Storm Forecast
Valid: Sat 13 Aug 2022 06:00 to Sun 14 Aug 2022 06:00 UTC
Issued: Fri 12 Aug 2022 22:17
Forecaster: TUSCHY

A level 2 was issued for parts of S France mainly for large to very large hail, excessive rain and severe wind gusts.

Two level 2 areas were issued for parts of Italy for large hail, severe wind gusts, heavy rain and a tornado risk.

Level 1 areas surround all level 2s for similar hzards but lower probabilities.

A level 1 was issued for NE Spain mainly for large to very large hail, severe wind gusts and excessive rain.

A level 1 was issued for parts of the Dinaric Alps mainly for heavy rain and large hail.

SYNOPSIS

Finally the European pattern turns more progressive - at least for the western parts as the long-lasting cut-off just W of Portugal makes contact again with the westerlies to its N and accelerates to the ENE. It should cross NW Spain until 00Z and approach SW France until 06Z from the WSW.

Further E a broad cut-off still covers most of the S-CNTRL and SE Europe with numerous smaller waves circling around its periphery.

No synoptic-scale front will play a role in today's outlook.


DISCUSSION

... S-France ...

Gradual lowering MSLP over W France is forecast, which assists in BL moisture advection from the Mediterranean and the Atlantic. This MSLP decrease is first driven by diurnal heating but it mixes betimes with supportive background conditions on the synoptic scale ahead of the approaching upper trough.

Until 18Z for S-CNTRL France:

SSTs in the 25-28 C range over the Gulf of Lion (3-4.5 K anomaly) helped to moisten up the marine air mass substantially for the past few days/weeks, which now advects N towards the S-French coast. 2m dewpoint readings in the upper tens to lower twenties are forecast up to the S fringes of the Massif Central with lower to mid twenties along the coast (Beziers to Montpellier). This BL moisture advects N beneath a pronounced EML with initially capped MUCAPE values in the 3500 to 4500 J/kg range! Until 18Z this area remains placed beneath the subtropical ridge axis but cap weakens towards the S Massif Central and ENS data indicates low-end chances for isolated CI probably due to gentle upslow flow from the S. Not sure why NPW guidance acts so reluctant with CI and keeps initiating storms rather short-lived - probably due to strong Q-vector divergence forecast for that time-frame and missing enhanced LL convergence signals. However any consolidating updraft would develop in an extremely unstable environment with 15 m/s DLS, which would favor large to very large hail (in excess of 5 cm) next to very heavy rainfall amounts. However this risk is on the isolated scale and depends on mesoscale support.

Overnight hours:

During the night, an eastward sliding and weakening subtropical ridge makes place for the approaching upper trough and attendant augmented large-scale lift. Ongoing rich BL moisture advection towards the Massif Central beneath steep mid-level lapse rates keeps MUCAPE in the 2500-4000 J/kg range with DLS in the 10-15 m/s range. Rather strong cap may keep CI suppressed until 00Z before initiation probabilities increase substantially from W to E. Initiating storms pose a risk for very large hail, severe wind gusts and excessive rainfall amounts. Wind profiles don't look that suspicious for training thunderstorm activity, but a few spots might see repeatedly developing storms, which would increase the flash flood risk significantly. Upscale growing MCS shifts E/NE towards Lyon until 06Z with an ongoing severe risk. A level 2 was added to highlight the risk of a few extremely severe events especially next to a few anticipated supercell storms.

Further W, over SW France, CI probably starts along the N-Pyrenees before moving N into an increasingly moist BL with dewpoints in the upper tens. DLS will be in the 10 m/s range, so clustering convection poses mainly a large hail and heavy rain threat.

... NE Spain ...

This region will see low chances for CI but potential high impacts with any initiating storm (NE Spain, next to Barcelona). Diurnal driven onshore flow of the aforementioned very moist marine air mass beneath steep mid-level lapse rates assist in the build-up of 2000-3000 J/kg MUCAPE (ICON up to 5000 J/kg). During the late afternoon into the evening this area resides along the W fringe of the ridge axis. Strong subsidence is forecast but also the passage of a low-amplitude wave, which adds at least some background forcing. This combined with enhanced convergence along the sea breeze and (dependant on how far inland this convergence zone advects) even some orographic support may assist in spotty CI (with GFS being on the most aggressive side). DLS around 15 m/s and mentioned extreme CAPE values would once again support large to very large hail, excessive rain and severe downburst wind gusts. This risk ends during the evening hours as synoptical driven onshore flow event begins with drying BL also along the coast.
A level 1 was added fow now because of the low confidence in CI. However any initiating storm could produce local level-2 conditions (which would be especially true for the GFS solution).

... Corse/Sardegna to W Greece to the Dinaric Alps ...

The severe event in the vicinity of the upper cut-off will be a bimodal one.
Ongoing organized convection from the previous night affects Corse/Sardegna and the far N Tyrrhenian Sea. During the daytime hours, this activity spreads SE towards CNTRL and SE Italy. MLCAPE in the 1000 to 2000 J/kg range with 25 m/s DLS and 10-15 m/s 0-3 km shear will be adequate for numerous organized multicells/a few supercells with all kind of hazards including large to very large hail, severe wind gusts and heavy rain. In addition with rather low LCLs in place, a few tornadoes will be possible. We added two level 2 areas where BL moisture will stay in place whereas areas like Pescara to Bari may see too much BL stabilization for a substantial severe risk.

The other risk extends beneath the upper cold-core low. We upgrade parts of the Dinaric Alps to a level 1 as moist SW-erly flow advects beneath the upper cold-core low and approaches the orography nearly orthognoal. Expect scattered to widespread slow moving and clustering storms with heavy rain and large hail to be the main hazard. The level 1 was expanded to the Adriatic Sea as very weak cap should not prevent CI away from the orography. 10 m/s DLS should support pulsating and upscale growing convection, which weakens during the night.

... SE/E Europe ...

Beneath the cut-off and along its fringes, enough CAPE/shear overlap exists for a vast area with thunderstorm chances. Regional better BL moisture support may push MUCAPE in the 1000-2000 J/kg range (e.g. S Ukraine) but weak shear precludes organized updrafts. Pulsating convection may produce a few hail and strong to isolated severe wind gust events next to heavy rain, but NWP spread in QPF maxima and modest CAPE/shear background preclude any upgrades for now.