Storm Forecast
Valid: Fri 10 Oct 2008 06:00 to Sat 11 Oct 2008 06:00 UTC
Issued: Thu 09 Oct 2008 19:26
Forecaster: TUSCHY

SYNOPSIS

What a show right now on the weather maps! Latest 12Z CMC upper atmosphere analysis has a potent cyclonic vortex just SE of Greenland with a 90-95m/s speed max along its SE quadrant. At the same time, an impressive 949hPa surface low was placed at roughly 61°N/28°W with a very tight gradient along its SE periphery. Measurements by scatterometry were off the maps, but 30-35m/s in the well mixed/slightly unstable S/SE quadrant assist model wind gusts forecasts of above 40m/s in a small corridor, keeping those 45-50m/s at 850hPa in mind. The next feature of interest developed just south of the huge vortex and evolved a large comma like structure. It became attached to the SW-NE elongated frontal zone and the whole process but also the appearance in different channels is evocative of an instant occlusion process around 6-10Z at 50°/35-40°W and since then further development occured. Downstream of this intense large-scale vortex SE of Greenland, strong WAA regime spreads to the east/northeast with geopotential height increase of locally more than 8gpdm/12h over W-CNTRL Europe. This process just overruns a weak upper trough, which was placed over W-Europe yesterday and a cut-off process over SW-Europe will be the result. A strong 50-70m/s 300hPa speed-max due to tight geopotential height gradients between the cold-core low and the ridge to the west continues to curve around the low. This process persists during the forecast period, resulting in a further drift to the SW, before stalling somewhere over Morocco/Western Sahara and starting a slow drift back to the north during the latter part of the forecast, as upper jet finally curves around. This process in turn starts to pump hot and dry desert air to the north with rising pressure over the Mediterranean area. So everything works to a 'grande finale' which will be a very strong and extensive high pressure area mainly at LL, covering most parts of central/S/SE-Europe. An upper trough is still placed over SE-Europe, so despite high surface pressure, conditions will be somewhat unsettled, although cold continental air should limit showers/thunderstorms to orographic supportive regions.

DISCUSSION

...Alboran Sea, Strait of Gibraltar, N-Morocco and Algeria...

This time one has to advance far into NW-Africa to get the whole picture of the environmental conditions. Not sure how well models like GFS gather the complex processes at all levels in the more or less data-sparse area, also visible in poor agreement regarding placment of the surface low pressure area, but all models agree in a gradual pressure fall over Morocco, western Algeria, western Sahara and N-Mauritania. Overall, the exact pressure field won't be that important this time as a broad area north and east of the upper/surface low will be strongly sheared, so smaller scale pressure disturbances could only enhance directional/speed shear in an already favorable environment. Strong WAA downstream of the surface low is expected and IR/WV animations support this trend as a thick cloud shield developed over Algeria due to strong isentropic lift. Cold front is more or less easy to localize despite very limited surface data access, as latest WV animation showed an impressive plume of a rapidly southward racing Shekeli (W-Algeria)/Sahel (Morocco) (all local names for their dust storms). However, very hot desert air advects northwards over Algeria, affecting the northern parts of Algeria and the Alboran Sea after 00Z, as mid-level lapse rates climb to 8-9k/km, so a well structured EML builds northwards. The main difficulty will be to highlight the area, where thunderstorms will start to evolve north of the Atlas mountains as forward trajectories still show a SE-flow over N-Algeria (regarding foehn effects/subsidence just north of the mountain range) and aforementioned WAA should help to cap the area during the day. But at 850hPa, winds will gradually back to the east, and a robust moisture transport will be underway during the day. Due to all those uncertainties, we go ahead and exclude the region just north of Algeria from the thunderstorm area.

Strong convergence, moisture advection and a cooler atmosphere should help to spark numerous thunderstorms further to the north, over/SW of the Balearic Islands. Modest DLS of about 15m/s should help to sustain multicell storms, but veering is weak and instability not too strong, so gusty winds/marginal hail will be the main risk. The region just offshore of SE Spain was included into the level area, as better LL-CAPE release/weak background flow could result in an isolated tornado/spout report, mainly during the morning and early afternoon hours, before DLS increases from the east.

Conditions become much better for organized storms over the Alboran Sea and the Strait of Gibraltar during the evening and night hours as next to the still well structured upper low the surface flow turns more cyclonic as surface depression builds northwards. 0-6km bulk shear is not excessively strong as main upper jet is still far to the south, but 20-25m/s winds below 3km result in very strong LL speed shear. More remarkable will be intense veering, pushing SHR1 and 3km values to impressive values of 250-400 J/kg/300-400 J/kg respectively. As moist airmass filters in from the east and lapse rates at mid-levels constantly get steeper during the day, widespread 800-1000 J/kg MLCAPE are forecast with weakly capped 2 kJ/kg SBCAPE mainly offshore. A deep parcel layer depth and a high helical inflow layer is forecast, supporting rapid thunderstorm organisation and rotation. Supercells, capable of producing severe wind gusts, large hail and tornadoes will evolve during the day and regarding the strength of the shear, even a strong tornado will be possible. Hence a level-2 area was issued, despite the overall weak DLS.

Instability rapidly vanishes onshore over S/SE/E-Spain, but elevated thunderstorms are still in a favorable environment to form rotating updrafts and isolated large hail well inland can't be ruled out.

A prolonged period of thunderstorm activity and high moisture content of the airmass point to a flash flood risk over SE/E-Spain.

...SE Europe...

A few isolated thunderstorms could evolve over orographically favored areas, but both weak thermodynamics and shear are the limiting factors for stronger/long-lived thunderstorm activity.