Storm Forecast
Valid: Tue 20 May 2025 06:00 to Wed 21 May 2025 06:00 UTC
Issued: Mon 19 May 2025 16:59
Forecaster: TUSCHY
A level 2 was issued for an offshore area W of the Gulf of Follonica mainly for a tornado, heavy rain, large hail and severe wind gust threat.
A level 1 surrounds that level 2 for similar hazards but with lower probabilities.
A level 1 was issued for NE Sicily for an isolated large hail risk.
A level 1 was issued for parts of E France mainly for heavy rain on a local scale and a low-end tornado risk.
A level 1 was issued for parts of S Ireland mainly for an isolated tornado and heavy rain risk.
A level 1 was issued for parts of Bosnia-Herzegovina and Croatia mainly for isolated large hail and severe wind gust risk.
SYNOPSIS
The main feature of interest (convective-wise) will be a zonally/eastward moving low-amplitude wave, which exits the Gulf of Lion E before crossing the Ligurian Sea during the day. A weakening trend is noted at mid and low levels as this feature gets accompanied by a diffuse LL vortex. Some lingering uncertainties exist with the impact of a potential MCV event from the previous night but overall modest support for this event keeps any impact on the low-end side. During the night, this wave slows down over N Italy and broadens as it awaits another impulse from the NW (a structuring trough over France).
A progressive trough crosses the Black Sea E and exits our forecast area, whereas otherwise rising heights become the dominant part over most parts of the Mediterranean into N Africa and from the far E Atlantic N towards Iceland.
DISCUSSION
... Italy, Gulf of Genoa, the Ligurian Sea and S towards the Tyrrhenian Sea ...
An coastal and eastward shifting MCS event continues from the previous night and affects far SE France during the morning hours before spreading into the Gulf of Genoa until noon.
Inflow into these thunderstorms features rich BL moisture as a plume of more than 10 g/kg mixed mixing ratios in the lowest levels resides to the S (e.g. W of Corse). There, capped MUCAPE in excess of 1000 J/kg is present as this moisture resides along the W fringe of a stout EML plume, which is centered from Tunisia into the Tyrrhenian Sea.
Near surface based activity is anticipated along the coasts and numerous organized thunderstorms pose an isolated hail and waterspout threat. In addition, temporal training is possible which induces a somewhat augmented heavy rainfall risk.
Of interest is the level 2 area, where a pronounced convergence zone W of the Gulf of Follonica sets up beyond noon in a 1000 J/kg MUCAPE and 15 m/s DLS environment. SR hodographs show 20 kt SR inflow into right moving cells, which is enough for organized and probably temporarily rotating updrafts. A few slow moving supercells pose a risk of a few mesocyclonic waterspout/tornado events next to heavy rain, severe gusts (downdrafts) and hail. Strong inflow offsets propagation, so expect training activity and a potential V-shaped (NE ward evacuating) MCS event. In case of CI over the Isle of Elba, a serious flash flood risk exists next to the mentioned risk of a tornado event and some hail.
We expanded the level 1 (surrounding the level 2) well inland into NW Italy to account for slow moving and clustering convection with heavy rain, gusty winds and some hail. The convection weakens during the overnight hours.
Isolated to scattered elevated thunderstorms are possible further S towards S Italy/Sicily but despite isolated hail with stronger updrafts, nothing severe is forecast.
However we added a tiny level 1 for the Etna area, where CI on a local scale cannot be ruled out during the day. The combination of rich BL moisture with 2m dewpoints in excess of 20 C (lowland) and steep mid-level lapse rates creates more than 2000 J/kg capped MUCAPE. If local wind systems manage to create a longer-lived updraft next to the Etna, large hail and heavy rain could become a risk. This however is a low-end risk and is not supported by all models.
... Bosnia and Herzegovina into Croatia ...
CAPE/shear space beyond noon is borderline for organized updrafts. Nevertheless, up to 500 J/kg MUCAPE and 15 m/s westerly DLS support a few multicells with isolated hail, gusty winds and heavy rain. This progressive activity weakens during the evening as BL becomes more stable.
... Ireland ...
In the vicinity of a quasi-stationary upper trough and atop modest BL moisture, expect a diurnal increase of MUCAPE in the 400-500 J/kg range. Especially ICON is rather bullish with 0-3 km CAPE build-up along the S fringe of the vortex, which looks reasonable with ongoing advection of an Atlantic airmass from the S with 2m dewpoints up to 10 C (a bit higher than yesterday). Initiating cells pose an isolated tornado risk but the more dominant hazard evolves betimes with clustering and slow moving convection, which brings heavy rain. This activity weakens beyond sunset.
... E France into far W Switzerland ...
A diurnal increase in MUCAPE is forecast admit good BL moisture with 2m dewpoints in excess of 10 C beneath neutral mid-level lapse rates. MUCAPE in the 400-800 J/kg range is forecast with near zero DLS. However, SR inflow in the 0-2 km layer is in the 10-15 kt range, which could increase the longevity of storms a bit. Overall, pulsating convection poses a local heavy rain threat with PWs in the mid twenties and near zero storm motion and mainly during cell merging and clustering phases. In addition some LLCAPE offers a chance for a short lived tornado or two. This level 1 is a marginal one and the risk diminishes after sunset.
