Extended Forecast
Valid: Sun 21 Aug 2011 06:00 to Mon 22 Aug 2011 06:00 UTC
Issued: Fri 19 Aug 2011 23:35
Forecaster: TUSCHY

A level 1 was issued for parts of France and Germany mainly for large to very large hail and severe wind gusts.

A level 1 was issued for parts of Spain and Portugal mainly for large to very large hail and severe wind gusts. An isolated tornado threat may evolve along the coastal areas.

SYNOPSIS

A cut-off low west of the Iberian Peninsula becomes trapped in the westerlies and starts to move northeastwards, eventually approaching the Bay of Biscay during the night hours. A sharp frontal boundary over parts of W-Europe will be the focus for isolated to scattered thunderstorm initiation. Stable and hot conditions prevail over most of the Mediterranean.

DISCUSSION

... France to Germany ...

Stalling stationary front from the 18th/19th cold front event continues to retreat northwards during the day and seems to be placed somewhere over central France and central Germany as an active warm front boundary. Air mass over far S-France/W-Mediterranean features a rich BL moisture content with surface dewpoint readings in the upper tens/lower twenties. As an upper low moves towards the Bay of Biscay, WAA strengthens over France and helps to sharpen the boundary during the day with EZMWF/GFS indicating a quite intense moisture gradient along this boundary....with moisture pooling bringing surface dewpoints well above 20°C on a local scale. Steep mid-level lapse rates atop this air mass feature a confined area with high CAPE build-up....probably in the order of 1-2 kJ/kg. However major uncertainties exist in the exact placement of this front due to numerous weak short waves crossing that boundary during the severe weather outlook day-1 period, each of them being able to generate strong to severe thunderstorms, which may modulate final boundary position.

Despite that, overall conditions favor rapid to explosive thunderstorm development on an isolated to scattered scale regarding thunderstorm coverage. Shear in order of 20-25 m/s (6 km bulk shear) will be more than adequate for intense multicell/supercell evolution with the main risk being large to very large hail and severe downbursts. Near parallel alignement of the deep layer shear vector to the frontal boundary also favors the development of numerous smaller scale and progressive clusters with a more widespread severe to damaging wind gust risk. However, major uncertainties in the meso-alpha/beta scale preclude any higher probabilities for now. Also, latest QPF outputs hint on more isolated thunderstorm coverage especially over central France, which also neglects a level 2 for now.


... Spain and Portugal...

As a negative tilted trough draws near during the day, the stage is set for scattered thunderstorms over the highlighted area. Main uncertainty that far out will be the final degree of instability. Models hint on enhanced moisture fields in the upper levels, which could limit diabatic heating due to the evolution of a thick Cs shield. Also, low/mid-level trajectories continue to back during the day, which brings much drier air from central Spain to the NW. Despite that, a cool down at mid-levels atop expected BL will result in at least 500 - 1000 J/kg MLCAPE. Large to very large hail and strong wind gusts will be the main risk. A slight tornado risk may arise along the coast, where surface based convection could materialize.