Storm Forecast

Storm Forecast
Valid: Wed 09 Nov 2011 06:00 to Thu 10 Nov 2011 06:00 UTC
Issued: Tue 08 Nov 2011 23:42
Forecaster: PUCIK / TUSCHY

A level 1 was issued for Southern Italy/Sicily and surroundings mainly for marginally large hail and excessive rainfall.

SYNOPSIS

A blocking situations has been prevalent over Europe for the past weeks, currently the high geopotential system is located over Scandinavia, amplified meridionally. Interestingly, to the south a shallow pool of lower geopotentials is found, with cool mid-level temperatures, inducing higher lapse rates. It is this factor that contributes most to the thunderstorm activity in the region. On the both sides of the ridge, very deep and prominent troughs will be observed with e.g. arctic outbreak over much of Eastern Europe.

DISCUSSION

... Western Iberia ...

As the upper and mid level trough approaches the region, in the warm sector ahead of the front models simulate low end latent instability with MLCAPE values mostly around 500 J/kg. Weakly unstable conditions will be accompanied by moderately strong vertical wind shear, possibly exceeding 20 m/s at 0-6 km layer, especially inland, where friction reduces low level flow. Nevertheless, weak forcing and meager instability will significantly reduce the severe weather risk. It looks like marginal threat of excessive precipitation might occur over SW Iberia, where the highest amounts of moisture and instability are predicted. Moist low levels and short period training pattern around the frontal zone might enhance the threat, but confidence in it remains too low to warrant a Level at the moment. An update might be requried however, later on.

... Central Mediterranean ...

A very complex setup evolves over this region, with satellite VW loops displaying as of Tuesday 21 UTC 3 separate vortices over the area. Each of the vortices does have an ability to initiate DMC in the region and two most important are discussed below.

The most interesting one is a warm core maritime low pressure system north of Corsica that has intensified thanks to the convection with sustained winds probably exceeding at maximum 80 km/h close to the center of the low as of writing the forecast. Most of the DMC in this low is displaced to the northwest and north of the low, suggesting dry intrusion above the center of the low, thus making it more assymetric. During the past few model runs, a constant weakening trend was indicated, which would be in line with decreasing SSTs in the Gulf of Lion (17C and lower) and more stable atmospheric conditions to the west. Despite that, shear decreases somewhat with weak ridging atop "01M", so this cyclone should produce further convective bursts next to the center. This may support at least some shallow warm core structure during the forecast period. Any deviation of the track to the south may bring this cyclone over warmer SSTs, which may assist in enhanced DMC, so the exact track will be followed closely. Despite those uncertainties, the center remains next to the coast of S-France, so bands of showers/isolated thunderstorms continue to spread inland with heavy downpours and strong to isolated severe wind gusts. ESTOFEX does not include the gradient winds of the cyclone in the final level decision and concentrates on its DMC probabilities. Due to the expected decreasing convective activity and yesterday's very limited thunderstorm activity in the convective bands of the cyclone, no level 1 was added. This cyclone should move onshore somewhere along the SW coast of France during the latter part of the forecast with rapid weakening onshore.

An update regarding a threat of excessive precipitation and severe gusts may be anticipated, if this cyclone reveals any significant changes in organization/strength.

Another vortex will progress from the Adriatic Sea to the Balkans. The most unstable area will likely be over the Ionian Sea, especially to the east of Sicily, where models hint on more than 1000 J/kg of MLCAPE, thanks to the high mid level lapse rates. Ahead of the trough, south to southwesterly prevailing flow is simulated with moderate wind shear values in the southern part of the tonge of instability. Local models show the development of shallow surface low over Sicily, which could enhance the cyclonic circulation over the region and support initiation. With steep mid-level lapse rates, moderate CAPE values and moderate wind shear, area including Sicily and its surroundings might see an isolated marginally large hail from stronger cells. Especially towards NE, weaker wind fields and moist low to mid levels with onshore flow might support few local heavy rain events. A low end Level 1 is issued for the area with the combination of these two threats.

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