Storm Forecast
Valid: Sat 19 Apr 2014 06:00 to Sun 20 Apr 2014 06:00 UTC
Issued: Sat 19 Apr 2014 05:01
Forecaster: TUSCHY

A level 1 was issued for parts of Spain mainly for large hail and locally excessive rain.

SYNOPSIS and DISCUSSION:

Two retrograde moving cold-core lows cause unsettled conditions from France to the Black Sea. Similar to yesterday, daytime driven CAPE build-up is forecast as moist/humid air at lower levels spreads west beneath mid-layer lapse rates of 7-8 K/km. MLCAPE remains below 500 J/kg for most regions but could peak at 800 J/kg in the immediate vicinity of any synoptic boundary. Stronger thunderstorms bring graupel and strong wind gusts. Effective PWs of 10-15 mm and tendency for storms to grow upscale into numerous small and disorganized clusters could locally result in heavy rain. Thunderstorms vanish until midnight, excluding the Adriatic Sea and a few storms over the SE Ukraine and NE-France/Belgium. The activity over NE France and Belgium will be monitored as forecast soundings show the potential for parcels to rise up to 4 km with adequate shear for organized but shallow convection. In case of a more constructive concurrence of low-tropospheric moisture and lift of a NW-ward sliding vortex, the reference of a risk of stray thunderstorm activity with an isolated tornado threat could be needed in further updates. A level 1 would be considered.

Over SW Europe, colder temperatures at 500 hPa spread east and cover most of Portugal and W/NW Spain during the forecast. Yesterdays persistent fog over Portugal reflects the high BL moisture content with this air mass which advects east and resides over NW and CNTRL Spain. Lapse rates of more than 7 K/km over the mountains and aforementioned moisture cause 500-800 J/kg weakly capped MLCAPE. Main change from yesterdays setup will be somewhat stronger mid/upper flow with 15 m/s 0-6 km shear but also stronger upper divergence as the left exit of a 50 m/s 300-hPa jet approaches from the west. Scattered thunderstorms evolve with a risk of large hail and strong wind gusts. An isolated funnel/short-lived tornado event is possible with LLCAPE in excess of 150 J/kg and LCLs below 1000 m mainly over W/NW Spain. Favorable upper flow pattern assists in upscale growth into a large but rather disorganized cluster with heavy rain the primary risk. The level 1 was expanded to the coast of the S-Bay of Biscay. The clusters thunderstorm activity becomes less active after sunset wit loss of diurnal heating and diminishing CAPE. A confined level-1 for large hail and locally excessive rain was added.
Thunderstorm probabilities also increase SW of Portugal between 00-06 Z. Onshore moving convection results in locally heavy rain with slow moving convection and PWs in excess of 15-20 mm.