Storm Forecast
Valid: Mon 25 Feb 2013 06:00 to Tue 26 Feb 2013 06:00 UTC
Issued: Sun 24 Feb 2013 22:00
Forecaster: TUSCHY

A level 2 was issued for the coastal areas of SW Albania, W Greece and NE Greece mainly for heavy to excessive rainfall amounts.

Two level 1 areas surround those level 2 areas mainly for an heavy rainfall risk, isolated large hail and an isolated tornado event.

SYNOPSIS

Vertically stacked cold-core low atop the W-Mediterranean shows no signs for any serious motion during the forecast with cold and dry air spreading south over most of the C/W Mediterranean. An extensive warm conveyor belt is present along its eastern fringe, which continues to tap into rather rich Mediterranean moisture over the eastern part of the Sea, with large-scale isentropic lift ongoing over most of SE Europe. Thunderstorm activity will be both WAA regime driven and thermally driven.
Over N-Europe dry air, surface high pressure and warming mid-levels keep the environment hostile for deep and electrified convection.

DISCUSSION

... Greece and Albania...

Phasing of the two major jets creates a broad, well structured and cyclonically curved 40 m/s mid-level jet streak over far N-Algeria/Tunisia with its exit region pointing towards Greece. In combination with persistent PVA and upper divergence, a prolonged period of initiation is expected within the highlighted area. The focus for initiation likely is an eastward moving and gradually dissolving frontal boundary, where most widespread thunderstorm activity should occur.

Until noon, scattered thunderstorms are forecast over the E-Ionian Sea, affecting the SW-coast of Albania and W/S-Greece. Leisurely eastward motion of the front and improving upper level support should keep an MCS going, which gradually builds south along the W-coast of Greece. As long as the front remains offshore, prefrontal 15-20 m/s southerly winds cause persistent influx of moist air into this system, so training activity is possible along the coastal areas. Latest data bring this boundary ashore during the late afternoon hours, which also marks the end of the training thunderstorm activity. Obviously, heavy rain with flash-flood problems will be the primary risk, although enhanced LL shear along the coast may result in a few supercellular tail-end-charlie storms, which could produce all kind of severe, including large hail, strong wind gusts and an isolated tornado event.

During the afternoon hours onwards, a similar scenario evolves over the NW Aegean Sea, where the approaching front and the prefrontal LLJ likely cause a cluster of onshore moving showers and thunderstorms. Again, the most extensive severe risk evolves along the coastal areas, where large hail and isolated tornadoes could accompany thunderstorm activity beside an heavy rainfall and flash flooding risk. This activity lasts well into the night and most likely until the morning hours.

Further inland over Greece, a thundery warm advection regime is likely, especially around noon, when some insolation is still possible. Northward spreading thick clouds of the aforementioned clusters and also attendant stratiform rain shields may lower the thunderstorm risk during the afternoon hours. Ongoing isentropic lift, adequate mid-level lapse rates and 15 m/s DLS create a set-up, where a few stronger storms may produce marginal hail and gusty winds. Again, heavy rain will be an augmented risk (either due to embedded thunderstorm activity in a straiform rain area or due to the slow storm motion), but current forecasts keep rainfall amounts below the level 1/2 thresholds.

... W/C-Mediterranean...

Despite dry air atop the Mediterranean, very cold mid-level air over the warmer SSTs supports ongoing plume of steep mid-level lapse rates, so any local convergence zone or the orography of the islands may serve as foci for initiation. Convective activity will be mostly sub-severe in respect to hail and wind, although isolated large hail can't be ruled out with some modest CAPE present in the hail/graupel growth layer. Of main concern will be the mesoscale, where local convergence zones may modulate the environment enough for an isolated tornado and heavy rainfall risk ... the latter risk already augmented due to the slow storm motion. The highest risk seems to run from the Balearic Islands towards Sardinia and Corsica mainly during the overnight hours. In addition, overlap of some low-level CAPE with robust vorticity signals point to an enhanced waterspout risk along any pronounced convergence zone and along the coasts of W-Corsica/NW Sardinia.

We thought about issuing a level 1 for this event, but the evolving vortex looks very healthy (seems like some polar-type low event with strong thermal gradients in the vertical) which could cause strong ageostrophic winds in the lower levels, more hostile for waterspout development. This scenario however may result in a pronounced convergence zone over/just east of the Balearic Island during the night, increasing the waterspout risk there. Ongoing model discrepancies and expected strength of the vortex kept this area level-1-free for now.