Mesoscale Discussion
Valid: Sun 20 Jul 2014 14:00 to Sun 20 Jul 2014 18:00 UTC
Issued: Sun 20 Jul 2014 14:18
Forecaster: PISTOTNIK

... Austria (E of Innsbruck) ...

Latest surface observations show exceptionally high dewpoints close to 20°C in the Alpine area of Austria and Northern Italy, likely yielding CAPE values between 1000 and 2000 J/kg or locally even more. Such a CAPE magnitude is extraordinary over mountainous terrain, where low-level moisture is usually mixed out quickly in response to daytime heating. Obviously the capping inversion has largely suppressed these vertical mixing processes today.
A cluster of intense storms has already formed in the bordering region between Bavaria and Western Austria, likely including an embedded supercell at its Southern flank. Another probable right-moving supercell has tracked from the Italian Dolomites region to Heiligenblut in Southern Austria, showing no signs of weakening yet. These storms can likely benefit from deep-layer shear around 20 m/s, more than the forecast models had suggested. The 12 UTC soundings from Cuneo and Milano (Italy) showed Southwesterly winds even around 30 m/s at 600 hPa, though it is questionable in how far the nose of this jet streak can catch up with the storms further Northeast.

Activity will continue to move Northeastward into larger parts of Austria, and new storms will likely initiate over orographic features ahead of it (where towering cumulus clouds are already visible on satellite imagery) or along its gust fronts. There is a pronounced risk of large hail, which might even exceed 5 cm in size locally. Heavy rainfall and localized severe downbursts pose additional threats.
Low-level moisture is similarly rich over the adjacent forelands, but the capping inversion will likely quench the majority of storms as soon as they leave the mountains (refer to the 12 UTC Vienna sounding). However, if a large MCS forms, it may survive some distance over flat terrain as well with an ongoing severe weather risk.

... E Switzerland, W Austria, S Germany ...

The current surge of storms has probably been fostered by a vorticity maximum which quickly translates northeastward. In its wake, temporary large-scale subsidence and increasing Southwesterly flow can result in a burst of South Foehn with significant drying in Eastern Switzerland, Western Austria and Southern Germany. There is no indication in observational data yet but limited-area models uniformly show such a scenario, which would decrease the risk of further storms.