Valid: Thu 26 Feb 2015 06:00 to Fri 27 Feb 2015 06:00 UTC
Issued: Wed 25 Feb 2015 19:53
A level 1 was issued for southern Greece and surroundings mainly for large hail and to a lesser extend severe wind gusts, excessive rain, and tornadoes.
SYNOPSIS / DISCUSSION
A subtropical ridge axis turns south-east into central Europe, so that the low geopotential across eastern and southern Europe looses its connection to the polar jet. The ridge axis will rapidly weaken due to a new intense Atlantic trough spreading into western Europe later in the period. Rather dry air masses will limit the chance of deep moist convection across most places, with the best chances below the low geopotential across eastern Europe and the Black Sea area. Further storms are possible late in the period to the west of Norway in a deeply mixed and rather moist maritime air mass.
Main concern is the central and eastern Mediterranean. In the range of the cut-off low, latest soundings indicate moist neutral profiles with steep low-level lapse rates (LIRE and LIBR ascends). Weak CAPE is associated with these profiles that are also expected on Thursday. Especially the Ionian Sea, Tyrrhenian Sea, and Adriatic Sea will be affected. Storms are forecast to go on through-out the period with a nocturnal offshore maximum and some onshore storms in the noon and afternoon hours. Organized storms are not forecast given weak vertical wind shear, but a waterspout is not ruled out given locally large low-level buoyancy and weak vertical wind shear.
Better organized storms can be expected ahead of the cut-off low across Greece and the Aegean Sea. Warm air advection from the south-east is associated with QG lift and quite strong vertical wind shear, exceeding 15 m/s in the lowest 3 km. Given elevated steep lapse rates and around 7 g/kg mixing ratio in low levels, CAPE can overlap with lift and favourable veering profiles. Multicells are expected to evolve with a potential of producing large hail and local severe wind gusts, especially in association with quasi-linear storms. Limiting factor will may be some capping, and initiation is quite uncertain in the eastern portions due to the lack of large scale forcing.
In the evening and night hours, the warm sector air masses to the south of Greece can become more unstable. Models agree that an elevated mixed layer advects offshore from the African land mass. Increasing low-level moisture will result in CAPE. This unstable air mass could support increasing thunderstorm activity along and west of the warm front and a few storms may also form within the warm sector.
Increasing vertical wind shear with veering profiles may support a few supercells, capable of producing large hail and severe wind gusts. Along and west of the warm front, storms may cluster during the night hours, posing a threat of local excessive rain and large hail. Tornadoes are not ruled out given rather strong low-level vertical wind shear.