Storm Forecast
Valid: Wed 28 Jun 2017 06:00 to Thu 29 Jun 2017 06:00 UTC
Issued: Tue 27 Jun 2017 22:52
Forecaster: PUCIK

A level 2 was issued for N Italy, W Slovenia, W Croatia mainly for excessive precipitation, large to very large hail, tornadoes and to the lesser degree for severe wind gusts.

A level 1 was issued for Pyrenees area mainly for large hail.

A level 1 was issued for W Germany and Holland mainly for excessive precipitation.

A level 1 was issued for S, Central and E Germany, Austria, Czech Republic, Slovakia, Hungary, Croatia mainly for severe wind gusts, large hail and excessive precipitation.

SYNOPSIS

At the beggining of the forecast period, a deep, negatively-tilted trough will stretch from W Ireland through S France towards the Balearic Sea. At the SW flank of the trough, a strong jet-stream with windspeeds of up to 60 m/s is forecast with a strong cyclonic curvature at the base of the trough. The base of the trough will eject NE-wards, crossing N Italy, the Alpine range, Austria and S Czech Republic by Thursday 06 UTC. In the southerly flow ahead of the trough, a large warm sector will cover much of S, Central and SE Europe ahead of the advancing cold front. By the end of the period, cold front is forecast to have reached Poland, Slovakia, Hungary and NE Balkans. Across much of the warm sector, abundant lower tropospheric moisture will allow at least for marginal CAPE. Thus, large part of Europe will be under a thunderstorm threat with several areas requiring closer inspection.

DISCUSSION

... Pyrenees...

Situated near the base of the trough, 15 to 20 m/s of 500 hPa will have overspread the region, resulting in DLS favourable for well organised multi or even supercells, especially to the S of Pyrennees, where shear will be stronger. Despite the relatively meager lower tropospheric moisture, modestly steep lapse rates thanks to the cold core of the trough may allow for a few hundreds J/kg of MLCAPE. This, in combination with rather cold mid levels, may allow for a few storms capable of large hail.

... N Italy, W Slovenia, NW Croatia ...

At the exit-region of jet-stream, in strongly diffluent flow regime at 300 hPa, strong synoptic scale upward motion is forecast over the area. With dewpoints around 18 deg C over the Po valley and around 20 deg C over the coastlines, MLCAPE values are expected to reach 1000 - 2000 J/kg and even more over the warm waters of the Gulf of Genoa and the N Adriatic Sea. MLCAPE will increase from N to S as the lapse rates increase towards the plume of EML from Sahara. With the strengthening flow aloft, DLS around 20 m/s is forecast, again increasing towards S. Hodographs look favourable for multicells or supercells with most of the shear confined tothe lowest 1 - 3 km of toposphere. Lower tropospheric shear will increase towards the evening hours especially over the NE Adriatic Sea, Slovenia and NW Croatia with LLS up to 15 m/s. CAPE - shear parameter space looks favourable for all kinds of severe weather threats. However, current thinking is that widespread storms may be already ongoing at the beggining of the forecast period. With southerly flow at the lower troposphere, upslope flow is forecast to result in numerous clusters of storms along the Alps, Gulf of Genoa and the N Adriatic. With abundant lower tropospheric moisture, training of cells may result in very high rainfall sums over some locations. While excessive rainfall seems to be the primary threat, any persistent and isolated supercell may be capable of large to very large hail and tornadoes. It is questionable for now how many of such isolated supercells manage to develop in the area with widespread initiation of storms. This remains the main uncertainty of the forecast.

... S and E Germany, Central Germany, W Czech Republic, Poland ...

In the belt of enhanced lower tropospheric moisture down there, with dewpoints ranging from 16 to 20 deg C and in the lapse rates around 6.5 K/km, NWP agrees on the marginal to moderate MLCAPE values, locally between 1000 and 1500 J/kg. Over the whole area, DLS around 15 m/s is forecast, which would be conducive for well-organised multicells. Transient supercells are possible, especially in the northern lee side of the mountains, where DLS could be slightly enhanced and over Central Poland, where forecast hodographs show significant veering of wind with height thanks to the WAA regime. A relatively early initiation, occuring already in the morning hours as suggested by models, may result in the fail to fully build up higher CAPE values over particular locations such as S Germany or W Bohemia. Furthermore, high to mid level cloudiness may hinder surface heating. Nevertheless, more isolated multi(super)cells may be capable of isolated large hail and that especially over S to Central Poland, where more "fat" CAPE profiles are simulated. Over Germany and W Czech Republic, one or two cluster of storms (possibly MCS) are likely with attendant threat of severe wind gusts and heavy rainfall. Towards the night hours, with the passage of the trough, most of the models simulate, despite absence of abundant CAPE, a heavy rainfall producing system with embedded thunderstorms travelling from S Germany through W, N Bohemia into SW Poland.

... W Germany, Netherlands ...

In the trough at the lower troposphere, a belt of enhanced lower tropospheric moisture will cross the area during Wednesday with marginal CAPE mostly below 500 J/kg. Despite rather weak CAPE and also moderate shear at best, elevated risk of excessive rainfall will exist due to the high precipitation efficiency of developing thunderstorms. The main reasons for high efficiency will be low cloud bases and high RH throughout the troposphere. A low-end Lvl 1 is introduced for the area.

... E Austria, E Czech Republic, Hungary, Slovakia, SE Poland ...

During the afternoon hours, rather similar setup to the area to the W is forecast with MLCAPE values locally above 1000 J/kg and moderate DLS. Local initiation is forecast over the mountains with a possible upscale growth into a few convective clusters capable of severe wind gusts. Stronger updrafts may also produce isolated large hail. Towards the evening, low level shear will ramp up with an increase in the southerly 850 hPa flow, which is forecast to reach 15 - 20 m/s. This would increase chances especially for severe wind gusts, if storms persist into the evening and overnight hours. Towards the morning, as the trough ejects NEwards, stronger mid to upper tropospheric flow will also reach the area, increasing DLS to 20 m/s. Thus, the best shear profiles will exist for the storms in the late night and morning hours. Models do not agree on the coverage of overnight convection and its exact position, but given the strength of the lower tropospheric shear, severe weather threat may be at maximum in the end of the forecast period over areas such as N Hungary and Slovakia. Tornado threat was considered as well, but will be comparatively lower due to stabilizing boundary layer overnight.