Storm Forecast
Valid: Tue 25 Aug 2009 06:00 to Wed 26 Aug 2009 06:00 UTC
Issued: Tue 25 Aug 2009 01:58
Forecaster: VAN DER VELDE

A level 1 was issued for NE Spain mainly for large hail and flash floods.

A level 1 was issued for France and Germany mainly for flash floods and a small chance of large hail / tornado.

A level 1 was issued for Ireland for tornadoes and gusts, if storms do develop.

SYNOPSIS

Early on Tuesday, a surface level depression develops over the east coast of Spain, in advance of the upper trough, drifting to the Balearic islands and further northeastwards. The airmass around this low is unstably stratified. A weak Atlantic cold front enters eastern France and western Germany during the period and will stall there. There is little quasi-geostrophic lift over it in GFS. The model plots convergence inside the warm airmass on the north flank of the Alps where CAPE can reach over 1000 J/kg. However, remains of the overnight MCSses may degrade the conditions compared to Tuesdays model output and lay the front more to the east.

DISCUSSION

...NE Spain...

A situation with a lot of synoptic scale lift (and thus a insolation-inhibiting cloud mass) and around 1000 J/kg CAPE. All models (GFS, ECMWF, WRF's) show convective precipitation continuing from the night into the morning in the Ebro valley area, backed by a strong mesoscale mid level vorticity maximum. As the depression moves offshore, however, after 12Z the warm moist air is pulling back to the sea with diverging northern winds, while southern winds still cause convergence on the eastern pre-Pyrenees, which has lower CAPE and more cap, initially. Ongoing convection will work to stabilize the atmosphere and GFS degrades the mid level lapse rates to near neutral values, while keeping good low level buoyancy. At the west side of the area, convergence and instability appear sufficient for some storms to remain into the evening. Offshore Girona/Perpignan, backed winds and remaining instability north of the low can sustain an MCS during the evening as the vorticity maximum moves in. Around 3 GJ/m² ICAPE is plotted over Mallorca at the end of the period, but it is capped or elevated in GFS. ECMWF however develops more precipitation just north of the Balearic isles, for this reason it is included in the threat level.

The shear environment looks rather complex around the vorticity and jet maxima. There will generally be around 20 m/s deep layer shear and 100-200 m²/s² 0-3 km SREH, which is sufficient for supercells, which after 12Z could be most favored near the Pyrenees where winds will be backed. But the shear is sufficient to organize strong multicells provided that the growth environment is not spoiled by early convection. Some storms will produce large hail. Large rain sums are possible locally since storm motion is forecast to be relatively slow, and anvils may accumulate downwind near the Pyrenees and cause prolonged rain.

...S and E France, W and N Germany...

Along the frontal zone, mixed layer LCL heights are below 800m and mid level lapse rates poor, 5-6°/km. Shear is enhanced to values sufficient for organized convection, 15-20 m/s DLS and >10 m/s LLS in places, but severe potential is limited by the poor profiles and storms will likely move parallel to the front, decay and spoil the environment for other storms. If not, and if there is some heating from the sun, then a stronger storm with isolated hail or perhaps a tornado could occur, but this chance seems low. The front-parallel shear and storm motion, combined with relatively high effective precipitable water content in the 30-40 kg/m² range, can locally cause flash floods especially when cells keep training over the same area.

...N Switzerland, S Germany...

If this area remains unaffected by remains of nocturnal storms, large instability can build up and be released in an environment with 15 m/s DLS, not as strong as to the west, but the better profiles would make up for this. Large hail may occur locally.

...Ireland...

During the night, an active front from a new Atlantic depression will affect the country. Some marginal instabilty is predicted at the warm front/occlusion point, in very strong shear conditions: 450 m²/s² SREH, 17 m/s LLS, and 20 m/s DLS. LCL is very low. One should be alert if indeed a convective cell forms, the primary risks would be tornado and severe gusts.