Storm Forecast
Valid: Sun 01 Dec 2024 06:00 to Mon 02 Dec 2024 06:00 UTC
Issued: Sat 30 Nov 2024 09:35
Forecaster: TUSCHY
A level 2 was issued for parts of E Greece mainly for extreme rainfall amounts.
A level 1 surrounds the level 2 for similar hazards, including an isolated tornado risk and strong gusts.
A level 1 was issued for far NW Spain mainly for heavy rain and an isolated tornado event.
SYNOPSIS
The overall positive westerly wave flux along 50-60 degree N reflects the near neutral NAO conditions although the progressivity is low with a shift of a quater of a wavelength to the E. Despite that ongoing motion, the European pattern features a rather blocked appearance with an impressive and only gradually eastwad shifting anticyclone atop CNTRL/E Europe.
A cut-off is placed along the anticyclone's southern fringe atop the Ionian Sea and is forecast to shift E towards the Aegean Sea during this forecast. NWP guidance has a rather stable handling of this deep and vertically stacked vortex. A bit more complex becomes the evolution within the BL as a dominant LL vortex seems to dissipate over the Aegean Sea with a new vortex reforming W of Greece, which then shifts towards the Ionian Sea. This scenario is well captured with a tightly clustered group of IFS-ENS members.
An extensive trough runs from NW to far SW Europe with an expected cut-off process near Madeira. This pattern causes a rather bullish CAPE/shear space over that region, as v-winds at 250 hPa approach record level for this time of year.
An extensive/wavy cold front is placed over W Europe and extends deep into the subtropics while the Greece cut-off is accompanied by an occlusion, which becomes the main focus for most extreme QPF amounts. The open warm sector shifts from the Aegean Sea E towards W-Turkey.
DISCUSSION
... Greece ...
An ongoing severe weather event over parts of Greece brings another round of heavy to extreme rainfall amounts.
The driver of this event is the cut-off with the re-structuring LL depressions, which keep an unstable and persistent onshore flow alive during this forecast.
Right now there is no connection into the tropics forecast, as rich moisture (mid/upper troposphere) atop the Red Sea remains decoupled from the airmass atop the E Mediterranean. Still, the advected airmass within the vortex' circulation has a long fetch atop the S-CNTRL Mediterranean, where SST anomalies regionally exceed 3K. A strongly convergent wind field points into a probably well estbalished TROWAL and occlusion, which assists in a band of convectively enhanced precipitation (pointing into the level 2 area).
A somewhat limiting factor could be the eastward displacement of the highest BL moisture, as the cold front crosses the Aegean Sea E. This lowers offshore MUCAPE more into the 400-700 J/kg range. A brisk NE-erly BL flow along the N Aegean Sea pushes against the vortex' background circulation (S-erly winds over the CNTRL Aegean Sea) and creates the mentioned quasi-stationary convergence zone, which pulls moisture into the orography. Expect embedded thunderstorms within this band of intense precipitation, which also affect the coastal areas. Ensemble and deterministic data point to 24h rainfall amounts in the 150-300 l/qm range within the level 2 and flooding becomes more widespread (keeping the rainfall amounts from the previous day in mind). Betimes however, dropping snow levels should help to bind some of the discharge.
An isolated tornado is also forecast offshore and along the coasts and we could see some strong gusts along the northern periphery of the level 2, where NE-erly flow is still able to mix to the surface.
This level 2 got framed by a broad level 1 for slow moving convection with heavy to locally excessive rainfall amounts but also for an isolated tornado risk.
... Madeira into far NW Spain ...
An atmospheric river ahead of the gradually eastward moving cold front roots deep into the tropics and this deep plume of moist air advects NE towards far NW Spain and overlaps with an robust EML, which emerges from NW Africa. The result is a broad tongue of prefrontal and elevated MUCAPE in the 1-2 kJ/kg range, which lowers to 300-700 J/kg near surface based CAPE over far NW Spain.
Overlapping 15-20 m/s DLS create favorable kinematic conditions for numerous intense thunderstorms, which could cause an unusual large hail and severe wind gust threat for Madeira and offshore areas to the NE/E. Similar shear values at 0-3 km and adequate SRH could also support a tornado risk along the vicinity of the front, which also includes far NW Spain.
Otherwise, the main risk for NW Spain will be heavy rain due to repeatedly onshore moving convection.
Some MUCAPE signals also exist over Spain, but placed beneath the ridge, no CI is forecast right now for most places (despite a more bullish EZ solution). The low probability lightning area was adjusted into W-Spain to account for a mid-level cooling trend as the ridge axis shifts E with a low-end CI probability not ruled out.
EZ is also rather aggressive in advecting a plume of MUCAPE deep into France towards Benelux. We added a confined low-prob. lightning area for the region, where the cold front intersects the NE ward surging moisture plume. Forecast soundings show some elevated MUCAPE with CAPE extending into the -10 C layer, so spotty events are still possible.
A few short-lived thunderstorms are also possible over NW Europe beneath colder mid-levels but this risk is too marginal for a lightning area.
