Storm Forecast
Valid: Mon 23 Jul 2012 06:00 to Tue 24 Jul 2012 06:00 UTC
Issued: Sun 22 Jul 2012 23:29
Forecaster: TUSCHY

A level 2 was issued for parts of C/S Italy and parts of the Adriatic Sea mainly for excessive rainfall amounts, large to very large hail, severe wind gusts and tornadoes.

A level 2 was issued for NE-Sardinia/SE Corse mainly for excessive rainfall amounts.

A level 1 was issued for all of the C-Mediterranean area mainly for large hail, heavy rainfall, strong to severe wind gusts and isolated tornadoes.

A level 1 was issued for N-Serbia and W-Romania mainly for isolated large hail.

SYNOPSIS

A cold-core low drops to the south atop the very warm Mediterranean and sparks widespread and well organized deep convection. A serious flash flood risk arises for parts of Italy with this kind of synoptic pattern.

Forecast models show a gradually evolving, quasi-stationary LL depression over the Tyrrhenian Sea. Latest ASCAT/synop data do not show any trustworthy signs of a closed surface circulation although a few SE-erly wind reports west of Rome could be a first sign. Despite an impressive vertical temperature gradient atop the warm Mediterranean, no model has yet a sign of a thermodynamic core modification of this evolving depression. We won't exclude some shallow warm-core structure atop the Tyrrhenian Sea during the end of the forecast, but the overall circulation of this deep vortex seems to be too broad for any rapid contraction/intensification of the low-level circulation center (LLCC) until 06Z. Also, thermodynamic stratification becomes worse from NW to SE as drier air temporarily filters into the cyclonic circulation and mid-layer warms by a couple degrees. In any case, due to the slow development and the placement well offshore, no serious risk for onshore areas is currently forecast until 06Z. We'll wait for tomorrow's HVIS data to define how healthy any potential LLCC will be.

Ridging builds in from the W over W/C-Europe and results in stable conditions.

DISCUSSION

... C-Mediterranean ...

Unseasonably strong cut-off low (500 hPa temperature dropping to -15°C or less) moves atop the warm Adriatic Sea (SSTs 23-26°C/positive SST anomaly of 1-3K) and the Tyrrhenian Sea (24-28°C/positive SST anomaly of 2-4K). Intense mid/upper jet streak circles around the deep cyclonic vortex and results in a more E-W aligned trough axis during the end of the forecast which also omits any southbound motion during the night. Iffy/erratic motion of the gradually consolidating LL depression atop the Tyrrhenian Sea is hard to forecast, but a slow drift to the SW-part of the Tyrrhenian Sea seems most plausible.

07645/16080 (all placed in the evolving CAA regime upstream of the southward moving upper low) show a dry air mass overspreading the Ligurian Sea/W-Mediterranean but this air mass remains displaced west of Corse/Sardinia. Worst forward calculated trajectories (regarding moisture) emerge from the N-Balkan States towards the W/SW with surface dewpoints in the lower to mid tens affecting N-Italy, Corse and Sardinia. Gradually strengthening convergent flow from the Adriatic Sea to S-Italy results in surface dewpoints in the lower twenties. Also no advection of dry/stable continental air from N-Africa towards the Ionian Sea is expected. Aforementioned BL quality beneath moderate mid-level lapse rates from the Tyrrhenian Sea to the north and steep lapse rates atop the Ionian Sea cause widespread 1000 J/kg MLCAPE with peaks up to 2000 J/kg over S/E-Italy/the Ionian and the Adriatic Sea. Persistent signals of augmented LL-CAPE/vorticity availability along the E-coast of Sardinia remain in the model fields, gradually spreading towards Sicily until 06Z.

A belt of intense shear accompanies mid/upper jet maximum, which circles around the cyclonic vortex. 3km and 6 km bulk shear in excess of 20 m/s spread over Sicily/S-Italy into the Adriatic Sea during the forecast with a rapid drop towards the Tyrrhenian Sea. Directional/speed shear in the lowest 1000 m remains weak during most of the daytime hours but increases during the evening hours onwards mainly from S-Italy into the Adriatic Sea.

This background information leads to the following regional severe risks:

... Corse, Sardinia and the Tyrrhenian Sea ..

Degree of LLCC intensification dictates the final risk (especially regarding organized lines of onshore moving convection). Waterspouts along the E-coast of Sardinia can occur all day long. With the depression becoming quasi-stationary to the east of Sardinia, prolonged period of onshore flow also causes heavy to excessive rain along the eastward facing slopes of both islands. A few model runs in the past had signals for a level 2 rain event, but with drier air filtering in from the north during the day, confidence remains below that threshold for most parts. However, life-threatening flash floods are possible as not much rain is needed to result in strong runoff (especially for those areas, where devestating forest fires occurred during the past few weeks). Hence we upgraded NE Sardinia/SE Corse (also keeping in mind that the thetae axis points to this area until noon).

... S-Italy ...

Shear/CAPE overlap is very favorable for rapid storm organization with long-lived multicells/supercells expected. Prolonged period of impressive forcing/divergent mid/upper flow regime assist in CI all day long (which may limit number and life time of discrete thunderstorm structures and increasing the risk for large storm clusters). Any discrete/tail-end storm will be capable of producing large hail (isolated very large hail events likely), severe wind gusts and very heavy rainfall. Although tornado probabilities remain clear during the forecast, LCLs at or below 1000 m and modest LL lapse rates may support an isolated tornado event. We upgraded that area to a level 2, given the chance for numerous extreme events (very large hail and excessive rainfall/flash flooding). Thunderstorms keep going all day long.

... C/E-Italy ...

Good news that past few model runs brought the steering cyclonic vortex more to the south over the Tyrrhenian Sea, so despite a prolonged period of easterly flow normal to the coast (with QPF amounts in excess of 300 mm/24h forecast by some models), current BL wind field only temporarily reveals the true easterly wind component towards the N-C Appennini Mountains. Nevertheless, very moist inflow from the Adriatic Sea (featured by SBCAPE in excess of 1500 J/kg) and upslope flow result in excessive rainfall amounts with an high-end flash flood risk. QPF of 100-200 mm/24h are likely. Any more discrete thunderstorm will see adequate shear/CAPE for organized updrafts with large hail and strong to isolated severe wind gusts. The tornado risk increases during the late afternoon hours onwards, when SRH-1 increases to 200-400 m^2/s^2 / EHI-1 of 3 (plus) and we don't want to rule out an isolated strong tornado event from Rimini to Foggia.

We added most parts of the Adriatic Sea including the E-coasts to the level 2 for the same risks like excessive rainfall, large hail, severe wind gusts and tornadoes.

... SW/W-Italy ...

Added a level 2 for excessive rainfall amounts for this part of Italy and an augmented flash flood risk can easily evolve from Naples to Rome due to the prolonged period of onshore moving storm clusters. The overall risk decreases somewhat towards Rome, where winds back to the SE/E, but storms moving along the coast may still result in a prolonged period of heavy rain. Hence the level 2 was expanded far NW-wards along the W-coast of Italy.

... Balkan States ...

Scattered thunderstorms are forecast beneath cool mid-levels. DLS increases from east to west with modest CAPE, so a few multicells with isolated large hail and strong wind gusts are forecast. Best CAPE probably evolves from W-Romania into N-Serbia, where a marginal level 1 was added. Also the level 1 from the Adriatic Sea was expanded onshore a bit to reflect a few strong to severe storms with 15 m/s DLS and 800 J/kg MLCAPE.