Storm Forecast
Valid: Fri 02 Feb 2018 06:00 to Sat 03 Feb 2018 06:00 UTC
Issued: Fri 02 Feb 2018 07:17
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for coastal areas of Croatia, Bosnia-Herzegovina, Montenegro and Albania mainly for excessive convective precipitation and to a lesser degree for severe convective wind gusts, large hail and tornadoes.

A level 1 is issued for parts of the west-central Mediterranean, Sardegna and the W coast of Italy for excessive convective precipitation, severe convective wind gusts, large hail and tornadoes.

A level 1 is issued for coastal Algeria for severe convective wind gusts.


SYNOPSIS

A highly amplified 500 hPa trough stretches from Finland to Spain at Fri 06 UTC and continues its slow eastbound translation. The associated surface cyclone fills up over Denmark. Its long cold front extends all the way from Belarus to northern Algeria. Several waves develop along this cold front and gradually merge into a diffuse secondary low over central Italy.
A deep and strong SW-erly flow advects very warm and moist subtropic air into southeastern Europe and even into the Ukraine. In contrast, maritime polar air fills the northwestern half of Europe.


DISCUSSION

... central Mediterranean, Italy into W and N Balkans ...

Steepening lapse rates ahead of the main trough spread over a moist maritime boundary layer and allow the formation of a few hundred J/kg of CAPE. The entire reservoir of slightly unstable air is under enhanced 0-3 km vertical wind shear (bulk shear between 10 and 20 m/s, storm-relative helicity on the order of 200 to 300 m^2/s^2) and under synoptic lift due to persistent warm air advection and positive vorticity advection.
Similar to Thursday, scattered organized storms are possible over a wide area. The lack of concentrated sources of synoptic lift makes it difficult to pinpoint offshore areas at the highest risk, especially as too strong capping may become an issue towards the south under the fringes of an African elevated mixed layer. There is some sort of agreement among the model pool that a subtle vorticity maximum may spark the bulk of the activity to the west of Sardegna in the afternoon, moving across Sardegna into the Tyrrhenian Sea in the evening and overnight. Storms may well organize into multicells, possibly even a few supercells with a risk of severe wind gusts and moderately large hail. Near the wavy frontal boundary, vertical wind shear may be enhanced enough already across the lowest levels to allow the formation of one or two tornadoes.
Confidence in a higher storm coverage increases along the west coast of central Italy, where the strong flow becomes exposed to persistent orographic lift. Flash floods with repeated and/or backbuilding storms then soon become the main risk.

Conditions are similar along the west Balkan coasts. The more "disturbed" flow and the even stronger orographic lift may indicate that discrete storms are less likely there, meaning that the risk of supercells with all kinds of severe weather is somewhat lower (though still enhanced enough to add to a level 1 risk). However, in return, forecast models largely agree on even more extreme rainfall accumulations, which are predicted to peak between 200 and 400 mm during this forecast period in Montenegro. Flash floods are expected to become quite numerous and possibly extreme, and the areas at highest risk are upgraded to a level 2.

Unseasonably mild and moist air will move far inland with the strong flow (e.g., 2m temperatures/dewpoints are predicted to rise to 16/6°C near the Croatian, Serbian and Hungarian border triangle!). The air still features patches of CAPE and is exposed to synoptic lift, hence at least isolated storms may form in vicinity of the cold front and might organize. However, the time frame for surface-based storms with a possibility of severe wind gusts and marginally large hail is brief, hence the level 1 is not extended further inland. Isolated storms may nonetheless drift as far as Hungary and western Romania in the evening and overnight, while they decouple from the surface and should not pose a severe weather risk anymore.

... Algerian coast ...

ECMWF and GFS agree on the formation of a transient meso-low near the tail of the cold front, which exhibits a very tight pressure gradient along its SW flank for several hours. A period of scattered to widespread severe wind gusts is expected in offshore and possibly coastal waters. It is unclear if thunderstorms will be involved, but a level 1 seems to be warranted.