Storm Forecast
Valid: Thu 21 Dec 2023 06:00 to Fri 22 Dec 2023 06:00 UTC
Issued: Thu 21 Dec 2023 04:04
Forecaster: TUSCHY

A level 1 was issued across parts of S Turkey mainly for heavy rain.

A level 1 was issued for parts of CNTRL Europe mainly for severe wind gusts and a few tornadoes.

SYNOPTIC DISCUSSION

Phasing of an anomalous deep trough crossing S Sweden into Poland during the forecast with a gradually weakening but still stout subtropical ridge over the NE Atlantic cause an intense mid/upper jet to point towards CNTRL Europe. Accompanying fronts cross this area and add a convective risk for a rather broad area.

Another wave affects SE Europe and causes a second focus for thunderstorm development.


DISCUSSION

... S Turkey ...

A lifting/filling upper wave crosses the area from SW during the forecast with a flattening mid/upper flow regime. Background lift on the synoptic-scale with persistent onshore flow ahead of an accompanying surface low push seasonable moisture ashore. Weak cap and persistent lift (also along the orography) result in messy but widespread convection with an eastbound moving cluster beneath coldest mid-levels, where DLS relaxes. Heavy rain remains the main hazard although an isolated coastal spout event can't be ruled out, where better LL thermodynamic profiles exist. The risk ends from W to E.

... CNTRL Europe ...

A dynamical forced shallow convective event unfolds from NW Germany to E Austria during the forecast. Synthetic WV imagery and forecast soundings including numerical IPV advection show a SE ward cutting dry slot, which intersects the SE-ward rushing occluding cold front. Placed beneath the left exit of a powerful 150 kt upper jet (3 sigma deviation from background climatology) abundant forcing exists out from the synoptic but also from some LL convergence along the SEward pushing surface front.

The general setup is not textbook-like with a somewhat softened thermal gradient at 0-2 km AGL including a low tropospheric wind max stapled along the S fringe of the LL depression and hence betimes well behind the progressive surface front. 0-3 km CAPE including prefrontal moisture is marginal with only patches of 100-150 J/kg CAPE and a rather diffuse cross-thermal T2m gradient. However, broken line structure in a setup with extreme kinematics and long/slightly curved hodographs is expected and hence a few instances with deviating storm mode or local surge events along the front cannot be ruled out. Therefore an isolated tornado risk exists. We kept the level 1 along the axis of strongest isallobaric response. We also cannot rule out some upscale growth into a NCFR betimes especially in the E part of the level 1 with better background orientation between flow/front.

Despite the isolated tornado risk, effective downward mixing should bring severe wind gusts down to the surface within confined swaths. Weak thermodynamics preclude an upgrade for longer lived/more robust updrafts with a higher confidence in more widespread downward mixing.