Extended Forecast
Valid: Fri 20 May 2022 06:00 to Sat 21 May 2022 06:00 UTC
Issued: Wed 18 May 2022 19:16
Forecaster: TUSCHY

A level 2 was issued for parts of NE France, Benelux, Germany to the Czech Republic mainly for all kind of hazards (possibly significant) but predominantly for severe wind gusts.

A level 1 surrounds the level 2 for similar hazards but with less coverage.

SYNOPSIS

Initial wavy flow pattern over Europe transforms into a more zonally aligned pattern as a cylonic vortex south of Iceland fills while strengthening ridging over CNTRL Europe moves S/SW. The upper trough (with the filling low attached) breaks up into numerous pieces with one piece translating E as a robust short wave. This wave crosses the English Channel until noon and affects N Germany during the overnight hours. Favorable upper dynamics (including a coupled jet) and interaction with a baroclinic zone over CNTRL Europe support constant strengthening of this wave especially over Benelux to the E. This is highlighted by elongated irrotational wind vectors at upper tropospheric levels (divergent upper-level outflow) downstream of a hook-shaped PV tongue. Resulting mass response causes lowering pressure down to the surface with most models now indicating LL cyclogenesis from Benelux E/NE.
There is not much change over E Europe with a quasi-stationary/ blocking trough still in place.

At the surface the rige gets framed by a wavy synoptic-scale front, which runs from N Spain to N France into N Germany and then E/NE to the Baltic States. Interaction of the short wave trough with that synoptic front adds enough forcing/lift for CI.

The surface response in all models converges to a developing depression over Benelux, which enters N Germany during the evening hours before moving E towards NE Germany until 00Z. Numerical guidance offers similar MSLP minima in the 1006-1010 hPa range. IFS-ENS has a zonally elongated spread of the potential track of this depression but with minimal intensity spread. The same for the synoptic front with overall good clustering of all the members. The developing depression along this boudnary results in a rapidly expanding warm sector from E France into the Czech Republic, which gets framed by a lifting warm front and a progressive cold front from the W.

Numerical guidance has an overall good handling on this set-up. While there are some differences in the exact track of the surface low and its intensity, models agree on the scenario.
Past 8 ICON runs had some differences in speed and amplitude of this wave which is not of surprise given ongoing uncertainties of the break up process of the upper trough and how fast the energy transfers E. Nevertheless, the past runs show a compact wave approaching Benelux/N Germany from the W. Similar uncertainties also exists with GFS/IFS in the past 5 runs. However, neglecting those timing issues, all models agree in a compact and intensifying/dynamic wave.

Summing up, a dynamic wave interacts with a wavy baroclinic zone. Attendant wind field overspreads an unstable warm sector in the highlighted area and hence organized convection is likely (also highlighted by anomalous high EFI CAPE/shear and IWV flux values). Main uncertainties remain the timing of this wave and the final track of the LL depression.

DISCUSSION

Not much technical discussuion that far out because convection from the previous night, mesoscale pressure/frontal features and aforementioned uncertainties (timing and track) still cast uncertainties where to place the highest severe risk.

Convection before noon should be confined to the lifting warm front (CNTRL Germany into Czechia) and the structuring cold front (NE France) with CAPE/shear space already supportive of severe wind gusts/large hail and heavy rain. An isolated tornado event is possible, especially over NE France.

Beyond noon, CAPE/shear improves with an impressive kinematic part.
Convection along the lifting warm front over CNTRL Germany could grow uspcale with a bowing segment moving into Czechia/SW Poland before outrunning best parameter space. This scenario is still uncertain and depends on how much WLA convection finally materializes.

Another focus resides along the cold front from the forenoon hours, which approaches the warm sector over Benelux into N/CNTRL Germany. Near normal aligned 25-30 m/s 0-3km AGL shear vectors pointing into clustering convection (due to strong linear forcing) with a 30 m/s 700 hPa jet approaching from the W and all this combined with 1 kJ/kg MLCAPE give an augmented chance for a derecho event (despite rather humid stratification of the low/mid troposphere but with rather steep LL lapse rates towards CNTRL Germany). Severe wind gusts (including extreme events) are likely.

The tornado risk depends on the final strength of the LL depression and how the BL wind field responds. However from noon onwards, any more discrete cell will be capable to rotate (mid-levels) with large/very large hail and severe wind gusts. A tornado risk is present, especially with latest GFS runs (although convective feedback problem may push this depression too much). Lowered LCLs along the front may support a (potential strong) tornado event within the area from Benelux to NW/N-CNTRL Germany. However it remains still unclear how long convection stays discrete. In addition, any tail-end charlie may pose a similar threat along lined-up storms.

Further N any severe risk ultimatively depends on the track/intensity of the depression. CAPE/shear space remains supportive of organized convection.

The level 2 was placed where current confidence is highest for CI and attendant severe, including ENS data. Certainly further refinements will become necessary with mentioned uncertainties.

Please note: strong to severe non-convective wind gusts are possible upstream of the LL depression from NE Germany into N Poland. This event won't be covered by our levels.