Schweizer Sturmforum

... Far E France, Switzerland into Austria and Germany and Benelux ...

The general pattern is favorable for an active thunderstorm day over CNTRL Europe. Diffluent steering flow turns more divergent during the day with improving jet configuration in a weakly/moderately capped air mass. (Sub)tropical history features TPWs in the 30-40 mm range with slack mid-level lapse rates. Squeezed height field between the French/English vortex and a NE-ward expanding subtropical ridge over Algeria causes a strong SW-erly wind field, pushing kinematics in the range supportive for better organized updrafts from E-France/Switzerland into Germany/W-Austria (0-6 km 15-20 m/s and 0-3 km 10-15 m/s, decreasing towards Benelux).

Current thoughts are that residual convection from the previous night with a diurnal weakness shifts from NE France into Germany until noon and is bound to the surface pressure channel (still able to produce a few heavy rainfall events over Belgium, Luxembourg into far W Germany). Drier BL air filters in from the SE and should suppress CI until it approaches NW/N-CNTRL Germany during the afternoon hours (then producing slow moving convection with rain issues). TPWs increase to 40 mm in its wake.

Another push of WAA is noted from France ahead of the steering depression, which spills rich BL moisture with Tds in the upper 10s/lower 20s into Luxembourg and W/SW Germany while TPWs decrease to 30-35mm - caused by drier air at mid-levels entering the scene from the SW, increasing potential instability (with weak/modest QG forcing from the SW) and offering the highest MLCAPE values of the day in these areas (1000-2000 J/kg from Belgium/Luxembourg to S Germany during the afternoon/evening).

Expect scattered CI beyond noon with organized multicells producing hail and healthy rainfall rates (probably a mixed mode). Current focus for CI resides along the second WAA feature/LL confluence zone and hence over NE France into SW/CNTRL Germany. Of concern is augmented 0-3km shear, which strengthens until the evening with some better curved hodographs in that layer. Hence, better streamwise vorticity for deviating cells increases the risk for rotating updraft with large hail/severe wind gusts. An isolated tornado is certainly possible in case a more discrete cell evolves in this low LCL setup. In case of upscale growth, bowing segments with strong/severe gusts are possible, too. Current focus for this enhanced wind gust/isolated tornado risk resides from N-Switzerland to SW/CNTRL Germany, but this could still change with new data.

Heavy to excessive rain is an issue everywhere especially with merging cells and initiating convection along the orography.

A lull in activity seems to become established towards W Germany into Belgium with competing forcing mechanisms. Overall I do not trust the very reluctant models as forecast soundings still show weak/non-existent capping and some diurnal heating. Subsiding synoptic conditions including temporal mid-level CAA seem to suppress CI in model fields, but this weakens towards the late afternoon hours with an approaching wave from the SW. In addition, the orography should do its job and hence we added this whole area into a level 1 mainly for excessive rain with slow moving cells (with an isolated hail risk in strongest updrafts).

The activity weakens during the night with some flare up over the S North Sea, where a nocturnal elevated cluster may evolve.

A level 2 was issued for Switzerland to S Germany into Austria and extreme N-Italy mainly for large/damaging hail, severe wind gusts and heavy rain.

Michael (Seon) hat geschrieben: ↑Fr 23. Jul 2021, 16:59 Estofex nun mit einem Level 2 für Samstag:
https://www.estofex.org/cgi-bin/polygon ... recast.xml