Extended Forecast
Valid: Thu 16 May 2024 06:00 to Fri 17 May 2024 06:00 UTC
Issued: Tue 14 May 2024 21:46
Forecaster: TUSCHY

A level 2 was issued for S Germany mainly for excessive rain.

A level 2 was issued for parts of NE Italy but also for parts of Croatia into SW Hungary/NW Serbia mainly for large hail, severe wind gusts, excessive rain and an isolated tornado risk.

A broad level 1 area surrounds the level 2 areas for a similar risk with lower probabilities.


SYNOPSIS

There is not much change to the previous SWODY 1 discussion mainly due to the static nature of the driving synoptic troughs/ridges.

The main change will be a more zonally aligned upper cut-off configuration over W-Europe, which builds into CNTRL Europe. This change in the cut-off's geometry drives the upper jet more towards the Alps and induces also better defined surface votices over N-Italy and S Germany.

This general scenario is well anchored in latest 4-6 NPW runs, which all show the general idea of lowering surface pressure from Bavaria towards N-Italy. Ongoing uncertainties exist, where to place the dominating LL vortices within this broad channel of lowered surface pressure. Latest IFS-ENS however has growing confidence in a confined LL vortex over S-CNTRL Germany with some ongoing zonal spread. The spread increases a bit towards Friday as this vortex lifts N/NW towards CNTRL Germany.
Further S over N-Italy, there is no clear signal of a dominating LL vortex so confidence there remains low right now.

DISCUSSION

... N-Italy ...

Abundant synoptic-scale forcing combined with CI along the orogaphy, residual outflow boundaries and moisture flux convergence zones but also with diurnal T2m increasing atop the convective temperature all pave the way for a messy but very active convective day.

Background kinematics are well in the range for organized multicells/a few supercells with DLS aoa 15-20 m/s. No real EML overspreads the rich marine BL air mass, so MUCAPE remains in the 800-1500 J/kg range. Only temporal diabatic heating is forecast but this is adequate for CAPE build-up and CI.

Ongoing DMC activity from the night is forecast over Venetio into Trieste area with excessive rain/an isolated tornado risk the main threat. This on/off risk continues during the day with any more discrete storm, which could bring severe hazards well inland.

Further W, the main show probably awaits the structuring upper trough axis, including the diffuse LL vortex. Widespread CI should limit long-lived discrete storm activity but CAPE/shear space is supportive for temporal supercell activity with all kind of hazards before growing upscale into bowing segments. NPW guidance agrees in enough forcing for an extensive MCS event to affect the N Adriatic Sea/NE Italy during the late afternoon/evening with heavy rain the main risk. Weakish 0-3 km shear and backed flow in the lowest 1-3km AGL lowers the gust and tornado risk but an isolated instance is still possible.

Due to the messy nature only a broad level 1 was issued for now, expect for the area with enhanced probabilities for a few longer-lived discrete supercells with all kind of hazards.

... Croatia into Hungary and NW Serbia ...

Another area of augmented concern exists from parts of Croatia into SW Hungary/NW Serbia, where 30 m/s DLS with elongated/looped hodographs overlap with aoa 1000 J/kg MUCAPE. A few more discrete supercells are possible due to weaker background forcing. The risk includes large hail, severe wind gusts and a tornado threat. This activity spreads NE/E and outruns best CAPE tongue betimes. Another level 2 was issued for this event.

... W/S Germany ...

Another round of scattered to widespread CI is anticipated along an extensive LL moisture flux zone, where BL moisture resides around 10 g/kg. Initiating storms once again have a limited time frame for becoming better organized with favorable LL inflow into growing updrafts. Collapsing SR flow aloft however supports growing cold pools betimes, with erraticly spreading small-scale clusters (in general a northward shift is forecast). Isolated hail/ a short tornado event rapidly turns into a more dominant heavy to excessive rain threat, especially with interacting clusters but also along the orography. There is a concern for clusters which move NE over SE Germany to take profit of the enhanced LL SR inflow, which could increase the flash flood risk due to a some training/a slow down of forward motion. Latest EPS data is already concering for numerous excessive and potential flash flood producing rainfall events.

Another area of enhanced flash flood risk exists from far W Germany into Belgium/Luxembourg along the periphery of a transient but closed mid-level vortex, which shifts W/NW. More organized bands of DMC activity could increase the flash flood risk on a regional scale but confidence is not yet high enough for a level 2 upgrade especially as convection to the SE could limit moisture advection into this risk area.

During the evening/overnight hours, the focus turns to far S Germany as the LL vortex is about to evolve. Model guidance is rather persistent in highlighting a confined region over W Bavaria into extreme E Baden-Wuerttemberg, along the W fringe of a Okubo Weiss maximum (centered over CNTRL/E Bavaria) with elongated dilatation axes espcially within its evolving deformation zone.
This kind of configuration combined with expected moist/unstable airmass and strong/deep SR inflow into growing clusters with embedded thunderstorm activity is well known for producing swaths of excessive rainfall. In fact latest model data already indicates 50-100 l/qm within this 12h time-frame. Keeping clustered ENS member of this vortex in mind, confidence is high enough for a flash-flood driven level 2 event (combined with the diurnal risk and admitting a nocturnal decrease of thunderstorm activity in this area).
Beyond 06Z this risk spreads NW along the periphery of the weak LL vortex.