Storm Forecast
Valid: Fri 28 Nov 2008 06:00 to Sat 29 Nov 2008 06:00 UTC
Issued: Thu 27 Nov 2008 18:01
Forecaster: TUSCHY

SYNOPSIS

Another arctic outbreak is underway, although this time more to the west or just west of Europe. An extensive vortex establishes over Europe, resulting in unsettled conditions as numerous disturbances rotate around this feature. Cool and stable conditions prevail over E-Europe.

DISCUSSION

... Sicily, Italy ,Tyrrhenian Sea, and the Adriatic Sea ...

Generally speaking, all models converge to the solution of a developing surface depression just off the coast of NE-Algeria already during the early morning hours, moving slowly to the north, before it strikes a new path more to the NE. They also agree in timing and path solution as this feature crosses N-CNTRL Italy during the evening and night hours. However the strength of this low remains still uncertain with WRF/GME on the more intense side ( sub-990hPa pressure core between Corsica and Italy ), whereas the rest of the model pool sticks with the more conservative/weaker solutions. Not the best data coverage over Algeria could be one reason for the remarkable spread. Final strength of the surface depression is crucial for vigorousness of return flow and hence instability release over the forecast area.However, as even more conservative models show enough CAPE for sustained convection, confidence has increased during the past few runs that organized thunderstorms are possible in the highlighted area.

...Sicily and S-Italy ...

Thermal ridge over Sicily and S-Italy flattens out quite fast with strongest CIN values well to south of Sicily/S-Italy, so atmosphere will be weakly capped. Both scenarios, a persistent moisture advection in the broad warm sector and some cooling at mid-levels will be more than adequate for widespread 400-800 J/kg SBCAPE with MLCAPE values running a bit lower. On the contrary, WRF has well above 1kJ/kg CAPE, so we think that plenty of instability will be in place, especially offshore.

Focus for initiation is quite difficult, as intense 40-45m/s mid-level streak sneaks in from the SW already during the morning hours, so initiation should already occur over Sicily during the morning hours and S-Italy around noon, which is in line with most of the models. Thereafter, a slowly eastward propagating cold front crosses the area from the west, affecting Sicily between 09-18Z and S-Italy between 15Z-00Z. Thunderstorm coverage will be widespread.

Shear values are impressive! (What a surprise for the winter time). As mid-level jet approaches, DLS boosts to 30-45m/s and 0-3km shear values reach up to 20m/s, so developing thunderstorms will gain organisation right away. It looks like a multiple severe weather threat for the upcoming period. Initiation in the prefrontal warm sector should be more of isolated nature, so discrete thunderstorms are well possible in an environment with LCLs below 500m, LL shear above 10m/s and SRH1 values in excess of 200m^2/s^2. In combination with aforementioned intene DLS, the environment is favorable for LL/mid-level mesocyclones with an attendant tornado threat. Even LL CAPE release increases during the day and we can't exclude strong tornadoes with such a set-up. Large hail and severe wind gusts are also likely with those supercells.

In addition, the cold front will spread eastwards and environment remains favorable for all facets of severe weather.More front-parallel flow, persistent and moist inflow from the S and a strong/progressive vorticity lobe hint on a mixture of a rapidly eastward racing and a southward/back-building MCS over S-Italy at 18Z onwards with a more widespread severe wind gust threat especially over extreme SE-Italy where latest GFS runs constantly had 35-40m/s at 850hPa during the cold front passage. Thunderstorms will finally clear the area to the east/southeast after 00Z.

...Tyrrhenian Sea, CNTRL Italy and the N/CNTRL Adriatic Sea ...

A strong and progressive cold front will reach the coastal area of W-CNTRL Italy between 9-12Z from the WSW. Prefrontal, stable airmass is strongly sheared with SRH1 of 400-600m^2/s^2. Passage of cold front coincides with the arrivial of a well evolved dry slot and strong lift, so there is the concern of potential instability release along the cold front and enhanced thunderstorm evolution just along the backside of the boundary. Those storms could still take profit of the intense LL shear/ abundant 0-3km CAPE release and tornadoes would be possible also well inland despite rapidly diminishing CAPE. We want to examine further model data before updating/upgrading.

The same high-shear/moderate instability environment evolves over the Adriatic Sea and especially along the NE/E coast. Right now we are somewhat concerned about the intense convection over S-Italy and attendant anvil debris/weakening mid-level lapse rates, which could affect most parts of the CNTRL/S-Adriatic Sea, limiting thunderstorm chances. However modest MLCAPE over the N-Adriatic Sea looks likely during the evening hours, as mid-levels continue to cool down. Rapidly strengthening DLS to well above 30m/s permits rapid thunderstorm organisation and an attendant severe wind gust / large hail / isolated tornado threat.

An augmented waterspout risk exists for Corsica and surrounding areas during the morning and early afternoon hours due to weak background shear and steep LL lapse rates.

... E-Atlantic and parts of Spain/Portugal ...

Outbreak of modified arctic air affects the area during the day. Airmass at lower levels remains quite moist as mid-levels cool down dramatically to well below -30°C at 500hPa. Increasing parcel layer depth and numerous, albeit weak, convergence zones should justify a broad thunderstorm area. Between 00Z and 06Z, the tornado/strong wind gust risk increases over extreme NW Spain / Portugal and the Strait of Gibraltar and level areas were drawn.