Storm Forecast
Valid: Wed 16 Apr 2025 06:00 to Thu 17 Apr 2025 06:00 UTC
Issued: Tue 15 Apr 2025 11:37
Forecaster: TUSCHY
A level 2 was issued for parts of NW Italy mainly for excessive rain.
A level 2 was issued for Corse/Sardegna mainly for excessive rain.
A broad level 1 encompasses both level 2 areas with a similar hazard but lowered probabilities. In addition an isolated tornado/severe wind gust and hail risk is forecast.
A level 1 was issued for parts of the W Mediterranean mainly for a few waterspouts/tornadoes and heavy rain.
A level 1 was issued for parts of Tunisia mainly for isolated large hail and heavy rain.
A level 1 was issued for parts of Germany mainly fo severe/damaging gusts, isolated large hail and heavy rain.
Some changes became necessary compared to yesterday's extended discussion, which is attached below the SWODY1.
The main change was performed over Corse and now explicitly mentioning/adding Sardegna, where a multifacet severe event unfolds. A very dynamic setup with a progressive/negative tilted trough sparks rapid LL cyclogenesis over the Tyrrhenian Sea, which shifts towards Corse before becoming stationary. IFS-ENS has a tight member clustering with MSLP readings in the 995-1000 hPa range. Some deterministic runs show even lower values and we now talk about negative MSLP standard deviations in the -3 to -4 range. No surprise to see numerous EFI parameters in deep red including substantial positive SOT values (e.g. water vapor/gusts/rain)
During the day, a prolonged period of training rain including enhanced/deeper convective elements affects both islands. During the evening and overnight hours, the approaching and rapidly deepening LL vortex adds a temporal overlap of better mid-level lapse rates and adequate low-tropospheric moisture for some MUCAPE build-up. Hence in addition to the more stratiform rain event we add clustering thunderstorms for the overnight hours with enhanced hourly rainfall rates. This excaberrates any flash flood issues and a few significant events are possible!
In response to the rapidly deepening LL vortex, an impressive LLJ event unfolds with Bft 9/10 gusts and Bft 11/12 gusts for the mountain areas and orographically favored regions. This increases the overall impact and despite only some convective elements, we decided to push this event into the level 2 area - mainly for the rainfall threat. An isolated coastal tornado event is not ruled out.
Another change compared to the extended outlook was to expand the S Alpine level 2 further S to add a risk of training convection along the coastal areas of extreme NW Italy/SE France. Within the level 2, we still expect 24 h rainfall amounts in the 100-250 l/qm range with regional peaks more like 300-400 l/qm (Aosta Valley into the N Piedmont area). This could cause some serious flash flood issues despite the area having experience with such rainfall events. Of main concern becomes the overnight hours, as BL flow/inflow from the E/SE increases to whopping 50-60 kt at 850 hPa - a result of the exceptional deep LL vortex to the S. Rich moisture at all levels (including the mid/upper levels per latest Layered PW data) gets advected into the complex orography, where 6h rainfall amounts around 100 l/qm are possible atop on the already impressive amounts from the daytime hours. This could support an elevated flash flood risk with a few serious events possible!
We also expanded the level 1 over N Italy more E to reflect enhanced probabilities for a nocturnal MCS, which evolves along the Apennines and races N. This activity and a few thunderstorms during the day pose some hail threat for the Apennines and surrounding areas. Otherwise this activity outruns best CAPE but experiences extreme shear values. Hence more elevated orography could probably see an ongoing severe wind gust risk next to heavy rain.
The level 1 over the W-CNTRL Mediterranean (for waterspouts/tornados and locally heavy rain) was shifted a bit W due some NWP correction regarding the upper trough's placement. We expanded the level 1 towards the French coast to reflect and enhanced waterspout/tornado risk with onshore moving convection.
We also added a separate level 1 for parts of E Tunisia mainly for hail including an isolated large hail event. Clustering convection poses a heavy rainfall threat, too.
Finally we also upgraded parts of S-CNTRL to N-CNTRL Germany for a confined risk of severe/organized convection. The best chance for CI occurs from CNTRL to N-CNTRL Germany but we expanded the level 1 more S to account for lowered EPS signals regarding CI. The main concern for CI arises along the W fringe of the diabatic low, which intersects a wavy N-S aligned frontal boundary. Rich LL moisture pooling assists in a confined belt of 500-1000 J/kg MUCAPE and virtually no CIN. NWP guidance remains very diffuse with some models showing no CI, whereas others offering scattered thunderstorms. However with some models indicating an eastward surging wind shift/pressure wave, confidence for CI has increased compared to yesterday.
Any evolving storm would move through a small but favorable CAPE/shear space with 15-25 m/s DLS and 10-20 m/s 0-3 km shear. A deeply mixed BL is forecast with strong/curved hodographs, offering strong and deep inflow into evolving cells. Any thunderstorm, riding this belt of CAPE/shear space could acquire rotation with a hail and severe to damaging wind gust threat. Even an isolated large hail event cannot be ruled out. Further N, clustering thunderstorms add an localized heavy rainfall risk.
Otherwise no major changes were performed.
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SYNOPSIS
A rather static pattern evolves over Europe in response to a constantly strengthening blocking ridge over E Europe, which extends all the way to Scandinavia. The long-wave trough over W Europe becomes quasi-stationary and breaks up into numerous lower-amplitude waves, whereas the steering vortex remains placed between Scotland and Iceland.
The wave of most concern exits Spain during the start of this forecast and shifts E thereafter before approaching Italy during the overnight hours. This wave cuts off of the frontal zone and evolves into a cut-off low, as it approaches Italy from the W. This wave keeps the negative tilt and its coupled upper jet configuration helps to evacuate mass with attendant mass response in form of an intensifying LL depression. This vortex shifts N from the Strait of Sicily and intensifies while crossing the Tyrrhenian Sea. It turns quasi-stationary somewhere over N-CNTRL Italy during the overnight hours.
DISCUSSION
... NW Italy ...
The main severe risk evolves in the Aosta Valley/Piedmont region, where persistent upslope flow is forecast during the day. Expect periods of heavy to excessive rain, especially with embedded convection. Compared to the day before, CAPE and BL moisture improve in an environment with deep moist profiles. Advected Layered PWs show a deep plume of moisture to work with and even some connection to the tropics exists, so a dominant collision coalescence process should support high hourly rainfall rates. During the night, the approaching vortex from the S induces an imperessive orographically forced inflow channel from the SE/E into the area of most concern, which keeps the excessive rainfall risk alive. Expect 24 h rainfall amounts within the level 2 area in the 150-350 l/qm range with (flash) flooding likely.
During the afternoon/evening hours, the risk of severe temporarily expands well to the S/SE into the Bologna area, where a few multicells/isolated supercells pose an isolated large hail and severe wind gust threat, which turns more to an heavy rainfall risk towards the Milan/Turin area. An isolated tornado event is possible.
We expanded the level 1 also to the W coast of Italy for clustering thunderstorm activity during the afternoon with heavy rain.
... CNTRL Mediterranean ...
A broad level 1 was issued to highlight a mixed severe risk. Corse resides next to the approaching and later-on also stalling vortex with slow moving convection the primary hazard. Heavy rain and an isolated tornado event become the main issue.
Otherwise, favorable high LLCAPE/weak shear setup exists beneath the upper trough and affects most of the W-CNTRL Mediterranean. A few waterspouts and slow moving thunderstorms with heavy rain and graupel/gusts are forecast.
... Parts of CNTRL/N Germany ...
Most of the airmass N of the Alps remains capped. However, CI is possible along a wavy frontal boundary, which bisects Germany from N to S and interacts with a structuring and slowly N ward moving lee vortex (shifting towards E-CNTRL Germany). Main issue right now seems to be modest CAPE with even lower ECAPE values due to rather dry low/mid-levels. Nevertheless, we issued a thunderstorm area, where latest EPS/deterministic has the highest probabilities for CI, which is the area of the front interacting with the diabatic vortex and with the orography. In case of development, strong to isolated severe gusts become the main hazard with a well mixed BL. Initiating storms may cause a few hail reports, too.
Thunderstorms ride the tilted frontal boundary and turn more elevated further W into NW Germany. Repeated thunderstorm development could bring some localized heavy rain. The risk extends into the overnight hours with a gradual weakening trend. A level 1 upgrade could become necessary in case of higher confidence in more widespread CI.
