Storm Forecast
Valid: Sat 20 Oct 2012 06:00 to Sun 21 Oct 2012 06:00 UTC
Issued: Fri 19 Oct 2012 23:03
Forecaster: TUSCHY

A level 2 was issued for parts of the Balearic Islands and a region north of those Islands mainly for large hail, severe wind gusts, heavy rain and tornadoes.

A level 1 was issued for an area west of the Balearic Islands mainly for a few waterspouts and heavy rain.

A level 1 was issued for the coastal areas of E/NE Spain mainly for heavy rain and an isolated tornado event.

SYNOPSIS

The long-wave trough over W-Europe starts to weaken and drips off into a well structured cut-off low over the far S-Iberian Peninsula. With another cold-core low over the Black Sea, an omega-like pattern becomes established over Europe. Strong ridging between both vortices keeps conditions dry and stable for most of C-Europe.

DISCUSSION

Yesterday's thunderstorm activity (or to state it more precisely: inactivity) confirmed how difficult CI forecasting can be, especially when peering on yesterday's 12Z sounding of Barcelona. As global models like GFS or EZ failed in showing this lull of activity (onshore), we think that subsidence was stronger than models indicated (onshore) and sufficient with warming mid-levels to preclude offshore CI (next to weaker BL confluence east of the Balearic Island confirmed by yesterday's OSCAT scans). We added those thoughts to this outlook, because those points still play a role at least for the start of the forecast period.

Time frame: 06-18Z
Marginal Level 1 for the coasts of E/NE Spain

Subsidence (although weakening) continues until noon over the area of interest. Partly offset by increasing upper diffluence and an evolving coupled jet configuration, probabilities for CI are on a slight increase. However, mid-levels (e.g. 500 hPa) remain warm with weak lapse rates, so MLCAPE remains on the lower-end side with 500-800 J/kg until noon (and probably until 18Z). With a brisk SE-erly flow beneath favorable upper diffluence, conditions are supportive of repeatedly onshore moving showers and isolated thunderstorms along the E/NE coast of Spain and far SW France. LL moisture remains a bit questionable with BL flow oscillating between S- and SE-erly directions and hence still prone to the intrusion of drier N-African air. However, models agree in higher LL mixing rations with some piling-up along the coasts. This combined with the aforementioned environmental conditons could locally and temporarily support training shower/thunderstorm activity with excessive rain. Mesoscale features likely dictate the final location/time. 20 m/s DLS overlaps the unstable air mass along the coast, where organized multicells could produce isolated large hail (despite unfavorable lapse rates in the hail growth zone) and strong wind gusts.

This time, we kept the 50-% lightning area along the coast, with lower probabilities ashore mainly due to rapidly weakening onshore CAPE fields. Overall, this period does not look really impressive for anything severe and therefore only a marginal level 1 was drawn for the E/NE coast of Spain.

Time frame: 18Z-06Z
Level 2 and the complete Level 1 area

After 18Z, a potent cut-off enters the area of interest from the SW with abundant forcing spreading rapidly to the north. GFS ensemble data gives adequate support of a gradually consolidating LL depression E of Spain. This depression starts to draw a strengthening cold front from SE Spain to the E/NE which enforces/resumes yesterday's inactive convergence zone just east of the Balearic Islands. Substantial moisture pooling along this convergence zone and a northward fanning EML assist in robust MLCAPE build up of widespread 1000 J/kg and peaks in excess of 2000 J/kg along that convergence zone/cold front. This front also marks a strong gradient of cap strength (increasing to the east) and temperatures/geopotential heights. As the upper low moves in from the S, a strong mid/high-level jet enters the W-Mediterranean and overspreads this baroclinic zone. 0-3km shear up to 25 m/s and 0-6 km shear in excess of 30 m/s next to impressive directional shear (veering along the front) support well organized DMC along that boundary. The area of main concern will be the Balearic Islands to SW-France where multicells and supercells can occur with large hail, heavy rain, severe wind gusts and an enhanced tornado risk. The level 2 was drawn with a large E-W extent to reflect uncertainties in the final boundary position. The level 1 over far SW France was also expanded more inland in case storms survive longer than anticipated.

West of the Balearic Islands, strengthening LL vorticity with increasing LL CAPE and weak DLS may increase the risk of a few waterspout events during the overnight hours beside a heavy rainfall risk with onshore moving showers/thunderstorms.

... SE Spain to C- Spain to N-central Spain ...

A quasi-stationary wavy frontal boundary impacts the area during the forecast period. Numerous showers and embedded thunderstorms affect the same area for quite some time. With the evolving depression just E of Spain during the night, an organizing deformation zone might also impact the same area during the night with heavy rain, probably long-lasting. Confidence however is too low for issuing an inland level 1 for most of E-Spain but despite that, heavy rain likely impacts most of E/NE Spain for some time of the forecast period. A low probability lightning area was issued well inland to reflect some elevated/embedded thunderstorm activity (also visualized by an inland advecting MUCAPE plume).

... E-Bulgaria, NE Greece and NW Turkey ...

A cold-core low approaches from the NE and affects the area especially during the night hours. Moist and warm maritime air from the W-Black Sea advects to the SW beneath rapidly cooling mid-levels. Therefore MLCAPE increases to 500-800 J/kg during the night with scattered showers and thunderstorms. The main risk will be isolated large hail and strong wind gusts. Storms continue beyond 06Z.