Storm Forecast
Valid: Sun 09 Jun 2019 06:00 to Mon 10 Jun 2019 06:00 UTC
Issued: Sat 08 Jun 2019 22:01
Forecaster: TUSCHY

A level 1 was issued across parts of E-/SE-Europe mainly for large hail, severe wind gusts and excessive rain.

A level 1 was issued for parts of SE-France and NW-Italy mainly for isolated excessive rain.

SYNOPSIS

An aberrant locked wave-train pattern continues with rising geopotential heights over and S of Greenland. Downstream trough is centered over far NW Europe with a steering vortex analyzed just to the NW of Ireland. A branch of lower heights extends over the Faeroe Islands to CNTRL Norway and Sweden with another well structured upper trough extending S over the Bay of Biscay to the Iberian Peninsula. The latter one remains mobile through the forecast but features only marginal zonal displacement.
Downstream ridging covers most of the CNTRL Mediterranean while a sharp upper trough resides further E over W-Turkey.

Locked/stable flow pattern caused unseasonable moist and unstable air to spread far N towards E-Finland into Russia during the past days with more seasonable values further south towards the Black Sea. Moisture content over the W-CNTRL Mediterranean is less favorable as drier continental air mixed south beneath ongoing synoptic-scale subsidence.

The main synoptic-scale fronts are a progressive eastbound moving cold front over NE-Europe and an undulating front over far N/NE Scandinavia into NW-Russia. All those fronts frame an extensive and unstable warm sector over E/NE-Europe.


DISCUSSION

...E-/SE-Europe ...

The extensive warm sector features numerous areas with enhanced thunderstorm probabilities in different thermodynamic/kinematic conditions:

NW Russia into SE Finland:

A N-ward lifting vorticity lobe brushes the thinning NW part of the warm sector. It adds some synoptic-scale lift and enhances frontogenesis temporarily with a strong cross-frontal thermal temperature gradient and with a near normal flow component along the progressive/eastbound moving cold front. Some moisture pooling assists in 800-1500 J/kg MLCAPE (despite meager mid-level lapse rates) along the cold front and the wavy zonally aligned front along the N fringe of the warm sector.

DLS remains weak (mostly less than 10 m/s) and hence disorganized clustering (probably ongoing from the previous night) is forecast with isolated hail and heavy rain. Temporal line-ups are likely over Belarus and to the N as the cold front pushes east, but decoupling from best (northward lifting) forcing results in a deterioration of the convective line. Don't want to rule out a cold pool driven severe thunderstorm event during mature stage of that line however.

Further S/E into the warm sector, CI becomes problematic as flattening ridge atop keeps any source for forcing displaced from 400-800 J/kg MLCAPE. In case of an isolated thunderstorm erupting along the orography, inverted-V profiles point to a severe downburst threat.
Lowering thickness during the night with passing mid-level waves increase the thunderstorm activity in the warm sector but low shear and lowering CAPE preclude anything severe. The general risk is too marginal for a level 1.

Ukraine to Bulgaria:

A moist/warm air mass beneath meager mid-level lapse rates offers widespread 1000-1500 J/kg MLCAPE. DLS increases from less than 10 m/s over the Ukraine to 10-15 m/s over Bulgaria, where tight ridge/trough spacing increase N-erly mid-level flow somewhat. CI is bound to the approaching but gradually decelerating cold front over the Ukraine and elsewhere to the orography or mesoscale boundaries (outflow or sea breeze fronts).
Scanning through forecast soundings, thermodynamic environment supports a large hail and isolated severe (downburst) wind gust threat with initiating thunderstorms. With upscale growing thunderstorm activity, excessive rain and a few severe wind gust reports should become the dominant threat. A broad level 1 will cover that risk.

Dynaric Mountains:

Despite being palced beneath the ridge, spotty CI along the orography can't be ruled out. Main question will be how robust updrafts are due to entrainment of drier environmental air. In case of a stronger updraft high CAPE/weak shear offers an hail and severe downburst threat. Low confidence in CI precludes a level 1 for now.

... Pyrenees to the Alps and surrounding areas ...

The neutral tilt of the leisurely eastbound shifting upper trough puts up an extensive but diffuse baroclinic zone from Spain to SE-France into N-Italy. In addition, onset of foehny conditions along the N-CNTRL Alps blurs the N/S cross-Alpine thermal gradient betimes.
Not much (short) wave activity is forecast at mid-levels but the flow becomes more disturbed at 700 hPa with numerous passing waves. An uptick in upper-level support is expected during the night, as the main upper trough tilts a bit into the negative with ane volving / broadening diffluent/divergent upper flow regime.
Yesterday's southbound rushing cold front (although rebounding N during the forecast as a warm front) cuts off any substantial moisture advection from the S. All ingredients like mid-level lapse rates, moisture, instability and forcing seem to be displaced most of the times. Hence only regionally enhanced thunderstorm probabilities are forecast.

Expect early and scattered disorganized thunderstorms over S-France to spread NE towards NW Italy and Switzerland in a consolidating WAA regime. Despite instances of hail and heavy rain over SE France, nothing severe is forecast.
A rapidly developing confluent low-tropospheric flow regime over S-France with augmented 0-3 km shear indicate an augmented moisture advection regime into the level 1 areas, where back-building /repeatedly developing thunderstorms pose an heavy rainfall threat. Model guidance remains below a level 1 (probably mainly due to modest moisture content) and degree of thunderstorm activity is also a bit uncertain, but we went ahead and issued two small level 1 areas. Would not be surprised to see higher QPF peaks than currently anticipated in an increasingly more favorable heavy rainfall setup. Upslope flow probably increases (thundery) rainfall threat in the eastern eastern level 1 (e.g. parts of the Piemont) during the night.

We expanded the low prob. lightning area far N into Benelux / Germany / NW Poland due to an extensive northward fanning MUCAPE plume. Passing subtle waves may spark a few nocturnal thunderstorms with no severe threat anticipated. Even a few disorganized/elevated nocturnal clusters could evolve along such waves (indicated by a few CAMs) but also along the structuring warm front (especially in the northern part of that lightning area). Confidence in placement is too low for issuing higher lightning probabilities.

On a local scale, conditions for one or two rotating updrafts may emerge over S Upper Austria during the evening. Foehny conditions along the N-CNTRL Alps are accompanied by surface pressure drop and an easterly low-tropospheric flow response along the N-fringes of the Alpine crest. Increasing BL moisture beneath NE-ward fanning Alpine EML plume create up to 1000 J/kg MLCAPE in a 20 m/s DLS environment. Large hail and isolated severe wind gusts are possible. Mixed signals regarding CI preclude an upgrade for now.