Storm Forecast
Valid: Sat 23 Nov 2013 06:00 to Sun 24 Nov 2013 06:00 UTC
Issued: Sat 23 Nov 2013 00:11
Forecaster: PISTOTNIK

A level 1 was issued for Corsica, parts of Sardegna and mainland Italy, and for coastal areas of Croatia, Montenegro and Albania mainly for excessive precipitation.

A level 1 was issued for the N Tyrrhenian Sea mainly for waterspouts.

A level 1 was issued for the Ionian Sea, Greece and the Aegean Sea mainly for excessive precipitation and severe wind gusts.

SYNOPSIS

A pronounced cut-off low resides over the Tyrrhenian Sea and controls the weather across the entire Mediterraean region. A jet streak circles its Southern periphery, and several vorticity maxima on its cyclonic flank overspread the areas from the Balearic Islands via coastal Algeria, Tunisia and Southern Italy to the Balkans. Model agreement on these mesoscale features and on the exact position of the surface low pressure center(s) is limited, which hampers the forecast confidence.
High pressure systems are situated over the British Isles and over Russia but they fail to connect. Cool and dry air has conquered most of Europe apart from the Southeast. Further North, an upper-level trough filled with arctic air starts to amplify and to dig southeastward over Scandinavia.

DISCUSSION

... Central Mediterranean region ...

Unseasonably cool and dry air has entered the Western and Central Mediterranean region in the range of the cut-off low. Where this air mass is exposed to a longer fetch or dwell over the sea surface, steepening lapse rates and some moisture recovery are sufficient for the evolution of a few hundred J/kg of CAPE. Widespread showers and scattered thunderstorms are expected throughout the forecast period. The weak shear precludes any higher degree of organization. However, localized heavy precipitation with a threat of flash flooding is possible in various regions with sustained onshore flow, which were covered by level 1 areas.
Waterspouts are possible near the dominant center of the surface low, which will most likely crystallize over the North Tyrrhenian Sea. A low-end level 1 was issued for these phenomena as well, though the rather dry air aloft and strong convective mixing processes keep the confidence somewhat limited.

... Eastern Mediterranean region ...

Ahead of the cut-off low, a diffuse cold front slowly moves eastward over the Ionian Sea and Greece, and deep-layer shear quickly increases to 20-30 m/s beneath the mid-level jet streak. Prefrontal Southerly flow advects better moisture northward, which will likely create CAPE around 500 J/kg. Scattered to widespread thunderstorms are expected. At least some of them will organize into multicells with a strong backbuilding tendency, posing a primary threat of excessive precipitation and severe wind gusts. Marginally large hail and an isolated tornado are not ruled out in case that tail-end storms can temporarily turn into supercells. The coastal areas of Western Greece seem to be most exposed, especially in the morning hours and then again in the evening and overnight. Limiting factors which preclude a level 2 instead of a broad level 1 are the limited instability and the model disagreement how far south- and eastward the lift support of the vorticity lobes aloft will swing.
Isolated thunderstorms might travel some distance inland into the Republic of Macedonia and parts of Serbia, where the frontal system occludes. They should quickly become elevated and not pose a severe weather risk any more.

Overnight, the flow is forecast to increase also over the Aegean Sea. Southerly low-level winds beneath the Southwesterly mid-level jet will moderately enhance deep-layer shear (15-20 m/s) and storm-relative helicity. Convective initiation is uncertain till the end of the forecast period, but some models give hints that the second surge of well-organized storms could travel far to the Northeast overnight. The level 1 area was extended over much of the Aegean Sea for this reason.
Before that, the Aegean Sea and coastal Turkey will see only isolated to scattered and unorganized storms during daytime. The only noteworthy threat is a continuation of flash floods from Friday night's storms into the morning hours over Rhodes island and the Turkish province of Mugla, which are situated near the nose of a very moist Southeasterly low-level jet. However, since the activity will mostly decay before Sat 06 UTC according to the bulk of the forecast models, no level 1 was added any more.

... N Norway ...

The strong cyclone beneath the base of the Arctic trough will likely attain some warm-core characteristics over the Norwegian Sea. With 850 hPa winds peaking just above 25 m/s, a few marginally severe wind gusts are possible if this cyclone moves onshore between Tromso and the North Cape late in the evening. The uncertain track precludes a level 1 for now.