Storm Forecast
Valid: Fri 11 Apr 2014 06:00 to Sat 12 Apr 2014 06:00 UTC
Issued: Thu 10 Apr 2014 16:55
Forecaster: TUSCHY

A level 1 was issued for parts of Spain, SW France and Portugal mainly for isolated large hail and severe wind gusts. Excessive rain over NE/E Spain and SW-France is possible.

A level 1 was issued for W-Turkey mainly for isolated large hail.

A level 1 was issued for NW Greece and Albania mainly for an isolated large hail and severe wind gust event.

... SYNOPSIS and DISCUSSION ...

Latest mid-/high-tropospheric analysis maps keep a disturbed streamline pattern in place over most of Europe. An extensive cold-core vortex tries to choke itself off the westerlies over SE-Europe. This vortex temporarily builds down to the surface, as most models agree in the development of a diffuse and broad LL depression over the W-Black Sea. Yesterday's soundings sampled that environment well and showed steep lapse rates and a rather moist mid-troposphere. Although a gradual mixing of the air masses in that vortex causes mid-layer lapse rates to weaken, infiltration of somewhat higher low-tropospheric moisture from the W-Black Sea should keep the atmosphere slightly unstable ... especially with some additional daytime heating. 0-3 and 0-6 km shear will be enhanced over Albania and NW-Greece, but the main concern is limited CAPE/updraft strength and therefore weak/short-lived thunderstorm activity. However, models still diverge substantially with MLCAPE calculations for this area and we therefore can't exclude a few stronger updrafts due to better BL moisture. Isolated large hail and strong wind gusts will be the main hazard. A similar setup evolves over W-Turkey, where another level 1 was issued.

Over NE Greece, Bulgaria and S-Romania, weak shear and marginal CAPE allow pulsating storms to peak in coverage and strength during the late afternoon. Marginal hail and gusty winds are possible.

A positively tilted and weakening upper trough still covers the northern parts of the Iberian Peninsula. A confined cyclonic vortex just west of Portugal weakens and broadenes to the E. Therefore, a moist marine air mass spreads east and covers most of N/CNTRL Portugal and Spain. At mid/upper levels, steep lapse rates with 8 to 9 K/km cover most of Spain and Portugal. Both ingredients overlap and result in a belt of 400 to 800 J/kg MLCAPE ... regionally higher peaks will be possible. Yesterday's soundings already sampled that air mass nicely with high LCLs, deep and dry subcloud layers and some mid-layer CAPE. Initiation of thunderstorms will be bound to diurnal heating over the mountains but also to weak mid-tropospheric impulses, which ride along the southern fringe of the upper level trough from W to E. Scattered to widespread thunderstorms are forecast. Shear outside the level 1 remains weak and increases to 15 m/s within the level area. Upper divergence seems to be maximized over E/NE Spain. Hence the following thunderstorm coverage/hazard is forecast:

a) N-Portugal and NW-Spain will see scattered strong to isolated severe pulsating storms. Upscale growth into numerous small and disorganized thunderstorm clusters is forecast. Rapid weakening is anticipated after sunset. Isolated large hail and severe downbursts are possible, but convection seems to stay too disorganized for confidentially issuing a level 1.

b) The level 1 area features the best overlap of ingredients for organized multicells and isolated supercells. Scattered initiation and favorable upper support/better shear probably cause more healthy looking clusters over NE Spain. 15kt NW-erly storm motion vectors could bring those clusters offshore to the Balearic Islands during the night. Large hail and severe downbursts are forecast. In addition, slow moving cluster could result in heavy rainfall amounts.

c) SW-France could see another cluster of storms, which evolves over the Pyrenees and builds slowly to the NE. Shear remains too weak for anything organized, but a slow moving cluster could bring heavy rain, next to an isolated large hail event.

During the night, onshore convection rapidly decays as the clusters over E/NE Spain move offshore while weakening.

The lightning areas over the SW Alps and over central Norway/Sweden highlight isolated and non-severe/short-lived thunderstorm activity.