Storm Forecast

Storm Forecast
Valid: Sat 24 Jun 2017 06:00 to Sun 25 Jun 2017 06:00 UTC
Issued: Sat 24 Jun 2017 05:57
Forecaster: TASZAREK

A level 2 was issued for S Austria , NE Italy and N Slovenia mainly for large hail, severe wind gusts and excessive precipitation.

A level 1 was issued for a corridor extending from NE Spain up to W Hungary and N Croatia mainly for the large hail, severe wind gusts and excessive precipitation.


A westerly circulation dominates over Europe. Jet streak stretches latitudinally from British Isles through CNTRL Europe, up to SW Russia. It separates warm and unstable air mass in the south and cold polar air mass at the north. Icelandic low approaches SW Norway. SW Europe is covered with a ridge that suppresses convective activity. A shortwave trough passes S France in the late evening and nighttime hours. In S and SE Europe pressure is horizontally blurred and the air flow is rather weak. The best overlap of LL moisture and lapse rates takes place over Alpine territory, and thus the main severe thunderstorm activity is concentrated on this area.

...Alpine area...

Moisture pooling (mixing ratios 11-13 g/kg) due to evapotranspiration and advection from France and Germany is taking place over northern Alps. Southern side over N Italy receives LL moisture advection from Ligurian Sea. Whole area is covered with an enhanced lapse rates exceeding 6.8-7.0 K/km. This combination provides ML CAPE ranging from 1500 over N Alps up to 3500 J/kg over S and E Alps with a healthy 1500-2000m parcel layer depths. DLS values range roughly from 8 to 12 m/s, but thanks to enhanced westerly flow at 700 hPa a 10-15 m/s MLS values are observed over N Alps. This suggests that developing discreet storms may merge into multicell clusters or even evolve into supercells given wind profile interacting with mountains. Synoptic scale lift will be low, but storms should evolve mainly due to strong diurnal heating and local convergence areas, most likely around 12-15 UTC when the cap will start to weaken. With decent instability and steep lapse rates, the main threat within these storms should be large hail (especially if mesocyclones will set-up) and downburst-type damaging wind gusts due to large values of delta theta-e (> 16 K). Some NWP mesoscale scenarios indicate a big storm system (possibly MCS) over S Austria/NE Italy/N Slovenia and moving into W Hungary in the late afternoon hours. Within these storms, a heavy rain causing local excessive precipitation and flash flooding cannot be ruled out, especially given rather weak storm motion. A level 2 is issued for the area where the probability for severe weather and the possible MCS is the highest. The main limitation of this forecast concerns the magnitude of the cap and the number of storms that will be able to overcome it. Currently CI seems to be the most likely over NE Italy and S Austria and N Slovenia.

...NE Spain and Pyrenees...

Moisture pooling thanks to sea breeze along Spanish coast and very steep lapse rates (> 7.5 K/km) provides ML CAPE up to 1000-1500 J/kg, but strongly capped. An enhanced DLS up to 15 m/s is also available thanks to the passage of the shortwave trough over SE France. Although some lift will be available, it is questionable whether thermals will be able to overcome convective inhibition over NE Spain. Current NWP scenarios indicate late CI in the nighttime hours. If storms initiate, they may be capable of producing large hail and downburst-type severe wind gusts. It is possible that 1-2 supercells may be present over this area. More pronounced CI is expected over Pyrenees where a convergence zone will take place. Within the support of diurnal heating, storms developing in the afternoon hours may produce severe wind gusts, large hail and heavy rain given their weak motion and PW exceeding 30mm. During nighttime they should move to the Mediterranean.

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