Storm Forecast

Storm Forecast
Valid: Fri 30 Sep 2016 06:00 to Sat 01 Oct 2016 06:00 UTC
Issued: Thu 29 Sep 2016 20:34
Forecaster: TUSCHY

A level 2 was issued for excessive rain, very large hail, severe wind gusts and isolated tornadoes for N-Tunisia, SE-Sardinia and far W-Sicily.

A level 1 surrounds the level 2 for the same hazards with lower probabilities of occurrence. Towards NW/N-Algeria, the main hazard will be excessive rain.

SYNOPSIS

A belt of strong westerlies runs from the Bay of Biscay over Germany all the way to far W-Russia. Embedded in this progressive flow regime, numerous mid-level waves / troughs move east and result in a vast region with unstable weather conditions. A broad upper low over extreme N-Africa moves to the NE and links to the progessive westerlies further north. The result is a leisurely eastbound moving large-scale upper trough, which affects the W-CNTRL Mediterranean. Downstream ridge suppresses DMC activity.

DISCUSSION

... France ...

Ahead of an approaching mid-level wave, BL flow turns southerly with modest moisture return flow forecast. This moisture advects beneath colder mid-levels and in addition, a weak EML plume from the Pyrenees advects to the NE towards CNTRL France. This differential temperature and moisture advection result in a confined region with enhanced MLCAPE build-up - not robust values but still 200-500 J/kg within the realms of possibility. DLS of 15-20 m/s is enough for a few better organized updrafts with isolated large hail being the main hazard. 0-3 km shear strengthens during the night to 20 m/s, but elevated nature of convection should limit the severe wind gust threat - although not to zero. Limited CAPE precludes higher probabilities for now and no level 1 was issued. Elevated convection - partially electrified - spreads east during the night and may even affect extreme E-France/ W-Germany with sub-severe activity.

... Ireland and SW UK ...

Eastbound moving upper low, filled with cold air or an H500 temperature of less than -25 C, affects the area of interest during the evening and overnight hours. An enhanced vertical temperature gradient between rather high SSTs (14-16 C or 1K positive SST anomaly) and cooling mid-levels cause 300-600 J/kg SBCAPE. Weak shear precludes organized DMC, but numerous thunderstorms with graupel and gusty winds are very well possible. Nothing severe is anticipated.

The same for parts of the North Sea and the Baltic Sea. Enhanced vertical temperature gradients result in numerous showers and thunderstorms with graupel and gusty winds.
LLCAPE of 100 J/kg or less with 15 m/s near BL flow are not favorable conditions for waterspouts, but I would not rule out a few funnel / short-lived waterspout events, where mesoscale assists.

... Sardinia, Corsica, N-Tunisia to Sicily ...

Most likely the most organized / severe DMC activity occurs in this broad area. Ahead of the approaching upper trough, south/southeasterly BL flow advects a very moist air mass from Malta to the NW - BL mixing ratios in excess of 13-15 g/kg will be common W/SW of Sicily. As the upper trough hesistates with its eastbound motion, mid/upper flow regime remains from the SW with only gradually increasing wind speeds from 500 to 300 hPa. Hence, an EML plume won't move that far to the north - most likely affecting Sicily and surroundings during the night. That's the area, where the highest MLCAPE is forecast, peaking with more than 2kJ/kg over NE Tunisia and just to the east. Further north, values gradually relax as mid-level lapse rates weaken. DLS intensifies during the forecast as mid-level flow intensifies with 10-20 m/s DLS from north to south. Capping is quite often an issue with such setups, but not this time with only modest warming within the capping layer. Hence, although weakly to moderately capped, initiation is likely between Tunisia and Sicily with thunderstorms spreading to the N and NE betimes. Not sure how far south (towards Malta) thunderstorms will build due to stronger capping and weak inflow from the south but expanded the level 1 a bit to the south.

Multicells and supercells are forecast with all kind of hazards, including very large hail, excessive rainfall amounts, severe wind gusts and an isolated tornado threat. Betimes, storms grow upscale into well organized clusters. Cold pool driven convection may also induce a confined region with enhanced severe to damaging wind gusts. In addition, growing clusters may bring torrential rainfall amounts with an augmented flash flood risk. Current lack of a confined LLJ core lowers confidence in where to pinpoint a confined area with training convection, but NE Tunisia and SE Sardinia may see highest probabilities for at least temporarily training convection. A broad level 2 was issued to highlight that overall risk.

Further to the west, over the W-Mediterranean and N-CNTRL Algeria, numerous thunderstorms are forecast beneath the consolidating upper trough, but weak shear should limit the severe risk. However strong CAPE signals and slow storm motions may result in flash flood producing storms. Hence the level 1 was expanded far west.

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