Storm Forecast
Valid: Sat 03 Nov 2018 06:00 to Sun 04 Nov 2018 06:00 UTC
Issued: Fri 02 Nov 2018 21:31
Forecaster: TUSCHY

A level 2 was issued across parts of the S-CNTRL Mediterranean mainly for excessive rain.

A level 2 was issued for parts of Sicily mainly for excessive rain, large to very large hail, tornadoes (a strong one is possible) and severe wind gusts.

A level 2 was issued for NE Algeria mainly for excessive rain. An isolated tornado event is possible.

A level 1 surrounds both level 2 areas with similar hazards but lower probabilities.


SYNOPSIS

An omega-blocking pattern exists over the Mediterranean. A ridge stretches over Greece and the Aegean Sea towards Romania and remains framed by two upper-level troughs. The western one over the W/ CNTRL Mediterranean will be the main "player" with respect of organized DMC activity due to overlapping ingredients, whereas the eastern one suffers from the advection of dry air from Turkey and the E Mediterranean region.
A mobile ridge displaces the polar front jet far north over N Scandinavia, where bone-dry air also precludes deep updrafts.
The quasi-stationary mid/upper low over N-Tunisia establishes a meridional alignement of the isotherms over most parts of CNTRL/S Italy. This baroclinic zone is characterized by a wavy front, which distorts in course of a developing and northward moving surface depression. The attendant cold front organizes between the Guld of Gabes and Malta and affects Malta/Sicily during the night from the SW. In the meantime a warm front pushes NW over Sardegna and Corsica.

DISCUSSION

... Corsica, Sardegna, Sicily, Malta and coastal areas of Tunisia and NE Algeria ...

During the daytime hours until 18Z ...

... the highlighted areas reside between a departing short-wave and ahead of a gradually developing impulse to the south (over Tunisia), which both circle the main upper trough. Most models indicate an ongoing thunderstorm cluster just northwest of Sicily, which shifts N/NW during the daytime hours. Uncertainties exist about this cluster as subsiding air along its periphery could hamper CI temporarily. In addition this cluster could depart from the most unstable air mass over the SE Tyrrhenian Sea. Most models indicate a weakening trend while drifting the cluster towards Corsica and Sardegna, but we would not be surprised about a constant E/NE ward drift in response to ongoing 25-30 kn SE-erly BL flow over the E/SE Tyrrhenian Sea. This could support the cluster to build into the more unstable air mass to its E/SE. Most of this activity stays offshore with excessive rain and a few waterspouts the main risk. W-CNTRL Italy has to keep an eye on this convection as parts of this activity could graze the coastal areas with the mentioned hazards.

A significant rainfall risk evolves over far NE Algeria, where persistent onshore flow of a moist marine air mass with up to 500 J/kg SBCAPE causes onshore moving showers and thunderstorms, which impact the same area for hours. Orographic lift results in additional lift for excessive rainfall amounts. A few waterspouts along a well defined convergence zone are forecast. Flash flooding is likely and a level 2 was issued.

A third area of concern exists over W/SW Sicily. Despite a temporarily relaxing 1-2 km AGL flow, conditions for slow moving or temporarily back-building convection remain in place and excessive rain will be a distinct possibility. During the afternoon and evening hours a gradual increase in DLS and curved hodographs indicate a growing chance for supercells with large (isolated very large) hail, severe wind gusts and tornadoes. The amount of LL shear with very low LCL heights also indicate a chance for an isolated strong tornado event. Hence a level 2 was issued for those areas.

NE Sicily was added to the level areas for temporarily back-building thunderstorms (mainly driven by orography and less by synoptic forcing). Stronger cap should keep this activity more isolated, but excessive rain and an isolated tornado/large hail risk is possible with this activity.

Marginal CAPE probably precludes organized thunderstorms over Tunisia, although an isolated severe (hail/wind) event can't be excluded. Favorable shear assists in updraft orgaization in case of regionally better CAPE and hence a broad level 1 was issued. Heavy rain due to slow movin convection remains the main hazard.

During the overnight hours until 06Z ...

... a developing and intensifying vortex crosses the Strait of Sicily from S to NW. GFS and IFS remain rather bullish with its intensification rate and hence a broad swath of 40-50 kn SE-erly 1-2 km AGL flow overspreads Sicily until 00Z and most of the Tyrrhenian Sea thereafter.
Until the cold front arrives at Sicily (around midnight), conditions over W/SW Sicily remain very conducive for severe thunderstorms with all kind of hazards. The main concern will be favorable conditions for training thunderstorms in the prefrontal air mass, which could produce torrential rain on a regional scale. In addition, DLS and LL shear support organized multicells/supercells with an attendant tornado and large hail threat (both could be significant). Severe wind gusts are also possible. Beyond 00 Z, NW-ward departing vortex leaves a trailing cold front behind, which impacts S/E Sicily during the second part of the night. A V-shaped MCS with an excessive rainfall risk (isolated large hail still possible) affects those areas and hence the level 2 was expanded E. This cluster also affects Malta with similar hazards.

Towards Sardegna and Corsica, DLS relaxes while low-tropospheric wind field responds to the deepening vortex over the SW Tyrrhenian Sea, so back-building convection with excessive rain impacts most of the Tyrrhenian Sea and the E-facing coasts of those islands. Especially E Corsica will probably see a prolonged period of onshore moving and temporarily back-building convection and hence a level 2 was expanded to the NW. We're not so sure about Sardegna due to a temporal lull in thunderstorm activity and less CAPE to work with. Nevertheless, the risk of excessive rain increases rapidly during the end of the forecast as the vortex approaches from the SE and hence we also included E Sardegna into a level 2.

An ongoing excessive rainfall risk exists over far NE Algeria, although the main activity gradually drifts E with a weakening trend during the night as the low-tropospheric flow turns more NW instaed of N.
N Tunisia was included into the level 2 to account for an excessive rainfall risk with onshore moving thunderstorms.