Storm Forecast
Valid: Fri 08 Jan 2010 06:00 to Sat 09 Jan 2010 06:00 UTC
Issued: Thu 07 Jan 2010 22:47
Forecaster: TUSCHY

A level 2 was issued for parts of Sicily mainly for tornadoes, severe wind gusts and large to very large hail.

A level 2 was issued for parts of Bosnia and Herzegovina, Montenegro and extreme NW Albania mainly for excessive rainfall and to a lesser extent for tornadoes.

A level 1 surrounds the level 2 areas with an isolated tornado, severe wind gust, large hail and excessive rainfall threat.

SYNOPSIS

The main player today is the impressive branch of the polar vortex, covering all of west/central/southwest Europe. A robust mid/upper jet streak curves the base of the trough, which indicates the strength and dynamic of this feature, as this process in general is suppressed by the ageostrophic wind (inertial-advective wind). This process however enables the streak to eject out of the base and therefore draws the main trough from a neutral into a negative tilt. This constellation results in a coupled jet configuration over west-central Europe. Due to the attendant high-level divergence and an active baroclinic zone over east Spain/France, a surface depression intensifies over the western Mediteranean during the forecast. Thunderstorms and heavy precipitation are expected in the sphere of influence of this depression but also heavy snowfall, especially over SE France and extreme NW Italy (southern Alps).

The eastern Mediterranean remains unseasonably warm/hot but dry.


DISCUSSION

... Sicily, Tyrrhenian Sea and S-Italy (~ 18UTC onwards) ...

The set-up features a progressive pattern over the central Mediterranean, as a strengthening depression moves into the western Ligurian Sea. WAA downstream affects the highlighted area all day long with isolated thunderstorms/showers possible although thermodynamic environment is not supportive for deep and long-lived storms.

Conditions change during the later afternoon hours, as the upper trough axis approaches from Algeria with gradually dropping surface pressure over N-Tunisia eastwards. This assists in a backing wind field and some moisture advection into a broad warm sector, which becomes increasingly unstable. The weakly capped warm sector itself and the cold front later-on will be quite active in repsect of thunderstorm activity and a combination of some LL moisture influx beneath mid-level cooling cause a tongue of robust 1000 J/kg MLCAPE.

ECMWF/GFS and WRF runs with different resolutions agree well in timing and strength of the surface low pressure channel, so timing of the cold front is roughly 18 UTC. Latest data supports the development of supercells with large to very large hail possible. However, the main reason for the level 2 is the quite aggressive wind field in the lowest 3km, e.g. 20m/s over the warm sector, increasing to 25-30m/s along the cold front and well above 15m/s 0-1km shear with strong directional shear in the warm sector. LCLs just around 500m and nearly uncapped conditions indicate an augmented chance for tornadoes and an isolated strong event is certainly possible with those parameters. Severe wind gusts are bound to strongest activity in the warm sector, but they could become more widespread during the cold front passage, especially near bowing segments. The risk spreads eastwards although instability is on the decline. Nevertheless, isolated tornadoes, severe wind gusts and large hail are also possible over S-Italy.

If GFS 18UTC trend continues in later runs, showing a sharp surface trough, affecting the SW-coast of Italy during the night, probabilities may need to be increased there due to an augmented tornado risk.


... Parts of Bosnia and Herzegovina, Montenegro and extreme NW Albania ...

Although the expected precipitation (intensity and duration) from the mean of all models is a marginal level 2 situation, we decided to stick with the aggressive solution due to the repeated, heavy precipitation events in the past and the positive rainfall anomalies at least along coastal areas (coarsely shown in TRMM data). Local HRM data is in line with most global models, showing an onset of the heavy precipitation during the afternoon hours. Later in the forecast, LL wind back to southerly directions, increasing the fetch for the LLJ significantly and therefore support the advection of a very moist south-Mediterranean air mass. Thunderstorms are forecast mainly along the coast and just offshore as instability drops off onshore. Next to excessive rainfall, tornadoes can occur along the coast, where LL shear increases rapidly. The bulk of activity shifts slowly southwards during the forecast

Isolated thunderstorms are possible over the N-central Mediterranean and N-Adriatic Sea, but expected coverage will be too low for a low-end TSTM area. The same for SW-Romania, where a very isolated thunderstorm is possible during the morning hours (9th Jan.)

Isolated to scattered thunderstorms are forecast over parts of Spain, the coast of Portugal, the Bay of Biscay and the western North Sea. Snow, sleet and marginal hail is the main risk with this activity.