Storm Forecast
Valid: Fri 02 Jun 2017 06:00 to Sat 03 Jun 2017 06:00 UTC
Issued: Thu 01 Jun 2017 21:59
Forecaster: TUSCHY

A level 1 was issued for N-Spain and far SW-France mainly for large hail and excessive rainfall.

A level 1 was issued for NE-Morocco and N-Algeria mainly for large hail and severe wind gusts but also for excessive rainfall.

A level 1 covers many parts of the Balkan States and parts of CNTRL-/W-Europe mainly for excessive rain, isolated large hail and an isolated severe wind gust event.

A level 1 was issued for extreme NE-Belarus und far W-Russia mainly for severe wind gusts and an isolated tornado event.

SYNOPSIS

The European mid-tropospheric geopotential height pattern features two significant cyclonic vortices. One is centered over the NE-Atlantic, whereas the other one is analyzed over far NE Europe. Between both vortices, ridging covers CNTRL-Europe with the ridge axis being centered from Germany to SW-Norway.

The Mediterranean is influenced by a broad upper trough over Italy, which moves east during the forecast. Upstream / downstream ridges of this feature remain ill defined but will propably be just strong enough to suppress convective activity somewhat.

At lower levels, weak surface pressure gradients prevail during the forecast. A more substantial pressure fall occurs over Benelux, W-Germany and E-France during the overnight hours due to the approach of an upper-level trough over UK, which rotates east around the large vortex. A N-S aligned surface pressure channel evolves over NE-France and parts of Benelux which could become a focus for thunderstorm activity during the night.
In respect of frontal boundaries, an occlusion affects the North Sea, E-France and NW-Spain during the day while a weak cold front over the S-Balkan States becomes more or less quasi-stationary in nature. Another front over N-Belarus and far W-Russia will be pushed rapidly to the SE.

DISCUSSION

This pattern results in widespread thunderstorm activity over many parts of S-/ CNTRL-Europe. In general, robust thermodynamics but mostly weak kinematics preclude a more widespread severe risk. However, a few regions will be highlighted for more concentrated swaths of severe mainly due to excessive rain and large hail.

... N-Spain ...

Gradually increasing synoptic forcing due to the approach of a positive tilted mid-level trough, diabatic heating and attendant enhanced upslope flow keep the focus for CI along the mountains of N-Spain. Thunderstorms increase during the afternoon and evening hours and a large cluster is expected to drift slowly to the NE during the evening while weakening/affecting far SW-France.

This part of our forecast area experiences strong DLS of 15-20 m/s. Combined with 400-800 J/kg MLCAPE, enough overlap of ingredients exists for a few organized multicells during initiation with large hail and strong wind gusts. Betimes, while clustering, the heavy/excessive rainfall risk increases somewhat. However, gradual decrease of CAPE should limit the overall severe risk.

... From N-Italy to Slovenia to Bulgaria/Greece ...

A broad upper-level trough is placed over a seasonable moist boundary layer. Meager (locally somewhat steeper) mid-level lapse rates (also due to numerous rounds of thunderstorms during the previous days) will be offset by the available BL moisture. Forecast soundings from that area show MLCAPE of 600 to 1200 J/kg while MUCAPE locally peaks at or above 1500 J/kg. The lowest CAPE values probably occur over N-Italy while the most robust CAPE (also in areal coverage) is forecast over the SE-Balkan States. The air mass is weakly capped with early CI forecast. This activity continues all day long. Outflow boundaries and the orography itself serve as foci for initiation but CI also occurs in proximity to the leisurely eastbound moving upper wave.

Thunderstorms will yield a large hail risk during their development phase (especially if they can remain more isolated for some time). Forecast soundings over N/NE-Italy also show a 1.5-2 km deep layer of dry/well mixed subcloud air, which would point to a few severe downburst events with maturing storms.
Rapid upscale growth into various disorganized clusters is forecast, as very weak DLS precludes organized convection. In fact, 0-6 km shear remains in the 1-8 kt range. During the clustering phase of convection, excessive rain with flash flooding becomes a distinct possibility due to slow moving convection.

We considered an upgrade for parts of that area, but confidence in pinpointing the area with highest severe remains challenging due to the dominant mesoscale impact on thunderstorm development/organization. Quality of CAPE and quasi-stationary storms may cause dangerous flash-flooding on a very local scale.

Thunderstorms weaken during the night especially over N-Italy but continue all night long ahead of the eastbound moving upper wave - mainly over the SE-Balkan States. With MUCAPE of 400-800 J/kg and slow storm motion, the isolated large hail and excessive rainfall risk continues.


... W-/NCTRL-Europe ...

A plume of moist air covers the broad area of interest. Highest values of the effective PW values are forecast over NE-France and Benelux into far W-Germany, where persistent confluence in response to the evolving surface low pressure channel occurs. Widespread mixed-layer mixing ratios of 11 g/kg or more offset meager mid-level lapse rates, so 400-800 J/kg MLCAPE is forecast. Peaks of 1 kJ/kg are likely on a regional scale with maximized BL moisture / moisture pooling. This plume of unstable air also extends to SW-Germany and Switzerland, where MLCAPE increases to 1 kJ/kg with MUCAPE of 1500 J/kg.

The following points support early (before noon) and scattered to widespread CI:

A diurnal driven weakening cap, colder mid-levels over the W-Alps and far S-Germany in proximity to the N-Italian upper trough and somewhat stronger mid-level flow over Benelux and NE-France, which turns more cyclonic betimes as a mid-level wave approaches from the W.

DLS remains very weak towards the Alps, but strengthens to 10-15 m/s over NE-France and Benelux. Current idea is that the most conducive environment for the development of a large and better organized cluster of storms exists over NE-France between noon and evening as widespread CI occurs and grows upscale. This activity then shifts to the NE and affects Benelux during the evening/overnight hours. We can't rule out that the cluster builds more to the SE (into the CAPE plume), which would bring it also into far W-/NW-Germany. We expanded the lightning and level areas accordingly more to the NE (all the way to the S-North Sea) as cold pool driven activity and modest steering flow push the cluster more to the NE.
The main risk from N-France to W-Germany will be large hail during initiation and with discrete storms along the fringes of any cluster. Betimes the risk turns to excessive rain with clustering and slow moving storms. In addition we can't exclude a low-end and localized risk of severe wind gusts in case of a temporarily better organized cold pools. Latest data indicates the highest risk of severe should evolve over far NE-France, Belgium, Luxembourg into W-CNTRL Germany. Excessive rain and large hail will be the main hazard.

Further to the SE, over S-Germany and the W-Alps, early CI with strong CAPE and very weak shear all point to slow moving clusters, which pose a risk of isolated large hail and excessive rain.
Again, an upgrade was considered for far S-Germany and the CNTRL Alps due to the CAPE signals in excess of 1500 J/kg MUCAPE and high coverage of storms. The main reason here to hold off a level 2 was the effective PW, which features only moderate values and uncertainty, where most dominant clustering occurs. Nevertheless, the complex orography of the Alps with very slow moving storms creates a favorable setup for flash flood producing storms! This includes also far S-Germany, where scattered thunderstorms with strong CAPE could result in numerous severe reports.

Finally, we have to mention SE-UK, where the northern edge of the CAPE plume sneaks beneath stronger mid-/upper-level flow. 15-20 m/s DLS and 400-800 J/kg MLCAPE may support a temporarily organized cluster of storms with an isolated large hail risk. However, the evolution of the activity further south (France) adds uncertainties. In case of the development of a large cluster over NE-France/Benelux, moisture advection/instability and initiation may be delayed or suppressed. Current model data gives enough confidence to stick with a level 1 for now.

... N-Belarus and far W-Russia ..

A eastbound racing upper trough pushes a front to the SE. Not much moisture advection occurs ahead of that front, but rapid cooling of the mid levels still results in daytime driven 200-500 J/kg SBCAPE. Placed beneath the left exit of a potent 45 m/s mid-level jet, a forced line of convection is possible to accompany the frontal passage. Forecast soundings from extreme NE/E-Belarus show CAPE in the 800-600 hPa layer with steep LL lapse rates, which supports enhanced downward transport of stronger winds. Hence severe wind gusts and a very isolated tornado threat will be the main hazard given 20 m/s 0-3 km shear, weak LL shear and low LCL heights. The evolution of the BL moisture over E-Belarus/extreme W-Russia has to be monitored closely as not much modification is needed for a more robust/widespread severe wind gust threat!The risk diminishes after sunset.

... N-Morocco and N-Algeria ...

Mountain convection with 400-800 J/kg MLCAPE and 10-15 m/s DLS supports a few organized multicells with large hail and gusty winds. Fat mid-level CAPE and a deep/dry subcloud layer indicate a chance for an isolated very large hail and severe downburst event, but degree of CI remains questionable. The risk diminishes during the night.