLCAPE with 20 m/s DLS could support an isolated strong event. Back-bending occlusion keeps air mass over Greece unstable for a prolonged period, so excessive rain should add a flash flood risk.

Further south, initially capped environment becomes more conducive for CI south of Greece, around Crete and over SW Turkey. Despite rather strong capping conditions, strong lift should help to initiate numerous storms, which grow upscale into eastbound moving clusters. 30 m/s DLS and more than 1kJ/kg MLCAPE support organized thunderstorms with large hail (an isolated very large hail event is possible), severe wind gusts, torrential rain and an isolated tornado threat). This area was upgraded due to the highest confidence of more widespread severe.

... N-Algeria to S-Spain ...

A quasi-stationary boundary, trapped along the Atlas Mountains, keeps BL moisture high. A confined area of 500-1000 J/kg MLCAPE and 15 m/s DLS with daytime heating assist in scattered thunderstorms with large hail the main risk. Clustering storms along/south of the Atlas Mountains could also bring excessive rain with flash flood problems. Weak positive thickness advection lowers the risk for thunderstorms from E to W during the night. Effective PWs of 15-30 mm, slow moving storms, dry soil and anticipated scattered to widespread CI increase confidence in a more serious flash flood risk. Hence the level 2. Towards Spain, less PWs and CAPE lower the severe risk and we kept this area in a broad level 1.