Storm Forecast
Valid: Sat 03 May 2014 06:00 to Sun 04 May 2014 06:00 UTC
Issued: Fri 02 May 2014 22:43
Forecaster: TUSCHY

A level 2 was issued for the N Aegean Sea mainly for large hail, severe wind gust and an enhanced tornado threat.

A level 1 was issued for the Aegean Se, N Greece and NW Turkey mainly for large hail, severe wind gust and an isolated tornado event. SE Bulgaria could see a few large hail events.

A level 1 was issued for parts of Slovenia, Croatia, N-Bosnia and Herzegovina, parts of Serbia, W-Romania, parts of Slovakia and Hungary mainly for excessive rain (especially the southern part of the level 1) and for a few large hail events (mainly Hungary to the north).

A level 1 was issued for CNTRL/S-Italy, the N-Ionian Sea and the S-Adriatic Sea mainly for excessive rain and an isolated tornado event.

SYNOPSIS

No major change in the synoptic pattern is anticipated. An upper low over the Tyrrhenian Sea starts to lift more to the E/NE as a 35 m/s mid-layer jet emerges from its base and spreads to the Ionian and Aegean Sea during the overnight hours. A ridge covers the far E-Atlantic and despite lowering mid-level heights, it continues to affect Ireland and UK during the forecast. A pronounced branch of the polar vortex remains in place over Scandinavia with cold low/mid tropospheric air. A few short-lived thunderstorms are possible over Scandinavia. Confidence in numerous lightning reports however is too marginal for a lightning area.
To sum up, the persistent blocking pattern is forecast to continue through the forecast period.

At lower levels an E-W bedded baroclininc zone runs from S-France to the Alps and all the way to the Ukraine. The wavy appearance of that front is due to the orography and also due to regionally enhanced thunderstorm activity from the previous day. In addition, a diffuse and broad LL vortex over the CNTRL Mediterranean drifts to the E/NE and some models show the development of other weak vortices over the E-Balkan States within that large-scale cyclonic flow. The main vortex over the CNTRL Mediterranean is accompanied by a wrapped up occlusion, which approaches the wavy braroclinic zone to its north. A complex interaction of those fronts is forecast from Croatia to Hungary and to the south.
Synoptic fronts also graze Ireland, UK and Scotland but they won't produce any organized convection.


DISCUSSION

... Slovenia, Croatia, parts of Serbia and S-Hungary ...

The main question will be how much insolation can be realized as this region resides between a curl of thick clouds associated with the occlusion of the Mediterranean's vortex and widespread thick cloud cover along the slowly southward sagging front over the Alps. Also, cirrus clouds from the overnight convection probably affect the area. Current thinking is that at least temporal diabatic heating can materialize. Weak mid-layer lapse rates (less than 7K/km for most places) will be offset by constantly increasing and deepening BL moisture due to strengthening BL convergence. Widespread 300-700 J/kg weakly capped MLCAPE are forecast even with limited insolation.

Shear magnitude oscillates around 10 m/s for the 6 km bulk shear. Also, with strengthening synoptic forcing and widespread CI, expect a mess of showers and thunderstorms, which grow upscale into numerous disorganized clusters. Initial storms could produce large hail on a very isolated scale. The main hazard will be heavy to isolated excessive rain with slow moving clusters. A temporal overlap of aforementioned CAPE and strengthening 30kt E/NE-erly 0-2 km winds with low LCLs and increasing SRH-1 could result in an isolated tornado event.

During the overnight hours, CAPE drops due to loss of diurnal heating, so expect thunderstorms to quickly decrease in strength and coverage. From now on, a lot depends on if and where another LL vortex tries to develop over the S/E Balkan States ahead of the main vortex over the Adriatic Sea. Most models agree in convergent flow with a plume of PWs in excess of 25 mm over Serbia and E Croatia. Convectively enhanced precipitation continues from Croatia to N-Serbia and models like GFS/WRF indicate spots with QPF well in excess of 100 mm/24h. The main reason against upgrading to a level 2 is the ongoing uncertainty about the evolving LL depression, but flash flooding should become likely. The event also seems to be stratiform in nature with embedded convection. The heavy rain may affect SW Romania during the end of the forecast.

...CNTRL/N-Hungary, Slovakia, SW Ukraine and W-Romania ...

Another round of widespread DMC activity along the wavy front is forecast. Environment features 0-6 km shear of 10 m/s or less with 500-900 J/kg MLCAPE and a weakly capped air mass. Initial storms produce a few large hail events, but betimes, clustering storms pose an heavy rainfall threat. This activity gradually diminishes after sunset. A level 1 for the mixed hazard was issued (and combined with the level 1 to the south).

... S-Italy ...

Beneath the upper low, scattered thunderstorms are forecast due to the augmented vertical temperature gradient between cold mid-levels and the warm SSTs. Most of the activity will be pulsating convection with marginal hail, gusty winds and bursts of heavy rain. An isolated tornado event is forecast mainly along the coasts, where enhanced BL convergence and LL CAPE overlap. However an isolated (onshore) event is also possible over CNTRL/S-Italy. CNTRL Italy was upgraded to a level 1 due to slow moving storms with excessive rain.

DLS in excess of 15 m/s from Sicily to the NE could support a few better organized storms with isolated large hail and a tornado or two. With persistent moisture advection from the W-Ionian Sea to the S-Adriatic Sea, a cluster of storms probably evolves around sunset and continues over the S-Adriatic Sea/N-Ionian Sea during the night. Excessive rain accompanies this cluster and affects extreme SE Italy. Tail-end storms over the N-Ionian Sea might see DLS in excess of 15 m/s, dependent on how far south convection builds. Therefore we don't want to exclude an isolated severe risk over SW Albania and far NW Greece in case this cluster approaches the coast. Excessive rain would be the main hazard.

... NE Aegean Sea, 00-06 Z...

Beyond midnight, the NE Aegean Sea will be placed along the cyclonically sheared side of a 35 m/s mid-layer jet with cooling mid-levels and strengthening synoptic forcing. The BL air mass over the Aegean Sea features an high moisture content, so the environment improves for DMC. DLS in excess of 30 m/s assists in rapid storm organization with multicells/supercells. Large hail, strong to severe wind gusts and an isolated tornado threat accompany the convection. The tornado threat will be maximized for storms, which approach the coasts where LL shear exceeds 30-35 kt.
We expanded the level 1 area well inland (all the way to SE Bulgaria) to reflect ongoing moisture advection and destabilization of the air mass well onshore. Latest data keeps CAPE too marginal for organized DMC over Bulgaria, but we won't exclude a few hail events with more elevated but organized convection.

... Crete, 00-06 Z ...

Crete and surrounding areas will be monitored for potential CI. Strong cap will be in place, but weakens from west to east. In addition, faint synoptic forcing grazes the region, so we can't rule out isolated convection. Shear would be intense, both directional and speed shear with severe wind gusts the main hazard. An isolated tornado event is possible in case storms root into the strongly sheared BL. With EZMWF still suppressing convection, we did not yet upgrade to a level 1.