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. Yesterday s 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 yesterday s 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.