Storm Forecast
Valid: Fri 28 Jul 2017 06:00 to Sat 29 Jul 2017 06:00 UTC
Issued: Thu 27 Jul 2017 18:10
Forecaster: TUSCHY

A level 2 was issued for the CNTRL-Ukraine into far W-Russia mainly for large/isolated very large hail and excessive rain.

A level 1 surrounds the level 2 mainly for a similar risk with lower probabilities. In addition, a few severe wind gust and an isolated tornado event are possible.

Two level 1 areas were issued for parts of NE-Spain mainly for an isolated large hail risk.

SYNOPSIS:

A persistent branch of the tropospheric polar vortex is situated over NW-Europe. This elongated vortex is centered between Iceland and Scotland and remains more or less stationary throughout the forecast. A negative tilted upper trough over NE-/E-Europe lifts northeast and leaves a quasi-zonal flow regime behind, which affects W-/CNTRL Europe. Embedded in this progressive flow are numerous short-waves, which approach the Iberian Peninsula and cross the CNTRL-Mediterranean.

A pronounced warm front over the CNTRL-Ukraine separates a hot and moist airmass to its east from a much cooler and drier airmass to its west. This front serves as main focus for organized DMC activity. An eastbound moving cold front over
CNTRL-Europe beneath the upper trough with cold mid-levels results in another area with scattered thunderstorms. Weak pressure gradients prevail over the Mediterranean Sea.

DISCUSSION:

... CNTRL-/E-Ukraine to far W-Russia ...

The airmass east of the warm front features high BL moisture content beneath moderate mid-level lapse rates, which results in MLCAPE of 800 to 1500 J/kg. A strong mid-level jet along the SE-fringe of that trough crosses the warm front and increases 0-6 km bulk shear to more than 20 m/s. The combination of strong upper divergence and sporadic diabatic heating should push CI quite early in the day. The most probable scenario is re-strengthening convection from the previous night, which intensifies during the day. Overall forecast hodographs feature a rather straight appearance atop stronger backing below 850 hPa and along the warm front itself. Rapid upscale growth into numerous large thunderstorm clusters is forecast. Large or isolated very large hail is likely especially during the initiation stage. Betimes, slow storm motion or temporal back-building increase the flash flood and strong to isolated severe wind gust risk. In fact, a localized serious flash flood threat is not ruled out along the warm front, where effective PWATs are maximized and training with strong low-tropospheric flow is possible. Thunderstorm clusters weaken gradually during the night while spreading slowly to the north. A continued large hail and excessive rainfall risk exists.

...Poland...

An eastbound moving cold front and ongoing PVA from the upper trough result in adequate lift to support scattered CI. The BL air mass along and behind the cold front is moist enough for moderate CAPE build-up beneath cold mid-levels. Temporal diabatic heating should push CAPE in the 400-800 J/kg range during the afternoon and evening. Forecast soundings show unsuspicious signatures with weak LL and deep-layer shear. Hence pulsating convection is forecast with temporal line-ups not ruled out along consolidating cold pools. Gusty winds and graupel will be the main risk. Given local CAPE peaks of 800 J/kg and temporarily broadening CAPE profiles in te hail growth zone, an isolated low-end large hail event can't be ruled out. Low LCLs and daytime driven increasing LLCAPE including locally enhanced LL shear along outflow boundaries may cause an isolated funnel/short-lived tornado event. Despite those possibilities, no level area was issued. This activity
decays rapidly after sunset.

... NE Spain ...

As the diurnal driven thermal low over Spain intensifies, a strengthening onshore flow advects a very moist airmass from the far W-Mediterranean towards NE-Spain. Orografic CI is hampered as this area resides beneath two short-waves with temporal mid-level warming and weak subsidence. Steep mid-level lapse rates aloft and the moist upslope flow may still support an isolated thunderstorm event. The environment would support robust/rotating updrafts with 800-1500 J/kg MLCAPE and 15-20 m/s DLS, so large hail is possible. Small level 1 areas were added to highlight that low-end but potential high impact risk. The time-frame with the best chance for one or two storms exists between 15-21 UTC.