Storm Forecast
Valid: Wed 25 Sep 2024 06:00 to Thu 26 Sep 2024 06:00 UTC
Issued: Tue 24 Sep 2024 20:14
Forecaster: TUSCHY
A level 2 was issued across N Portugal into NW Spain mainly for excessive rain.
A level 1 surrounds that level 2 mainly for similar hazards but with lower probabilities.
Numerous level 1 areas cover parts of the Tyrrhenian/Adriatic Sea mainly for a localized excessive rainfall and tornado threat.
A level 1 was issued for parts of the N Balkans mainly for some hail/gust reports.
A level 1 was issued for parts of the coastal areas of the E-Black Sea mainly for heavy rain.
SYNOPSIS
A positive tilted long-wave trough is placed from the Norwegian Sea to the NE North Atlantic. Numerous compact waves with variable amplitude circle that trough and bring unsettled conditions for many parts of Europe.
Of interest is the ingested airmass into the periphery of that trough, which features offshore TPWs in the 40-55 mm range and hence a more or less unmodified tropical airmass, which converges along a zonally aligned thermal/moisture boundary at roughly 45N. This airmass advects E and pushes EFI IVT to its limit from N-Portugal deep into France (including broad areas with SOT 1 and more). This slightly unstable airmass gets accompanied by a strengthening mid/upper level wind field with DLS in the 50-70 kt range. Embedded fronts and waves along this atmospheric river cause widespread rain with regionally impressive amounts.
DISCUSSION
... CNTRL Mediterranean ...
Another round of coastal convection occurs during the daytime hours during the passage of a mid-level wave and attendant thermal trough. A substantial mid-level warming trend occurs from the W during the evening onwards, which suppresses DMC activity.
Until 18Z however, 500 to 1500 J/kg MUCAPE overlap with 15-20 m/s DLS, so numerous confined areas along the coast will see training convection, once again with impressive spatial rainfall gradients. Numerous level 1 areas were issued, where NWP guidance agrees on local 24h rainfall amounts in the 50-200 l/qm range. Similar to yesterday, a few coastal tornadoes are forecast with that activity. A weakening trend is anticipated during the evening hours onwards.
... E-Black Sea ...
Moist/unstable onshore flow continues behind the dissolving vortex with an ongoing heavy rain issue in the level 1 area. The activity and the attendant risk diminishes during the overnight hours.
... N-Balkan States to Belarus ...
A belt of slighlty unstable air with MUCAPE in the 200-600 J/kg range advects NE. Combined with diurnal heating, isolated to scattered thunderstorms occur with some gusts, graupel and localized heavy rain.
A better CAPE/shear space exists from Croatia into Serbia and S-Hungary with MUCAPE in the 600-1500 J/kg range. DLS up to 15 m/s assists in a few better organized storms with some hail and gusts. A confined level area was added, where a temporal/borderline level 1 risk exists during peak heating.
... N-Portugal into the S Bay of Biscay, W-CNTRL France and up to the S-UK...
The mentioned atmospheric river/warm conveyor belt event with IVT peaks well in excess of 1000 kg/ms points into the level 2 area. Passing fronts and the orography support a prolonged period with excessive rain and widespread 24h amounts in the 80-150 l/qm range with higher peaks on the local scale.
Despite the more dominant statiform rain, some MUCAPE is present with temporal peaks in the 300-500 J/kg range - enough for embedded thunderstorm activity (beside slantwise instability release, resulting in enhanced non-thundery rainfall rates in embedded convective bands). This activity continues beyond 06Z.
During the night, an unstable airmass advects deep into France but the vertical CAPE distribution most likely ends above -10C, so despite very efficient hourly rainfall rates it remains uncertain, if a few thunderstorms manage to form. Right now we expanded the level 1 well inland combined with a low probability thunder area to reflect that risk. Heavy rain will be the main concern although nowcast decides how unstable LL become in an extreme shear environment (25 m/s 0-3 km shear) regarding gust/tornado probabilities. Latest guidance keeps this risk rather low however.
We expanded the lightning areas well N into S-UK, where cooling mid-levels atop a moist marine layer cause a broad pool of 200-400 J/kg MUCAPE. DLS in the 10-15 m/s range is enough for a few better organized evening/nocturnal storms with some graupel, gusty winds and heavy rain.
... N-Germany ...
A favorable kinematic but weak thermodynamic setup may spark a few shallow mesocyclones during peak heating but any severe risk should stay sub-severe/too isolated for any upgrade.
