Valid: Fri 12 Jan 2018 06:00 to Sat 13 Jan 2018 06:00 UTC
Issued: Thu 11 Jan 2018 09:16
A level 1 was issued across SE-Greece for a low-end tornado risk and excessive rainfall.
A level 1 was issued across W-Greece for a waterspout risk and excessive rainfall.
A level 1 was issued across SE-Italy for a localized excessive rainfall risk.
An high-over-low blocking pattern continues during the forecast. A strongly positive 500 hPa geopotential height anomaly over Scandinavia forces numerous troughs to retrograde W over E-Europe. In the meantime, a broad upper trough swings east over the CNTRL-Mediterranean and serves as main focus for thunderstorm activity over the warm Mediterranean. Yet another highly amplified upper-level trough over the far NE-Atlantic approaches W-Europe during the night. No serious thunderstorm threat exists with this feature despite a few short-lived thunderstorms on the cyclonic side of the upper-level jet (offshore).
... S-Greece and surrounding areas ...
A vertically stacked vortex approaches S-Greece during the daytime hours and impacts the region especially during the night. A gradually intensifying upper jet to the south causes improving upper divergence over the area of interest during the night. In addition, GFS is the most aggressive model in developing a reforming LL vortex near or just over SE-Greece, which would improve the overall deep-layer lift. Finally a southbound moving cold front probably stalls somewhere over the N-Aegean Sea, which could also serve as additional lifting mechanism. The net result will be enhanced overnight convection over SE-Greece, which takes profit of a SE-/E-erly inflow of an unstable marine air mass. Main lightning activity probably stays offshore with a low-end chance for a few elevated onshore thunderstorms. Heavy rain will be the main threat although this precipitation turns to snow in higher elevations betimes. A confined level-1 for excessive rain was issued with a low-end tornado risk along the cost/just offshore.
Another area with enhanced convective precipitation exists along the W-coast of Greece with a prolonged confluent flow regime parallel to the coast. With strong synoptic-scale forcing in place, repeated thunderstorm development with locally excessive rain is forecast. Would not be surprised to receive numerous waterspout reports with augmented LL CAPE/LL convergence regime in place. A level 1 was issued to cover the waterspout/rainfall threat. This activity weakens beyond 00 Z.
... SE-Italy ...
Slow moving or quasi-stationary showers/isolated thunderstorm could bring locally heavy rainfall amounts to SE-Italy. We expanded the level 1 to SE-Italy to account for this risk.