Storm Forecast

Storm Forecast
Valid: Fri 12 May 2017 06:00 to Sat 13 May 2017 06:00 UTC
Issued: Thu 11 May 2017 21:35
Forecaster: PUCIK

A level 1 was issued for N Italy mainly for large hail and severe wind gusts.

A level 1 was issued for NE Croatia, SE Austria, Hungary, NW Romania mainly for large hail and to the lesser degree for excessive precipitation and severe wind gusts.


At mid and upper troposphere, a trough is forecast over France and Iberia, slowly rotating to NE. At its base, a strong jet-stream is simulated with windspeeds over 30 m/s at 500 hPa. Jet will spread E-wards towards Balkans during the day as a short-wave trough shifts from Spain towards N Italy and N Adriatics during the forecast period. At the same time, a weak ridge will cover NE Germany, Denmark and parts of the Northern Sea. To the E, a deep trough over Russia and N Scandinavia will continue moving NE-wards. At the surface, a pronounced frontal boundary separates arctic airmass over Russia from polar airmass over Central Europe. WAA regime is forecast over much of Western, Central and especially SE Europe.

Numerous locations will see thunderstorms during this day. Activity may be split into two basic regimes:
1. Under the core of the trough over W and SW Europe in weak to moderate shear and cold mid-tropospheric temperatures.
2. In the WAA, in moderate to strong shear under or in close proximity to the jet-stream.

Both regimes deserve closer inspection below.


... Iberia, France, BENELUX, W Germany ...

Under the cold-mid tropospheric airmass, fairly steep lapse rates are simulated, resulting in marginal to moderate instability over these areas. The highest values of MLCAPE, exceeding 1000 J/kg, are forecast for central and N France, as well as Belgium. However, with DLS below 10 m/s, weakly organised storms in form of single cells or chaotic multicells are forecast. Stronger pulse storms may be capable of marginally large hail, heavy rainfall or gusty winds. Over Iberia, weaker instability is forecast, however with moderate shear in between of 10 and 15 m/s, supporting multicellular convection. Again, marginally large hail may accompany stronger storms. Combined threat seems a bit too low to warrant a Lvl 1 atm over both regions. Storms will be diurnally driven, forced by local boundaries or topography and are anticipated to weaken quickly after the sunset.

... N Italy, NE Croatia, SE Romania, Hungary, NW Romania ...

In these two areas, located underneath the jet, strong DLS is forecast, in between of 15 and 25 m/s, increasing towards S. Such degree of vertical wind shear would be supportive of supercells. While the high degree of shear is fairly certain in the forecast, degree of instability and convective initiation remain questionable. Over N Italy, models simulate rather weak latent instability with only a few hundreds J/kg of MLCAPE. Number of storms is not certain atm while the initiation may occur only in the evening hours with the approach of the short-wave trough from WSW. If storms manage to initiate, large hail will be the primary threat, especially from supercells, with severe wind gusts as the secondary threat. Over Croatia and Hungary, storms will initiate in the WAA regime and/or along the weak warm frontal boundary. Degree of instability is uncertain given the potential overestimation of dewpoints by models. Nevertheless, if storms develop, they will be capable of large hail and possibly severe wind gusts. Excessive rainfall is not ruled out over the northern part of area where cells may cluster. Towards the south, initiation becomes more questionable and isolated, while shear becomes stronger, so that any developing cell may become a long-lived supercell.

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