Party time ;D
"An outbreak of severe thunderstorms is probable over S-Italy, the S-Adriatic Sea and adjacent areas after midnight with all kind of severe possible, including flash flooding.
... S-Italy, parts of Sicily, S-Adriatic Sea, N-Ionian Sea, parts of Albania, Bosnia and Herzegovina, S-Croatia and Montenegro ...
A progressive trough approaches from the western Mediterranean during the evening/night hours with robust geopotential height falls spreading rapidly towards the east. The main high-level support for initiation will be a strong wave, ejecting out of the base of the neutral tilted trough, placed just east of the Balearic Islands at 00Z (15th May). This wave crosses Sicily and most parts of central/south Italy from roughly 21Z onwards with potent UVV maximum lifting northeastwards thereafter. Initiation over the southern parts of the level 2 area may be more conditional in nature, as best forcing grazes the area to the north.
At 18Z onwards, atmopshere destabilizes rapidly with strong WAA regime at 900-800hPa and attendant increasing mid-level lapse rates (next to EML layer spreading northeastwards), so a vast area with increasing MUCAPE evolves over Sicily north/northeastwards. Betimes, surface based CAPE is on an increase over the Tyrrhenian Sea and the central Adriatic Sea after 21Z, along the cyclonic shear side of the mid-/high-level jet with GFS/WRF pointing to 400-800 J/kg SBCAPE, maximized offshore/along coasts.
As the upper trough gradually acquires a more negative tilt throughout the night hours, already intense hyperbaroclinic zone continues to get more intense with a phasing polar/subtropical jet. 1-6km shear of 30-60m/s, 1-8km shear topping out at 50m/s, 0-3km shear of 20-35m/s and SRH-3 values of 300-600m^2/s^2 overspread the area from the SW during the evening/night hours and are more than adequate for strong and long-lived mid-level mesocyclones. Final degree of potential MUCAPE magnitude may be re-analyzed in respect of shear parameters being too extreme for updrafts.
Models agree well in the rapid development of a surface depression, moving off the coast of NE Algeria/N-Tunisia with a movement to the NNE, later straight northwards over central Italy/central Adriatic Sea The final strength still remains a bit unclear due to model discrepancies with the data-limted area over N-Africa. In any case, strengthening depression assists in a rapid ageostrophic deflection from the background flow in the lowest 1-3km, increasing LL speed/directional shear to 10-20m/s / 150-300 m^2/s^2, maximized along the east coast of the Adriatic Sea and over S-Italy.
Current scenario of the expected event foresees a gradual increase of elevated supercells over most parts of the level 2 area during the late afternoon hours. Overlap of steep lapse rates, strong directional shear and extreme speed shear point to a large hail threat with any thunderstorm with significant hail also well possible in long-lived updrafts/mid-level mesocyclones. Thunderstorm coverage and intensity becomes more widespread/intense after 21Z over the Tyrrhenian Sea , where surface based convection may form, spreading rapidly east/northeastwards. This convection may pose a risk for significant hail, severe wind gusts and tornadoes and a long-lived and significant tornado can't be excluded, given rapid storm motion and LCLs at or below 600m. Those storms haste towards the central Adriatic Sea, Bosnia and Herzegovina/Montenegro and S-Croatia after midnight.
In response to the deepening surface depression, a 20-30 m/s LLJ evolves ahead of the eastward shifting cold front, covering the Ionian Sea and the S-Adriatic Sea until 06Z. All ingredients are present for excessive rainfall over N-Albania, Montenegro, S-Croatia and parts of Bosnia and Herzegovina, as 12-13 g/kg 1 km ML mixing ratios advect northwards beneath incoming intense UVV maximum and high-level divergence. Combined with topographic lift and aforementioned LLJ magnitude, a backbuilding MCS may evolve with training storm activity.
No level 3 was yet introduced, due to the uncertainty in respect of the final magnitude of SBCAPE build-up. However, forecast soundings/windgrams reveal a tornado-prone kinematic environment overlapping with modest CAPE, so this region definitively has to be monitored.
This event continues well after 06 Z. "
