01-06-2017 | Estofex | m.b.t. 02-06-2017 t/m 03-06-2017
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.
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 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.
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.
... 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.
Hagel(-schade) | Onweer/blikseminslag | Verwachtingen/waarschuwingen | Wateroverlast/(zware) neerslag | Wind/storm (-schade)