Mesoscale Discussion
Valid: Tue 22 Jul 2014 13:00 to Tue 22 Jul 2014 18:00 UTC
Issued: Tue 22 Jul 2014 13:45
Forecaster: PISTOTNIK

On the Northern flank of the Italian cut-off low, a pronounced convergence zone stretches roughly along a line from Regensburg (SE Germany) to Nitra (Slovakia). Moderate Northeasterly winds (5-10 m/s) to the North of this boundary are opposed by a weak Westerly "barrier jet" along the Alps and mostly Southerly winds in the bordering region between Hungary and Slovakia. The resulting convergence zone can be interpreted as an old occlusion which wraps around the low and in whose range a tongue of warm and particularly moist air pushes southwestward again.

12 UTC station observations and soundings showed surface dewpoints around 19°C, almost saturated profiles with a little CAPE (50-300 J/kg), weak vertical wind shear (<10 m/s) and very high precipitable water (35 - 45 mm) in the range of the convergence zone. The situation looks prime for disorganized storm clusters which are capable of producing severe flash floods. Numerous showers and thunderstorms have recently initiated and started to grow upscale. Activity in the Southern Czech Republic looks particularly menacing, as it (1) shows a tendency to merge into several lines which are almost parallel to the flow, and (2) is moving into the bordering regions with SE Germany and Austria which have received 70-110 mm of stratiform precipitation in the past 24 hours.
In addition, localized vorticity maxima along the boundary can be concentrated into a few funnel clouds and possibly a weak non-mesocyclonic tornado. Chances are highest between Linz and Vienna (Austria), where the past few hours have brought some sunny spells and not so much convective activity yet, and where the convergence line is also particularly sharp.

Large clusters of heavy rain and embedded thunderstorms will continue to move WSW-ward into Northern Switzerland, Southern Germany and Austria in the evening hours. Beyond the expiry time of this Mesoscale Discussion (18 UTC), the associated flash flood risk should gradually decrease in the Western part of the highlighted area, while it remains high well into the night in Eastern and Southeastern Austria, where orographic lift will prolong the precipitation from the storms "imported" from Slovakia and Hungary.
Behind the convergence line, gradually drier air from the Northeast starts to reduce the risk of further storms.