European Thunderstorms Summer 2014

Harps wrote: »

Crazy conditions in Germany at the moment, watching the Dusseldorf airport webcam when the camera got blown over, tornado or severe dornburst there at the moment with constant flashing

http://www.dus.com/dus/webcam/

A level 2 and level 3 were issued for Central Italy across the Central Balkans into much of Romania and Bulgaria for severe wind gusts, large hail, tornadoes and to a lesser extent for excessive precipitation.

DISCUSSION

... Italy across the Balkans into Romania and Bulgaria ...

A pronounced Saharan elevated mixed layer moves northeastward, sampled by many of the Tuesday 12z soundings (e.g. Brindisi, Pratica di Mare, Ajaccio, Decimomannu, Trapani, Tunis, Dar El Beida). Its Northern fringes overspread increasingly rich low-level moisture in the range of the frontal boundary. Over Italy and much of the Balkans, surface dewpoints can be expected to rise to around 20C on Wednesday. This results in moderate to high CAPE (1000-2000 J/kg, locally even more) under exceptionally strong 0-6 km shear (25 to 40 m/s) and 0-3 km storm-relative helicity (in large areas higher than 300 m²/s², with simulated maxima up to 600 m²/s²). Large-scale lift in the left exit region of the jet streak overspreads a belt from Northern Italy into the Northern Balkans, Hungary and Romania, while it is mostly absent further South.

A few storms may still be active (or already form) in the morning. More and more storms are forecast to initiate along old outflow boundaries or over orographic features in the course of the day. A quick organization into supercells with large or even extremely large hail and severe downbursts can be expected. Strongly enhanced 0-1 km shear and helicity over a large area, especially from Bosnia into Northern Bulgaria, indicates also a risk of several tornadoes, a few of which may be strong.
Later in the afternoon, storms will likely merge into a squall line that races eastward and poses a primary threat of widespread severe to extreme wind gusts, sustained by a forming Westerly low-level jet (20-25 m/s at 850 hPa). The bordering region of Bulgaria and Romania seems to be at the highest risk of being in the path of such a feature, and despite remaining uncertainties, the exceptional overlap of instability and shear warrant a level 3 for this region. Further convection, often in form of supercells with an ongoing severe weather risk, may continue to initiate all evening long, as outflow boundaries are expected to push Southward into Southern Serbia and Southern Bulgaria. In addition, prolonged periods of heavy precipitation pose a threat of flash floods with larger clusters that form.
Activity will slowly fade overnight as the storms either move onto the Black Sea or encounter a strenghtening cap beneath the elevated mixed layer towards the South.

Further North, storms may be active through much of the forecast period in the vicinity of the frontal zone over Northern Italy, the Northern Balkans and Northern and Central Romania. With an almost front-parallel motion, very high rainfall accumulations and resulting flooding seem to be the dominant risk. Areas of particular concern are the Western and Southern Carpathian mountains in Romania. Current thinking is that this activity will be partly embedded and may decouple from the surface in case it is undercut by too cool air. However, if more discrete storms manage to form and stay surface-based, large hail and severe wind gusts may occur as well.