Storm Forecast
Valid: Tue 15 Apr 2025 06:00 to Wed 16 Apr 2025 06:00 UTC
Issued: Mon 14 Apr 2025 09:15
Forecaster: TUSCHY
A level 1 was issued across parts of SE/E France, SW Germany and W Siwtzerland for a mix of heavy rain, some hail and strong to severe gusts. An isolated tornado is possible over parts of SE France.
A level 1 was issued for parts of NW Italy mainly for heavy rain.
A level 1 was issued for NE Spain mainly for isolated hail and strong to severe gusts. Heavy rain is possible.
A level 1 was issued for parts of N Algeria into NW Tunisia mainly for some hail, gusts and heavy rain issues.
SYNOPSIS
The long lasting blocking anticyclone over Scandinavia breaks apart and clears the way to an extensive upper trough over NW/W Europe. This negative tilted trough amplifies and approaches N Algeria druing the end of the forecast. In the meantime, another extensive and long-lived blocking ridge becomes established over most of E Europe with another upper low gradually rotating towards the Middle East.
In response to this configuration a general pressure/height decrease occurs from Ireland all the way towards the W Mediterranean. A broad/dominant vortex over N France lifts N/NE with another weaker imprint E of Spain and one over Algeria.
An extensive wavy frontal boundary runs from England S/SE towards CNTRL/E France and NW Algeria. This boundary becomes the main focus for organized convective activity. Elsewhere, meager BL moisture keeps thunderstorm probabilities on the lower end side.
This forecast period marks the start of a multi-day heavy rainfall event especially along the S Alps.
DISCUSSION
... SE to E France, parts of Switzerland into SW Germany ...
Rapid BL moisture recovery is forecast along and E of the frontal boundary with 2m dewpoints in the lower tens for most places, increasing into the mid-tens over far SE France. Mid-level lapse rates remain rather ill defined with some recovery forecast towards W Switzerland in response to a N-ward fanning weak EML from the Swiss Alps.
CI becomes messy and rather widespread around noon onwards along the Massif Central (orography interacting with the front) and to the E/SE of the front during the afternoon hours. 15-20 m/s DLS favors multicells and a few supercells just after CI mainly between Burgundy, NE Massif Central, Jura and Vosges into W/NW Switzerland. Rather thin/elongated CAPE profiles overlap with modest SRH3, as forecast hodographs feature rather straight signatures. However, modification of the wind field by the orography could support a few longer-lived rotating storms with some hail and strong to isolated severe gusts. The main hail risk resides along the Jura, far W Switzerland and the French Alps. An isolated tornado event is possible mainly E/NE of Lyon, where somewhat enhanced LL CAPE could interact with semi-discrete cells. As convection moves into SW Germany, it gradually outruns CAPE with a decrease in storm organization and attendant severe.
S of Lyon, the main risk gradually transitions into bands of training convection during the evening onwards with rather unidirectional shear and slightly elevated convection. More discrete cells pose a temporal hail and gust threat but the main risk turns into swaths of heavy rain (e.g. S/SE Massif Central and the Provence), where 12h rainfall amounts could reside in the 50-100 l/qm range.
A broad level 1 was issued to cover this risk as no maximized storm coverage or chance for extreme severe is noticed for now. However, a few storm tracks could produce hazards in the level 1/2 periphery.
... Parts of NW Italy ...
The main area of interest evolves over the Aosta Valley and parts of NW/N Piedmont during the overnight hours, where training convection becomes established. Amount of BL moisture and CAPE along the inflow area of those storms seems to be still manageable, so we kept this risk in a level 1 for now. 12h rainfall amounts are forecast to approach 50-100 l/qm until 06Z with local higher amounts. Flash flooding on an isolated scale is possible.
... NE Spain ...
We issued a confined level 1 for NE Spain, where modest CAPE overlaps with strong DLS. Main limiting factor for a more robust severe risk becomes meager CAPE, with even lower ECAPE values due to some mid-level drying. Nevertheless, a few multicells and an isolated supercell are possible with isolated hail and gusts. Clustering convection turns the risk into heavy rain as it approaches the SE Pyrenees.
... N Algeria to N Tunisia ...
A level 1 was added for scattered thunderstorms during the afternoon, which grow upscale into an eastbound shifting cluster. Meager CAPE should limit the overall severe risk but a few strong to severe gusts, isolated hail and locally heavy rain are forecast.
... Germany and Benelux ...
Scattered thunderstorms evolve during the day with isolated hail and rain/gust issues. The overall risk does not justify any upgrade yet.
