Storm Forecast

Storm Forecast
Valid: Fri 10 Nov 2017 06:00 to Sat 11 Nov 2017 06:00 UTC
Issued: Thu 09 Nov 2017 16:04
Forecaster: PISTOTNIK

A level 1 is issued for parts of the Norwegian Sea, parts of the North Sea, Denmark and the S Baltic Sea, and adjacent coastlines mainly for severe convective wind gusts.

A level 1 and level 2 are issued for Malta, parts of Sicily, and surrounding sea waters mainly for excessive convective precipitation and to a lesser degree for tornadoes, large hail and severe convective wind gusts.

A level 1 is issued for parts of the Tyrrhenian Sea for non-supercellular tornadoes (waterspouts), and for the E side of Corsica for excessive convective precipitation.

SYNOPSIS

Forecast maps over Europe start to look increasingly wintry. An intense mid-level trough over Scandinavia continues inching southward towards central and eastern Europe. In its wake, polar air surges southeastward over the North Atlantic and ensures vivid low-level cyclogeneses along the main frontal zone.
Low 500 hPa geopotential is also present all the way from the Ukraine to the western Mediterranean Sea, where a cut-off low separates and escapes deeply into Algeria.
The cyclonic conditions keep a day of active weather in store for much of Europe. Deep convection may occur in some areas discussed below.

DISCUSSION

... Norwegian Sea, North Sea, Baltic Sea ...

Main feature of interest will be a warm-core cyclone ("polar low") beneath the base of the polar trough over the Norwegian Sea. It is predicted to follow a G-shaped track and approach the coast of central Norway late in the forecast period. Severe wind gusts will become widespread along its western, southern and eastern flank, and they may easily exceed hurricane force over open water, at least in confined areas.
Like usual in these situations, it is not clear if convection will grow deep enough to produce lightning. Since GFS predicts equilibrium (cloud top) temperatures below -20C, a rather large area with at least low lightning probabilities, and consequently a rather large level 1 area, are issued. However, remaining uncertainties about the path and intensity of such a mesoscale vortex presently preclude a level 2, despite the high-impact weather it may bring.

Otherwise, scattered to widespread showers with sporadic electrification are expected in the stream of cold air over the North Sea, Denmark and the Baltic Sea. CAPE is limited below 100 J/kg and vertical wind shear is weak, but near the southern rim of the highlighted area, the background wind field is strong enough (20-25 m/s at 850 hPa) to support isolated severe wind gusts even with disorganized convection. This risk is highest over Denmark in the first half and over coastal areas of Poland, Kaliningrad and Lithuania in the second half of the forecast period.

... along the frontal zone: from the Baltics and Poland to Ireland ...

A long and wavy frontal zone forms the southern boundary of the polar air outbreak. It intensifies towards the west, over the North Sea and the British Isles, where it is also overspread by a mid-level jet with up to 40 m/s at 500 hPa.
One surface cyclone will form in the wake of the southern Norwegian mountains and move towards southern Finland. It is followed by a pronounced frontal wave that will cross Ireland and England late in the forecast period.
The formation of narrow convective rainbands is not ruled out along both cold fronts in the night hours, when they are favourably overrun by strong synoptic lift. The areas of interest are NE Poland and Lithuania (placed in the left exit of the jet streak) and Ireland (placed in its right entrance). However, forecast models do not indicate any CAPE. At the moment it appears unlikely that this convection will produce any lightning. Nonetheless, isolated severe gusts are not ruled out in case it grows deeper than anticipated.

... central Mediterranean region ...

Ahead of the southern cut-off low, an elevated mixed layer from the Sahara spreads over Malta and surrounding sea waters towards Sicily, creating CAPE on the order of 1000 J/kg on top of the warm and moist maritime boundary layer. Wind profiles are favorable for organized convection in the warm air advection regime with 0-3 km bulk shear around 15 m/s and veering wind profiles. As synoptic lift erodes the cap, a few supercells with all kinds of severe weather may form, but near the southern rim of our domain or even outside of it.
Further north, the forecast models predict a sharp wind shift between Sicily and Malta, which separates southerly and northerly surface winds, is initially stationary and then starts moving southward as a cold front. Clustered convection will likely go on along this boundary during much of the forecast period. WRF simulates precipitation peaks in excess of 150 mm in southern and eastern Sicily, where dangerous flash floods are likely! Other severe weather is possible in case of more discrete storms, though vertical wind shear is somewhat lower than further south.

Scattered thundery showers will also be active in the colder air over the Adriatic, Tyrrhenian and west-central Mediterranean Sea. CAPE on the order of a few hundred J/kg and weak vertical wind shear make severe weather rather unlikely. However, a few waterspouts are possible along any of the numerous convergence zones. A level 1 was extended over parts of the Tyrrhenian Sea, where this risk appears highest due to more robust CAPE and a strongly converging wind field.
A belt of moist air is advected westwards over the Tyrrhenian Sea and impacts the east side of Corsica, where heavy rainfall with embedded convection may occur.

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