Storm Forecast
Valid: Fri 22 Feb 2013 06:00 to Sat 23 Feb 2013 06:00 UTC
Issued: Thu 21 Feb 2013 20:45
Forecaster: TUSCHY

A level 1 was issued for the Aegean Sea, Crete and W/SW Turkey mainly for heavy rain, large hail and an isolated tornado event.

A level 1 was issued for parts of the SW Mediterranean mainly for severe wind gusts and an isolated tornado event.

A level 1 was issued for extreme SW Spain and parts of Portugal mainly for an isolated tornado event.

SYNOPSIS

A major high over low blocking pattern is in progress and affects the weather over most of Europe during the following days. Before this pattern finally establishes, a belt of numerous more or less organized vortices extends from the Atlantic all the way to the eastern Mediterranean. Gradually strengthening and structuring of a dominant vortex over S-France/N-Italy finally generates a better defined focus for CI over SW Europe. Also, with more healthy looking WAA further downstream, weak troughs over the E-Mediterranean start to dissolve while shifting a bit north towards the N-Aegean Sea.

North of that belt of vortices, conditions remain very hostile for DMC and even better defined shower streets over the Baltic Sea won't be a focus anymore for sporadic lightning activity as high pressure continues to strengthen with increasing subsidence. Therefore any thunderstorm activity remains confined to the Mediterranean and adjacent areas.

DISCUSSION

... Crete and surrounding areas ...

Negative tilted trough with a gradual decrease in strength dictates the thunderstorm development and scattered thunderstorms likely evolve during the forecast. An ill defined LL depression over SE Greece still produces enough convergent flow over the Aegean Sea to offer mean LL mixing ratios in excess of 8 g/kg. This moisture beneath steep mid-level lapse rates of 8 K/km and more helps to boost MLCAPE to 400 - 800 J/kg with local peaks AOA 1000 J/kg.
Kinematics reveal adequate strength for organized DMC with 15 m/s (3 km) and up to 20 m/s (6 km bulk shear) next to enhanced SRH-3km especially along the NE fringe of the aforementioned LL depression (e.g. along the SW coast of Turkey).

An UVV maximum grazes the area to the south but inserts enough lift for CI and hence, scattered thunderstorm development will be already underway during the start of the forecast and continues all day long. There will be a gradual decay of convection from west to east as backside subsidence of the through spreads east. The main risk will be large hail, especially with the more discrete storms over Crete and potential tail-end charlie storms along the SW coast of Turkey. Inland moving clusters of showers and thunderstorms are expected at least until the afternoon hours, as a 20 m/s LLJ points ashore before backing/weakening thereafter. Beside the hail risk, heavy rain will accompany clustering storms. With the weakening trough, dynamics also do not look impressive and hence the storm clusters should not reveal any higher degree of organization but slow eastward movement of the clusters should offer enough time for some flash flood problems and hence the level 1 was expanded far north along the W-coast of Turkey. The tornado risk is not augmented, but given the expected strong to severe storms atop the Mediterranean with some LLCAPE and enhanced LL shear along the coast, a "mesoscale accident" can't be ruled out and an isolated tornado event is therefore possible.

... Portugal, parts of Spain and the W-Mediterranean ...

The structuring vortex over S-C Europe connects with another vortex over the E-Atlantic, which approaches the Iberian Peninsula during the forecast from the west. This finally fosters the development of an extensive low pressure channel, which runs from Spain to the NE. A plume of subtropical moisture advects to the east ahead of this trough and supports some onshore CAPE although weak mid-level lapse rates within the evolving warm conveyor belt keeps magnitude marginal at best. Hence only 100 - 500 J/kg MLCAPE with slightly higher MUCAPE values are expected. Despite strong shear (6 km bulk shear in excess of 25 m/s), marginal CAPE will likely limit the severe risk and only a few strong thunderstorms are anticipated over S-Portugal/SW Spain. Gusty winds and marginal hail accompany that activity. A marginal level 1 was issued for far SW Spain, where nocturnal thunderstorm activity may see some increase in LL buoyancy. Onshore moving storms therefore may be accompanied by an isolated tornado event.

Cold-core convection further upstream (e.g. most of N-Portugal) evolves in a weakly sheared environment. Nevertheless the lower levels reveal some buoyant signals with stronger LL shear along the coasts so a few cold-core funnels and isolated waterspouts can be expected with that activity. Further inland, an increase of the T-Td spread in the BL may limit that risk somewhat. The main time frame of interest will be 12-00Z with peak activity during the afternoon hours. That's the time, for which a marginal level 1 was issued.

... W-C Mediterranean ...

Over the W/C Mediterranean, numerous cyclonic vortices support an enhanced thunderstorm risk mainly south of the Balearic Islands towards the Tyrrhenian Sea. The main feature of interest will be a disturbance, which moves off Spain at roughly 00Z and moves atop a barolcinic zone just south of the Balearic Islands. A developing depression will be the result which spawns an unstable and sheared warm sector (MLCAPE of 500-1000 J/kg and strong LL shear and DLS). Latest data show the warm sector just to the south of the Islands, but only a small model deviation to the north places the region in the strongly sheared warm sector. A few well organized multicells are forecast with strong to severe wind gusts and isolated large hail. The tornado risk is not zero as SRH remains enhanced within the warm sector but also along an E-W aligned boundary over the Balearic Islands where storms might ingest adequate helicity for some updraft rotation. The activity translates rapidly east and approaches Sardinia during the end of the forecast period.