Storm Forecast
Valid: Sun 11 Aug 2019 06:00 to Mon 12 Aug 2019 06:00 UTC
Issued: Sat 10 Aug 2019 21:55
Forecaster: TUSCHY

A level 2 was issued for NE-Spain mainly for large hail, severe wind gusts and excessive rain. An isolated tornado is possible.

A level 2 was issued for NW-Italy/SE-Switzerland mainly for large to very large hail, severe wind gusts and excessive rain. An isolated tornado risk is possible.

A level 1 surrounds both level 2 areas with similar hazards but lower probabilities.

A level 1 was issued for Slovakia to far N-Romania mainly for large hail and severe wind gusts.


SYNOPSIS

A deep trough is placed over Ireland/Bay of Biscay with broad downstream ridging in place.
A wavy surface front is analyzed from N-Morocco to the Alps and further to the NE. This front serves as main focus for organized DMC activity.


DISCUSSION

... NW-Italy, Switzerland, SE-Germany into W-Czech-Republic ...

The wavy synoptic-scale front over Switzerland transforms into a leisurely northward lifting warm front over Switzerland and far S-Germany to the W-Czech Republic as a subtle impulse from S-CNTRL France lifts NE towards Switzerland at around 18Z and the N-Czech Republic until 00Z.

Surface pressure fall occurs over NW-Italy (prolonged period of near unimpeded diabatic heating) and in the path of the NE-ward lifting short-wave (Switzerland and S-Germany). The front's anafrontal nature fosters an increased thermal gradient with up to 6 hPa difference between the Vosges and areas futher east. This setup supports easterly LL flow over NW-Italy and S-Germany beneath strong SW-erlies along the frontal zone.

The air mass over NW-Italy features deep low-tropospheric moisture with yesterdays' 12Z readings showing dewpoints in excess of 15 C up to 1400 m AGL. This moisture offsets meager mid-level lapse rates, which increase a bit next to the orography with an Alpine EML plume. MLCAPE in the 1.5-2.5 kJ/kg range is forecast.
Further north, towards Switzerland/far E-France, moisture recovery and good diabatic heating until the afternoon push MLCAPE into the 500-1000 J/kg range but final CAPE magnitude depends heavily on cloud distribution.
Over S-Germany, substantial BL moisture recovery is needed behind the past frontal passage as latest soundings show well mixed 0-3 km layer with no residual moisture source available N of the Alps. Moisture recovery is forecat with forecast soundings indicating rising dewpoints below 3km AGL until 18Z. A pretty healthy looking cap at 700 hPa is forecast by most models but cooling in this layer occurs during the evening and beyond.

Kinematics support organized convection in the aforementioned regions with DLS in excess of 20 m/s and 0-3 km shear in the 15-20 m/s range. In addition strong directional shear is present along/east of the synoptic-scale front.

With strong cap holding steady over NW-Italy and Switzerland until the afternoon, good diabatic heating is forecast over the area of interest and late CI is anticipated (afternoon onwards). Highest confidence in severe thunderstorm development resides in the level 2 area, where upslope flow/passing short-wave should initiate a few long-lived supercells/multicells. Large hail, severe wind gusts and excessive rain are likely and I don't want to rule out a significant hail event with mature supercells despite meager lapse rates/warm mid-levels. These storms - surface based with strongly curved hodographs - also bear a tornado threat. I want to emphasize that convection remains tied to the orography with prolonged period of very moist/unstable and uncontaminated inflow pointing into ongoing convection, so don't want to rule out local extreme rainfall reports/flash flood events with training/slow moving convection.

Over far E-France, the eastward surging front (behind the short-wave) interacts with the unstable prefrontal air mass along the French/Swiss border and scattered thunderstorms evolve. Strong speed/directional shear supports organized updrafts and initiating stroms could become severe for a couple of hours with large hail, severe wind gusts and an isolated tornado risk. This activity grows upscale into a NE-ward moving cluster - at this time it looks like a messy setup over SW-Germany/NW-Switzerland with lots of rain but otherwise a gradually lowering hail and tornado threat. Any more discrete storm however poses an ongoing severe risk with isolated significant severe not completely ruled out.

Finally CI is also anticipated during the late afternoon onwards along the Swiss/German border in the narrowing warm sector (bound by the cold front and the Alps). Diffuse foci for LL convergence keep confidence low in where and when exactly CI occurs, but a few organized storms probably travel rapidly to the NE. Diurnal moisture recovery and placement next to the frontal zone dictate how elevated and severe convection becomes (with strongest speed/directional shear below the 700 hPa). Large hail and severe wind gusts will be the main risk with this activity.
However we would not be surprised to see a spatiotemporal SW-NE aligned stripe from the Lake of Constance to the NE, where thunderstorms take profit of the the wind field below 700 hPa as cap erodes from the W. Multicells and a few supercells could then bring large hail, severe wind gusts and heavy rain closer to the Alps.
This activity approaches NW-Czech Republic during the night with a gradually weakening trend as thunderstorms outrun the MUCAPE plume. We broadened the level 1 and added the SW-Czech Republic for developing nocturnal thunderstorms in a WAA regime with an isolated large hail risk.

A level 1 was issued for parts of SE-France due to slow moving/back-building evening/nocturnal convection. 15-20 kn S-erly LL flow advects rich BL moisture into this activity along the gradually S-ward sagging cold front. Excessive rain becomes a concern and we don't exclude a local threat of significant flash flooding if convection remains tied to the orography for a few hours. A level 2 was considered, but uncertainties in how fast the front accelerates to the SE precluded an upgrade for now.

Beside the rainfall threat, initiating or more discrete storms pose all kind of hazards before clustering.

... NE-Spain ...

Evening/nocturnal thunderstorm development is forecast with a structuring coastal convergence zone and an gradually approaching upper trough from the W. Point source forecast soundings show 1500 J/kg MLCAPE and 25 m/s DLS with deep/moist profiles. CI occurs due to upslope flow and LL convergence. Multicells/supercells bring large to very large hail, severe wind gusts and excessive rain. An isolated tornado event along the coast is possible.

... Slovakia to far N-Romania ...

Despite rising thickness, a subtle wave passes between 12-18Z from W to E, so isolated to scattered CI along the orography due to WAA and LL upslow flow along the W fringe of an anticyclone is forecast. 15-20 m/s DLS and 1.5 kJ/kg MLCAPE with rather straight hodographs inidcate a risk of multicells and a few splitting supercells with large hail and severe wind gusts.

... Baltic States ...

An eastward surging occlusion slides beneath cold mid/upper-levels and this moisture source beneath that thermodynamic stratification supports widespread 500-1000 J/kg weakly capped MLCAPE. However, synoptic-scale subsidence at mid/upper levels keeps CI on the isolated scale. Any stronger updraft takes profit of 15 m/s DLS and hail/strong wind gusts area possible. No level area was added due to the limited coverage.
Further north, towards Estonia, shear decreases and slow moving thunderstorms bring locally heavy rain.