Valid: Sat 15 Sep 2012 06:00 to Sun 16 Sep 2012 06:00 UTC
Issued: Fri 14 Sep 2012 21:56
A level 2 was issued for parts of Greece and parts of the W-Aegean Sea mainly for excessive rainfall, large to very large hail, severe wind gusts and an isolated tornado.
A level 1 surrounds the level 2 mainly for a lesser risk of aforementioned weather hazards.
A level 1 was issued for the Ionian, Adriatic and parts of the Tyrrhenian Sea mainly for isolated tornadoes/waterspouts and locally heavy rainfall amounts.
A zonal flow pattern evolves over N-Europe with numerous troughs racing from west to east. DMC probabilities remain low due to marginal moisture content in the lower troposphere.
A quasi-stationary upper low remains active over the Ionian Sea. Due to its very slow eastward motion, attendant surface cold front also remains in place over Greece and the Aegean Sea. Organized convection is forecast with that activity.
... Greece and Aegean Sea ...
The leisurely eastward moving upper low still dictates the weather over parts of the SE Mediterranean. At the surface, an eastward moving cold front seems to stall somewhere over E-Greece with no real net motion seen in forecast model fields. As long as the lower tropospheric vortex still moves southbound atop the Ionian Sea, no reason is seen for any boundary shift in W-E direction, so expect this boundary to linger over Greece all day long.
Rich BL moisture and a NE ward advecting EML create a supportive environment for 1000-2000 J/kg MLCAPE along and to the east of the cold front (e.g. E-Greece and Aegean Sea). CAPE diminishes further to the west and north (within the surrounding level 1).
A strong temperature/geopotential height gradient remains present along the eastern fringe of the vortex, so a brisk SSW-erly flow still affects that boundary with 0-6 km DLS AOA 25 m/s and only marginal weaker 0-3 km shear forecast. As the cyclonic vortex starts to weaken in the lower troposphere during the day, surface wind field becomes less cyclonic (or even opens up), which also results in weaker LL shear. Nevertheless, BL flow remains disturbed along the boundary with locally augmented LL shear/SRH-1 (but substantially higher SRH-3 values).
Both forcing and high-level diffluence support initiation all day long, so expect repeatedly DMC development within the highlighted area.
Given aforementioned parameters, well organized multicells and supercells ride along that boundary to the north/northeast. Main uncertainty will be the final storm mode, as either a broken line of DMC or numerous solid MCS events may evolve. Also, the severe risk may decrease over the N-Aegean Sea due to rain cooled air from a broad stratiform rain shield, which may evolve in conjunction with any MCS activity. The main concern will be the level 2 area, where either training storm activity may produce excessive rainfall amounts or where multicells/supercells may produce large to very large hail, severe wind gusts and an isolated tornado next to excessive rain.
It has to be emphasized that this kind of pattern may support locally life-threatening and flash flood producing rainfall amounts especially in the level 2 area!
The level 1 was expanded to the north, to account for pockets of enhanced diabatic heating and suface based activity. Shear and CAPE would be adequate for organized storms over the N-Aegean Sea/N-Greece with all kind of severe, but final placement and development of any MCS event remains obscure to go with a level 2.
Isolated and mainly elevated thunderstorm activity expands all the way to Romania/SW-Ukraine and is confined to the afternoon hours. With 20 m/s DLS forecast, we can't rule out an isolated large hail event (e.g. N-Romania), but expected severe coverage remains too marginal for a level 1.
... Tyrrhenian, Adriatic and Ionian Sea ...
The risk of waterspouts continues, given expected LL CAPE and vorticity fields. However, yesterday's soundings and synop data indicate that LL CAPE may be a bit lower to what GFS forecasts and LL lapse rates also show a weakening trend during the forecast, so the overall spout risk might be on the isolated side. Still a level 1 was added.
That level 1 was also expanded to a few onshore areas, where heavy rain may still occur due to repeatedly onshore moving showers and thunderstorms. Isolated 100 mm/24 h amounts are still possible, but with a weakening structure of the cyclonic vortex, bands of DMC also fail to organize as well as yesterday.