Storm Forecast
Valid: Sun 16 Oct 2016 06:00 to Mon 17 Oct 2016 06:00 UTC
Issued: Sun 16 Oct 2016 00:36
Forecaster: PISTOTNIK

A level 1 and level 2 were issued for Albania and W Greece mainly for excessive convective precipitation and to a lesser extent for large hail, tornadoes and severe convective wind gusts.

A level 1 was issued for parts of the Ionian Sea for large hail, tornadoes and severe convective wind gusts.

A level 1 was issued for parts of the Adriatic Sea for (non-supercellular) tornadoes.

A level 1 was issued for Wales and parts of England for severe convective wind gusts and to a lesser extent for tornadoes.

SYNOPSIS

The blocking high pressure system from Scandinavia continues its slow SE-ward shift and is centered over the Baltic States on Sunday. To the west it is flanked by a mature cyclone near the British Isles and to the east by an upper-level trough filled with polar air which exits from the E Ukraine into Russia.
Areas to the south of the high-pressure system are not fully stable, either: On the one hand, a stationary front, which separates the polar air to the NE from much milder air to the SW, produces clouds and stratiform rain in a belt from S Scandinavia to Romania and the Black Sea. On the other hand, a vorticity maximum travels SE-ward over the Adriatic Sea and the Balkans, which will be the main focus for possible severe storms.

DISCUSSION

... S Adriatic Sea, Ionian Sea, Albania, W Greece ...

ECMWF and GFS agree that CAPE reaches around 1000 J/Kg in coastal areas surrounding the Ionian Sea and rises to more than 3000 J/kg over open waters towards the south. With a Saharan elevated mixed layer on top of a very moist maritime boundary layer, capping will be very strong, though. The Saturday 12z Brindisi sounding samples the environment very well. It also includes strong vertical wind shear (both speed and directional shear) across the lower and mid-troposphere, which will gradually relax on Sunday but will still easily suffice for storm organization until 18 UTC.
Lift ahead of the upper-level trough and particularly by onshore and upslope flow will likely keep a strong MCS going, which rides SE-ward along the Albanian coast. Especially coastal areas will by prone to flash floods with such a system, but e.g. the Saturday 12z runs of the Greek BOLAM and WRF predict high precipitation amounts also well inland towards the bordering mountains to FYROM. In addition, the coastline may also be affected by tail-end storms, which could be supercellular and pose a risk of large hail, severe wind gusts and even a tornado.
Outside of the range of onshore and upslope flow, the strong cap will likely suppress any convection despite the impressive CAPE reservoir to the south. Since both the lift of the trough and the strength of the upslope flow will fade during the day, the severe weather risk will steadily decrease and should be mostly over after 18z, when remnants of the storms will affect Greece.

... central Adriatic Sea, Bosnia-Herzegovina, Montenegro, Serbia ...

Beneath the upper-level trough, some CAPE is still available over the Adriatic Sea, and a little CAPE may also develop over land in response to daytime heating. Vertical wind shear is weak. Scattered showers and thunderstorms will stay disorganized, but may bring a few waterspouts along the coast of S Croatia (Dalmatia), Bosnia and Montenegro.

... British Isles ...

The Atlantic cyclone steers an occluding frontal system across the British Isles, followed by well-mixed maritime air. With moderately enhanced vertical wind shear (10 m/s across the lowest 1 km and 15 m/s across the lowest 3 km), a few severe wind gusts or an isolated tornado are not ruled out in case convection organizes linearly along the cold front or in a following comma. A level 1 was drawn over the area where the strongest 850 hPa wind field (~20 m/s) and considerable forcing for lift ahead of a 500 hPa short-wave trough overlap.