Storm Forecast
Valid: Thu 09 Jan 2025 06:00 to Fri 10 Jan 2025 06:00 UTC
Issued: Wed 08 Jan 2025 20:51
Forecaster: TUSCHY

A level 1 was issued for parts of S Germany into Hungary mainly for strong to severe gusts and a low-end tornado threat.

SYNOPSIS

A positive tilted trough over S Norway/Sweden points towards NE France and shifts E during the day while its tilt turns more neutral, even positive betimes. This sharp upper trough features an impressive Q vector couplet, indicating deep/intense QG forcing, which overspreads the level 1 area from W to E.
Augmented ageostrophic geopotential flux into the ridge over SW Europe increases positive height anomalies over that part of Europe. This height gradient sustains an healthy 50 m/s upper jet along the SW fringe of the trough, which points towards the Alps.

Elsewhere, a much less dynamic pattern features an upper trough over N Libya, which lifts NE towards the extreme E Mediterranean. Anticyclonic flow dominates SW and far E Europe (into Russia) with a building ridge over far NW Europe.

The mobile trough over CNTRL Europe gets accompanied by a progressive cold front, which shifts from CNTRL France all the way to far E Poland/Hungary until 06Z. This front becomes the main driver for an augmented severe risk.

A broad/diffuse lee cyclogenesis occurs over N Italy but the upper jet core rapidly crosses this vortex, whicht takes it on the anticyclonic flank of the jet. This suppresses further development (including strong synoptic scale subsidence during the overnight hours).


DISCUSSION

... S Germany/far E France into W Hungary ...

The cold front is forecast to organize over far E-France during the late forenoon hours, which then enters CNTRL Germany around noon. Taking cross sections trough this cold front for that time, we see a tight LL theatae gradient, assisting in impressive LL frontogenesis well in excess of 15 K/100km/3h. Further up, the tilted frontal surface becomes less organized. Forecast soundings support a sharp surface front passage up to 825 hPa with a building frontal inversion atop.

Hence a developing narrow cold frontal rain band is forecast and this line runs ahead of the strongest 0-3km shear, wich doubles to 30 m/s in the postfrontal airmass. Hence, it remains unclear, if convection already becomes deep enough for lightning activity and effectve downward transfer. 15 m/s LL shear should already support strong wind gusts.
We added a 50% lightning area, where some increase of the BL moisture depth is forecast, mainly driven by the orography (enhanced advection). Forecast soundings reveal a better chance for thin CAPE reaching into the charging region.

During the afternoon and into the evening hours, the SE ward racing cold front interacts with the enhanced orographic flow along the Alps and we finally see an overlap of rather intense 0-3 km shear along the line. A decreasing LL moisture influx ahead of the front due to te orography should induce a constant decrease of already marginal LLCAPE, so we could see a break up into shorter line segments, as this front moves into SE Bavaria. Despite the loss of structure, a more effective downward transport of the stronger flow aloft should finally increase the risk of severe gusts, as this line crosses S Bavaria and moves into N Austria thereafter (until midnight). A low-end tornado risk can't be ruled out in case of more separated line segments.
Despite low confidence in lightning activity, we issued a low probability area, as not much modification is needed for a few events. Also, past events with marginal BL moisture but intense lift often overperformed to what NWP guidance proposed, so would not be surprised about more lightning activity in the end.

The level 1 was expanded well into W Hungary, where some re-intensification of the line (including lightning probabilities) is forecast due to the interaction with better LL moisture. The severe wind gust risk diminishes betimes.

Right now the peak severe gust risk seems to evolve over extreme SE Bavaria, over the mountainous area of SE Lower Austria and probably into Vienna. No upgrade was performed due to the temporal lull in convective activity with meager BL moisture (but better evaporative cooling).

The level 1 was expanded far N into the Czech Republic mainly for the convectively driven wind risk with deeper but probably mostly non-electrified convection. Again, not much modification is needed for a few thundery events and hence we added a low-prob. lightning area.

... Other lightning areas ...

Numerous areas were highlighted with some non-severe tunderstorm activity. A SE-ward shifting trough over the North Sea could spark transient swirls, but latest data indicates too diffuse LL PV anomalies for any polar low feature (despite cooling upper-levels).
A more focused lightning coverage is forecast over far NW Italy with the evolving LL vortex, but nothing severe is anticipated.