Extended Forecast
Valid: Mon 10 Jun 2019 06:00 to Tue 11 Jun 2019 06:00 UTC
Issued: Sun 09 Jun 2019 02:18
Forecaster: TUSCHY

A level 2 was issued across parts of Germany, the N-Czech Republic and NW-Poland mainly for severe/damaging wind gusts, large to very large hail, excessive rain and a tornado threat.

A level 1 surrounds the level 2 mainly for similar hazards with lower probabilities.

A level 2 was issued across parts of NW-Italy mainly for severe/damaging wind gusts, large to very large hail, excessive rain and a tornado threat.

A level 1 surrounds the level 2 mainly for similar hazards with lower probabilities.

A level 1 was issued across Benelux and far NW-Germany mainly for isolated large hail.

A level 1 was issued across parts of S/E-Romania mainly for large hail.

SYNOPSIS

A stable geopotential height field features an extensive upper trough over W/NW-Europe with broad/NE-ward building ridging over the CNTRL Mediterranean into E-Europe. Between both protagonists, a broad SW-erly flow regime stretches from the Iberian Peninsula to CNTRL-Germany. A first piece of energy along the E fringe of the main trough (the negative tilted, deamplifying and gradually lifting upper wave in the SWODY-1 discussion) lifts N towards W-CNTRL Europe with a second impulse from Ireland entering the base of the main trough during the night. This keeps the broad upper trough more or less in place during the extended forecast range, assisting in a prolonged advection regime of a moist/warm air mass towards CNTRL-Europe. Trajectories east of the trough emerge from N-Algeria with an EML plume fanning N towards the Alps (with additional adiabatic warming N of the Alps due to foehny conditions and hence an additional increase of mid-level lapse rates).

Further north over Scandinavia into N-Russia, a progressive flow regime pushes a cold front rapidly and a zonally (near parallel to the background flow) aligned and wavy boundary slowly to the east during the forecast period. A decelerating cold front over E-CNTRL Europe turns into a structuring and gradually N-ward panning warm front over CNTRL-Europe. Its warm sector moistens rapidly with influx of rich/deep low-tropospheric moisture from SE-Europe. The intersecting tip of the N-ward fanning EML with the BL moisture creates an extensive plume with substantial CAPE over parts of CNTRL-Europe.

DISCUSSION

... Germany, parts of the Czech-Republic into NW-Poland ...

Yesterday's mentioned rather disorganized baroclinic zone in the SWODY-1 discussion becomes much better defined in response to the strengthening foehn along the N-Alps and turns into a leisurely N-ward lifting warm front. On the one hand, this plume of adiabatically warmed air results in a strong 750-850 hPa cap with a loaded gun situation due to 10 g/kg (plus) mixed BL mixing ratios beneath steep mid-level lapse rates and capped MLCAPE in the 1.5 to 3.5 kJ/kg range. On the other hand, the improving baroclinic conditions along the periphery of the adiabatically warmed air with enhanced low-tropospheric convergence plus falling surface pressure beneath the foehny air mass induce a constant increase of LL forcing - maximized along the W/N fringes of the CAPE plume.

Northward lifting piece of mid-level energy, numerous subtle low to mid-tropospheric waves and the improving LL forcing increase confidence in scattered CI along the baroclinic zone and along the apex of the impressive CAPE/thetae/TPW plume (N-Czech Republic to E-Germany) during the late afternoon onwards.

The wind field, attending the baroclinic zone/structuring and N-ward lifting warm front features enhanced 6 km and 3 km bulk shear (15-20 m/s and 15 m/s respectively). In addition uncontaminated/pre-convective forecast soundings reveal gradually improving directional shear with curved hodographs up to 3 km (but with some weakeness aloft in the 3-6 km layer).

Given subtle synoptic-scale forcing, initially more discrete CI is likely with organized multicells/supercells forecast, especially in the level 2 area. Large/very large hail and severe wind gusts are possible. LCLs drop rapidly below 1km AGL next to the warm front and any storm riding that boundary could also produce a tornado.
Near parallel alignement of shear/storm motion vectors hint at rapid upscale growth into numerous organized clusters, which can produce swaths of severe to damaging wind gusts, large hail and excessive rain. We expanded the level 2 far north accordingly but also quite far E to account for convection building more aggressively into the CAPE plume (compared to what latest models indicate).

Can't exclude a few orographically induced updrafts down to NE Austria and despite still limited support by numerical guidance we already upgraded to a level 1. In addition, a few LAMs inidcate a weakness of CIN in this area, which increases confidence in at least isolated CI. In case of further rising confidence an upgrade to a level 2 may be considered in later outlooks due to the very unstable environment. Also a broadening of the level areas may be considered in this area due to cold pool driven convection with large/significant hail and severe wind gusts.

... N-Italy ...

An approaching mid-level short-wave and diffluent/divergent upper flow regime plus some descending (foehny) air behind the N Apennines result in falling surface pressure along the Po valley and in responding ageostrophic E-erly low-tropospheric flow. Advection of rich moisture beneath steep mid-level lapse rates creates MLCAPE in the 1-2.5 kJ/kg range. DLS of 20-25 m/s and SRH-3 in excess of 250-300 m^2/s^2 complete the favorable setup for organized DMC activity. Latest idea is initiating convection from W to E in the level 2 area with long tracked supercells possible. Large to very large hail, severe wind gusts and excessive rain are forecast. The tornado risk, although mitigated a bit by LCLs around 1 km AGL, can't be excluded - especially towards the evening, as the BL cools down with decreasing LCL heights.
Capping could be an issue and hence issued a tight southern level gradient.

An excessive rainfall event unfolds during this forecast along the Swiss/Italian border (e.g. N-Piemont) and was highlighted by a northward expansion of the level 2.

... Benelux and NW Germany...

The approaching de-amplifying upper trough affects the area of interest during the afternoon hours and hence during peak diabatic heating. Despite rather thick cloud cover, models break up the clouds during the afternoon with temporal insolation anticipated. A plume of moist air from the E and some diabatic heating should push afternoon SBCAPE in the 400-800 J/kg range. DLS in the 20 m/s range and somewhat augmented SRH-3 next to the synoptic-scale front indicate an isolated large hail threat with temporarily organized convection.
Of interest is the bimodal QPF of the numerical guidance. Global models agree in CI with clustering onvection whereas most of the LAMs keep CI on the isolated scale. However with the dynamic nature and the agreement in CAPE during most of the models we added a level 1 for now.

... Romania, Bulgaria and Turkey ...

An upper trough with cold mid-levels atop a moist and warm BL air mass assists in a broad area with augmented CAPE. Most areas see 500-1000 J/kg MLCAPE with S/E-Romania probably exceeding 1500 J/kg due to the highest BL moisture content(moisture pooling along the S/E fringes of the Carpathian Mountains). Passing subtle mid-level waves and the orography spark numerous thunderstorms with isolated hail and gusty winds in a weak DLS environment. S/E Romania were upgraded due to the thermodynamic setup. Initiating thunderstorms can produce large hail. During the evening and night, disorganized clusters bring heavy rain and gusty winds to the highlighted area with a gradual decline in severity.

... NE-Spain ...

During the night, onshore flow of a marine air mass beneath cold mid-levels creates regionally enhanced SBCAPE in the 400-800 J/kg range. 20 m/s DLS and enhanced directional shear indicate an environment supportive of a few organized multicells with large hail and severe wind gusts. An southeastward expanding cluster could evolve into the warm/unstable inflow from the SE. An upgrade could become necessary in the next update cycle but confidence in severity is still too low for a level 1 (rapid clustering and veer-back-veer profiles in forecast hodographs).

...SE-UK ...

A convectively enhanced heavy rainfall event is possible within the tip of the westward wrapping occlusion/plume of unstable air. However uncertainties remain high with latest guidance showing barely any CAPE to work with. Active convection upstream could also impact moisture advection. Nevertheless heavy rain affects SE UK and right now the low prob. lightning area was expanded into this area.