Storm Forecast

Storm Forecast
Valid: Wed 03 Sep 2014 06:00 to Thu 04 Sep 2014 06:00 UTC
Issued: Tue 02 Sep 2014 20:51
Forecaster: PISTOTNIK

A level 1 and level 2 were issued for the Aegean Sea, E Greece, NW Turkey, Bulgaria and SE Romania mainly for excessive precipitation, tornadoes, and to a lesser extent for large hail and severe wind gusts.

A level 1 and level 2 were issued for parts of the Balkans, Greece, the Adriatic Sea and Italy mainly for waterspouts and excessive precipitation.

SYNOPSIS

A stationary cut-off low over the Adriatic Sea and an extensive high over North-Central Europe introduce another persistent blocking situation. The main frontal zone is deflected far to the North and only affects Northern Scandinavia.
Hot conditions are confined to Iberia and Turkey. Substantial CAPE can develop where the steep lapse rates of this hot air partly overlap with seasonally rich low-level moisture from the Black and Mediterranean Seas.
The advection of warm and moist air from the East returns into Romania and East-Central Europe and allows limited CAPE buildup in response to daytime heating in these areas as well. Otherwise, moist and rather cool air over much of the continent gets slowly reheated under anticyclonic influence.

DISCUSSION

... SE quadrant of Europe ...

Large-scale lift associated with warm air advection and various travelling short-wave troughs overspreads the warm and moist air from the Aegean Sea into NW Turkey, the Black Sea, Bulgaria and SE Romania, which can provide CAPE on the order of 1000 J/kg or more (refer to Friday 12 UTC's Athens and Istanbul soundings). Besides, enhanced deep-layer shear (10-20 m/s) and 0-3 km storm-relative helicity (up to 250 m^2/s^2) are forecast to be in place, maximized near the Bulgarian-Romanian border.
Scattered to widespread thunderstorms can organize into multicells, supercells and large clusters. The abundant low-level moisture, with dewpoints above 20C in coastal areas, indicates that flash floods and tornadoes (especially over warer) will be the main risk. In addition, localized large hail and marginally severe wind gusts are possible with discrete and particularly well-organized storms, though low cloud bases will limit these threats.

At the Northern and Western flank of the cut-off low, scattered showers and thunderstorms will form under low to moderate CAPE and weak shear over the rest of Romania, the Balkans, the Adriatic Sea and Italy, plus to a lesser extent in Hungary, Slovakia and Austria. This activity could partly be embedded in warm air advection cloudiness. With precipitable water between 30 and 40 mm and slow storm motion, localized heavy rainfall is possible. The Italian coastlines may see a number of waterspouts overnight, especially the Adriatic coast between Manfredonia and Otranto where the center of the surface cyclone lingers around.

Storms activity will exhibit characteristic diurnal cycles with an afternoon and evening maximum over land and night/morning maxima over sea.

... Spain ...

Scattered high-based afternoon storms are expected over mountains across Northern Spain. In the inland areas, deep and dry sub-cloud layers could favor isolated microbursts.
Better Mediterranean moisture is advected onshore along the East coast, where latest observations indicate dewpoints around 22C. The sea breeze regime will feature plentiful CAPE on the order of 1000 J/kg and enhanced 0-3 km shear around 15 m/s. If this moisture proceeds far enough inland to be fed into upslope circulations, a few well-organized storms with a risk of isolated large hail and severe wind gusts may result. However, current thinking is that the capping inversion will be too strong to allow an involvement of this moist air into inland storms. Hence no level 1 is issued.

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