Mesoscale Discussion

Mesoscale Discussion
Valid: Mon 10 Jun 2019 11:00 to Mon 10 Jun 2019 14:00 UTC
Issued: Mon 10 Jun 2019 11:30
Forecaster: PISTOTNIK

This Mesoscale Discussion (MD) is issued to describe latest developments of the pre-convective environment across central Europe according to latest observational data and its implications for the possible imminent severe weather outbreak.

Mid-level clouds and scattered elevated convection are moving NNE-ward in the strong warm air advection regime from Switzerland to SW Germany, N Bohemia and E Germany, i.e. across the northwestern half of the highlighted area. This elevated convection is intensifying in Switzerland, where a band of active thunderstorms (with a risk of high rain accumulations due to its almost flow-parallel orientation) is moving in from Italy and a first leading storm has also just crossed the border to Germany at Lake Constance. Current thinking is that this activity will continue to move into SW Germany in the next few hours, but will - at least preliminarily - stay elevated due to cool near-surface air (2m temperature/dewpoints values around 18/14C at 10 UTC and only gradually rising).
Previous elevated convection over Germany is gradually breaking up or departing to the north, and the boundary layer is recovering under increasing insolation. While the ECMWF and GFS forecasts of 2m temperature seem to be largely on track, forecasted 2m dewpoints are 2-3K too high until 10 UTC near the nose of the warm air advection regime from central Germany into NW Poland, indicating yet more adverse conditions for surface-based convection. Nonetheless it is not ruled out that additional elevated thunderstorms will from from central to E and NE Germany already in the next few hours.
Though this described activity could already organize well under strong vertical wind shear, its severe weather risk remains low as long as it does not root down to the surface, which is not anticipated yet until 14 UTC.

Further east in the eastern parts of Austria and the Czech Republic, warm air advection is almost completed with 2m temperatures approaching 30C and 2m dewpoints of 17-20C, confirming or even slightly surpassing the model forecasts. It appears likely that the aggressive model forecasts with CAPE on the order of 2000-3500 J/kg are indeed about to materialize. However, capping is very strong, and even in case the cap can once be broken, entrainment from rather dry environmental air at mid-level can make it difficult to sustain updrafts.
The surface wind field indicates that shallow, thermally direct circulations have developed across the broad temperature gradient. A leading surge of westerly winds has reached a line from Prague to Salzburg by 10 UTC, and a second, slightly stronger surge with wind speeds around 5 m/s has just passed Munich. Together with a dryline along the northern Alps, they form an L-shaped zone of fairly pronounced convergence that engulfs SE Bavaria, N Austria and Bohemia and should be monitored for possible convective initiation. Though vertical wind shear is weaker than further west and north (0-6 km shar ~15 m/s), any storm that forms can soon turn severe with a high risk of large hail and severe downbursts owing to abundant CAPE.

All in all, there seem to be hints that the most severe convective activity beyond the validity of this MD might affect Bohemia rather than Germany, though later data need to be awaited to confirm or reject these trends. We will try to cover the further development with new MDs as our time and the information content of observationsal data permit.

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