Extended Forecast
Valid: Wed 31 Jul 2024 06:00 to Thu 01 Aug 2024 06:00 UTC
Issued: Mon 29 Jul 2024 15:52
Forecaster: TUSCHY

A level 1 was issued across parts of France, Belgium, Luxembourg, SW Germany, W Austria, NW Italy and Switzerland mainly for heavy/extreme rain, large to isolated very large hail and strong to severe downburst events.


SYNOPSIS

There is no major change on the synoptic stage compared to the severe weather outlook day 1 (SWODY 1). A persistent subtropical ridge extends from Spain to the northern Balkans with the highest 500 hPa height anomalies down in E Spain/far W Mediterranean and Morocco/NW Algeria.

With weakening heights over CNTRL Europe, a low-amplitude wave, initially over NE France/Belgium, slides E into Germany and further SE into Austria during the night. This wave flattens the mid-level flow regime so we end the day with a more zonal flow pattern over CNTRL Europe (although with an anticyclonic touch).

Low height anomalies persist from Iceland/S Greenland to the Norwegian Sea to W Russia into the E Mediterranean.

Once again, no major frontal boundary affects DMC activity. This will be driven mainly by the low-amplitude wave itself and mesoscale forcing/the orography.


DISCUSSION

... W-CNTRL Europe ...

Similar to the SWODY 1, this forecast period is shaped by unusual moist conditions throughout most of the troposphere. TPWs oscillate around 40-50 mm with peak values from E France to W Germany. Once again, this equals a PW anomaly in the 2-4 sigma range.

Along the surface, NWP guidance continues to show a zonally aligned pressure channel, which runs from NW to NE France and into Belgium/Luxembourg and W Germany. Very rich BL moisture with mixing ratios between 14-16 g/kg accumulate in this channel with BL moisture input probably also caused by ongoing positive SST anomalies in the Atlantic/Mediterranean. A grazing Spanish/North African EML plume pushes MUCAPE once again in the 2000-3000 J/kg range, with higher peaks on the regional scale. EFI-CAPE peaks with SOT values in the 2+ range over the mentioned area.

DLS will be in the 10 m/s range with 5-10 m/s values in the lowest 3 km AGL.

Capping issues decrease as temperature values along the inversion drop a couple degree Celsius during the passage of the low-amplitude wave.

CI is a mess and remains one until the nowcast. The main uncertainty will be how the convection evolves from SWODY 1 and if a residual MCV impacts the forecast (including potential laid out outflow boundries). If we just take the timing of the mid-level wave, a high-coverage thunderstorm event could unfold for E France into W/CNTRL Germany.

Initiating convection poses a risk of large to very large hail (temporary) and extreme rainfall rates (probably in the 40-60 l/qm/h range) with flash flooding possible on the local scale. Betimes with clustering and outflow dominant convection, the risk turns to heavy rain, isolated hail and strong to isolated severe gusts. As a side-note the lightning activity could become exceptional high with this CAPE/moisture distribution.

Right now a broad level 1 should be the best approach for this event. If a more focused corridor of severe can be worked out tomorrow, a regional upgrade could become necessary. This upgrade would be driven by rain and hail.

... Other lightning areas ...

Mostly sub-severe convection is forecast over N Europe into Russia.

A belt from the Stockholm/Uppsala area to Estonia, E Latvia into the Pskovskaya Oblast could see a regional upgrade in later updates in case the more bullish GFS/ICON forecasts verify. Slow moving storms could bring heavy rain and isolated hail with GFS even showing a time window for more organized convection over Estonia. NWP guidance remains too scattered with height geometry and CAPE distribution so now upgrade for now.

The convection along the E Black Sea could produce some healthy rainfall rates but more on an isolated scale and partially offshore.