lundi 8 septembre 2014

European Storm Forecast Experiment

Storm Forecast
Storm Forecast
Valid: Mon 08 Sep 2014 06:00 to Tue 09 Sep 2014 06:00 UTC
Issued: Sun 07 Sep 2014 14:18
Forecaster: TUSCHY
A level 2 was issued for the Pyrenees and S-France mainly for excessive rainfall, large hail and severe wind gusts. An isolated very large hail event is possible.

A level 1 surrounds the level 2 for similar risks but lower coverage.

A level 1 was issued for Albania, Montenegro, Kosovo, FYROM and parts of Greece mainly for a few excessive rainfall events.

A level 1 was issued for waterspouts and an isolated excessive rainfall event.


An asymmetric cold-core low over the North Sea drifts east. No intensity change is forecast as it crosses Denmark and S-Sweden during the rest of the day. For the remaining forecast area, weak mid-layer height gradients prevail. Confined but substantially weaker cold-core lows dictate regional upswing of thunderstorm probabilities. One low exits N-Spain and emerges over the SE Bay of Biscay. Another one is placed over the W-Balkan States and a third one crosses Sardegna from W to E (although models keep this disturbance as a wave). This supports widespread thunderstorm activity over parts of Europe.



The cold-core low approaches Denmark from the west and moves atop increasingly warm SSTs of 16-18 °C. Temperatures at 500/850 hPa drop to -30/+4 °C respectively and cause widespread LLCAPE on the order of 100 J/kg or more and roughly 500 J/kg SBCAPE. Stronger updrafts, which move over coastal convergence zones (forced by friction) may cause a few waterspout events. In addition, heavy rain may accompany slow moving storms, which tend to affect same areas for some time. During the night, storms and attendant severe weather risk shift east.

Pyrenees and S-France

A leisurely northeastward moving synoptic front becomes more or less quasi-stationary north of the Pyrenees. Deep convergent flow of that modified subtropical air pushes effective PWs up to 30-40 mm. In addition, weak SE-erly flow advects a moist marine air mass from the W-Mediterranean well onshore (e.g. mixed-layer mixing ratios in excess of 13 g/kg). Dissolving elevated mixed layer over that BL air mass features MLCAPE in excess of 1kJ/kg ... peaks up to 2 kJ/kg are possible. The disturbance, weak cap and some diabatic heating assist in rapid and widespread thunderstorm initiation after noon. Ahead of the approaching disturbance from N-Spain weak ridging spreads east and affects most of S-France. No steering flow exists and with storm motion vectors of 5 kt or less, any storm poses an excessive rainfall threat. Significant flash flooding is well possible! Initiating storms may bring large hail in excess of 3 cm due to aforementioned high CAPE, but gradual upscale growth into numerous clusters should shift the hail risk to an excessive rain risk. Forecast soundings also show deep, moist profiles with some DCAPE, so water-loaded downdrafts may produce a few (wet) downbursts.

The W-Pyrenees experience stronger flow along the base of the aforementioned low with 15 m/s DLS expected. Forecast soundings in this region indicate robust CAPE of 1500 J/kg, deeply veered flow and moderate shear. Organized multicells and a few supercells with large to very large hail will be likely. An isolated very large hail event (5 cm in diameter) can't be ruled out. Excessive rain and severe wind gusts also accompany those storms. Further east, shear cuts in half and the excessive rainfall risk comes to the fore. Due to the miscellaneous severe weather threat a large level 2 area was issued.

Balkan States, Alps, the Czech Republic and Poland

Mainly daytime driven thunderstorm activity occurs over the W/SW Balkan States with forced convection over the Czech Republic and Poland. The Alps experience a mixture of diurnal driven activity and increasingly forced convection due to the approaching cold front from the NW. Neither PWs nor QPF outputs point to the need of a broad level 1. Still, slow moving storms result in spotty heavy rainfall reports. The only area with an augmented heavy rainfall risk exists over the SW Balkan States in the range of a disturbance, where somewhat better upper-divergence exists.

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