Storm Forecast
Valid: Wed 18 Sep 2024 06:00 to Thu 19 Sep 2024 06:00 UTC
Issued: Tue 17 Sep 2024 16:47
Forecaster: TUSCHY

A level 2 was issued for parts of the Adriatic into the Ionian Sea mainly for excessive rain.

A level 1 surrounds that level 2 mainly for heavy to excessive rain, a few tornadoes and isolated hail.

A level 2 was issued for the far NW Aegean Sea mainly for excessive rain.

A level 1 surrounds that 2 mainly for heavy/excessive rain, a tornado and hail risk.

A level 1 was issued for the far W Mediterranean Sea mainly for heavy rain and an isolated tornado risk.

SYNOPSIS

A persistent high-over-low pattern features an impressive anticyclone over Scandinavia with constantly rising 500 hPa height anomalies, centered from the Shetlands to the Gulf of Bothnia.

To the south an extensive/zonally aligned channel with low geopotential heights affects most of the Mediterranean. A quasi-stationary upper low is centered between the Corso-Ligurian Basin and the Gulf of Follonica with a slight eastbound motion forecast due to an evolving Fujiwhara effect with a more progressive retrograde moving upper low to its NE/N. Some deterministic solutions show a weak surface low response but f.ex. IFS ENS reflects the low probablities of a developing LL depression with a strong zonal spread of only a few supportive members. It is more probable to see a broad area with lowered MSLP and numerous more or less defined embedded vortices.

The main frontal activity is confined to the North Atlantic up to the Barents Sea. The DMC activity is mainly driven from the upper vortex and LL convergence zones.

A progressive upper low over E-Europe sparks some thunderstorms, but these storms should stay mostly sub-severe. This activity affects parts of the flooded areas but adds only low-end rain on a very local scale.

DISCUSSION

... Central Mediterranean ...

A rather favorable setup for excessive rainfall (convectively enhanced) evolves over the Adriatic Sea with one focus for the Marche into Emilia Romagna regions.

Peripheral of the upper low and the diffuse/broad LL vortex a W/SW-erly aligned cyclonic flow regime affects Corsica into the Adriatic Sea. This flow interacts with the deep easterlies to the N between the blocking anticyclone and the mentioned vortex, which creates a broad area with persistent/deep confluent flow. Latest data takes this convergent flow into the Marche/Emilia-Romagna region but any passing vortex could temporarily shift this zone to the S and N. Growing confidence in this region comes from a rather stable run-to-run handling of latest NWP guidance including UK10/ID2/WRF.

This convergent flow regime is based in an unstable offshore airmass atop the Adriatic Sea, which features MUCAPE in the 1-1.5 kJ/kg range. TPWs reside in the 30-35mm range, which is shy above the background climatology (120%). CAP layered moisture products indicate pockets of enhanced mid/upper-level moisture just S of Sicily, which probably gets advected into our area of interest. This adds additional absolute moisture for this event.

Expect repeated showers/thunderstorms in an environment, supportive for efficient rainfall amounts (deep warm cloud layer and effective collision-coalescence process, deep saturation of the troposphere and a persistent slow moving convergence zone). Limited mass response due to a missing dominant LL vortex results in only modest inflow values, also reflected in modest EFI values for the IVT. Rather strong bora winds may balance this limited parameter however. Nevertheless, EFI signals for rainfall remain impressive with positive SOT values still present.

EPS data indicate reasonable probabilities for 50-100 l/qm/24h but certainly we cannot rule out peaks in excess of 200 l/qm (probably in less than 24h), either along the orography or where persistent training of convection occurs. A spout/tornado risk exists along the coastal regions as more intense updrafts interact with the vorticity-rich convergence zone/coastal convergence.
A level 2 was added with the main focus for the most organized band of convection from the afternoon into the overnight hours, as the upper low approaches from the W.

Otherwise a broad belt from Corsica/Sardegna to the Adriatic and Ionian Sea experiences conditions, which support slow moving/training convection. One focus will be beneath the upper low over Corsica/Sardegna, where heavy rain and a tornado/spout risk accompanies stronger storms. A nocturnal uptick in convective activity along the upper low's center could result in a rather active overnight period with semi-discrete convection capable of producing local flash floods beneath slow moving convection.

Another focus exists over the Ionian into the S/CNTRL Adriatic Sea (mainly offshore and along the coasts), where repetead convective activity is forecast. Background conditions support organized convection with a constant increase of DLS into the overnight hours. In addition, any storm interaction with mesoscale boundaries/coastal convergence zones could support a tornado risk but could also increase the risk of training storm activity with excessive rain on the local scale. Most intense storms could also produce isolated hail. The hail risk increases from Tunisia into the Strait of Sicily/Malta with more robust ML lapse rate/moisture overlap, which broadens mid-level CAPE profiles in strong background shear.

Highlighting any hot-spots is tough but right now we expanded the level 2 from the NW Adriatic Sea S into the Strait of Otranto, also adding some coastal areas W of Greece due to rather high storm coverage.
More (nocturnal) clusters are possible (e.g. over the Tyrrhenian Sea and around Malta), but confidence in any level 2 upgrade remains too low for now. N-Corsica is a high-end level 1. In case of a favorable placement of the upper low, ongoing training convection from the previous night could linger until noon.
Otherwise a broad level 1 was issued.

... NW Aegean Sea ...

Another level 2 upgrade was performed over the far NW Aegean Sea into the Thermal Gulf and adjacent areas. Responding to lowering MSLP to the W/NW, a persistent convergent flow regime keeps going from the previous night. Forecast hodographs indicate rather favorable conditions for a cluster of organized convection affecting the level 2 area, including numerous semi-discrete (tail-end) supercells. A tornado and excessive rainfall risk exists next to isolated hail. This risk diminishes until the afternoon hours, as BL winds turns N, which ends the E-erly onshore flow regime.

... Gulf of Valencia ...

The main focus for organized DMC activity occurs before noon in response to a passing short-wave impulse, which interacts with a zonally aligned LL convergence zone and moist/unstable (500-800 J/kg MUCAPE) conditions to its W/S. This area bends E/NE towards the Balearic Islands. Slow moving convection occurs and DLS around 15 m/s is adequate for a few better organized storms. Main focus is reserved for any storm, which rides this vorticity-rich boundary, which could support a supercell event including an isolatd tornado/spout risk. Otherwise heavy to excessive rain on a local scale remains the main hazard.

During the daytime hours, more storms erupt over E-into CNTRL Spain with an isolated gust/heavy rainfall/graupel risk.