Storm Forecast
Valid: Fri 14 May 2010 06:00 to Sat 15 May 2010 06:00 UTC
Issued: Thu 13 May 2010 23:21
Forecaster: TUSCHY

A level 2 was issued for the Balearic Islands mainly for large hail, an isolated significant event possible.

A level 1 was issued for the Balearic Islands and surrounding areas mainly for excessive rainfall.

A level 2 was issued for parts of Italy, parts of Sicily, parts of the Adriatic Sea and adjacent regions mainly for large hail, severe wind gusts and tornadoes ( significant events possible ).

A level 2 was issued for parts of S-central Italy, Montenegro, N-Albania and S- Bosnia and Herzegovina mainly for excessive rainfall.

A level 1 was issued for NE-Poland, W-Belarus, Lithuania and Latvia mainly for excessive rainfall and an isolated tornado.

A level 1 was issued for SE-France, N-Italy and parts of Austria mainly for large hail.

SYNOPSIS

A potent upper trough enters the W-Mediterranean during the morning hours with a rapid translation towards the east. Conditions become favorable for the development of a gradually intensifying surface depression over Italy, which reaches the central Adriatic Sea until 06Z (15th May). Another surface depression over NE-Poland/W-Belarus lifts northwards and later-on northwestwards during the day. Cool and quite stable air overspreads most parts of W/W-central Europe, suppressing deep convection.

DISCUSSION

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.

... The Balearic Islands 06-18Z ...

The main story will be the potent trough, rotating through the highlighted area during the day. An overlap of 20-30m/s DLS, mid-level lapse rates of 7.5 to 8 K/km and some directional shear in the lowest 3 km hint on well organized multicells/supercells with a large hail threat and even a significant event can't be ruled out. LL shear is weak, but this limiting factor for LL mesocyclone development may be offset by abundant of LL CAPE and augmented shear along outflow boundaries, so we also inctroduce tornado probabilities. The severe risk may linger until 15Z but gradually translates into a heavy rainfall risk, as conditions become better for training thunderstorm development/rapid clustering beneath the upper trough. Background shear relaxes with mid-level cooling atop still moist BL air mass, so showers/thunderstorms may last until the evening hours. Combined with severe convection at the morning, locally excessive rainfall may justify heavy rainfall probabilities, so combined with the large hail threat, a level 2 was added. Activity will finally vanish after sunset.

... NE-Poland, W-Belarus, Lithuania and Latvia ...

The main risk will be excessive rainfall with slow moving thunderstorms. A weak depression over Belarus lifts northwards during the forecast with good moisture pooling forecast in a confluent streamline pattern. Latest synop data reveals dewpoints of 15°C and moisture remains in place throughout the night/tomorrow. Mid-level lapse rates remain near moist adiabatic with weak shear at all levels, so rapid clustering of thunderstorms is forecast. Model output (e.g. GFS) seems to be overdone with dewpoints near 20°C, therefore SBCAPE forecast will be cut in half - 800 to 1600 J/kg expected. Slow storm motion, moist BL, enhanced convergence as surface low draws near from the south, rapid clustering and persistent/moist influx from the SE point to an augmented excessive rainfall risk. Given abundant LL CAPE and very low LCLs, an isolated tornado event can't be ruled out along intersecting outflow boundaries or beneath a rapidly strengthening updraft. Also, hail may accompany stronger storms with atmospheric conditions looking supportive not for large hail but copious amounts of small hail with a deep-moist column and strong updrafts (and weak shear/lapse rates).

This large cluster of storms will move offshore during the late afternoon/evening hours in a more stable environment over the central Baltic Sea, so a constant decline in thunderstorm activity is forecast. Nevertheless, heavy rain may reach SE-Sweden during the later night/morning hours.

... SE-France, N-Italy and S-Austria ...

Daytime driven thunderstorm development is forecast with 200-800 J/kg SBCAPE and 15-30m/s DLS. A few large hail reports may be received beneath better organized multicells. Overall risk diminishes after sunset.