Storm Forecast
Valid: Fri 14 Oct 2011 06:00 to Sat 15 Oct 2011 06:00 UTC
Issued: Thu 13 Oct 2011 13:47
Forecaster: TUSCHY

A level 1 was issued for the S-Tyrrhenian Sea and the extreme N-coast of Algeria/Tunisia mainly for excessive rainfall.

A level 1 was issued for S-Italy, Sicily and the W-Ionian Sea mainly for severe wind gusts, large hail, excessive rainfall and an isolated tornado event.

A level 1 was issued for the central/E-Ionian Sea, parts of Greece, Albania and Macedonia mainly for excessive rainfall amounts. Waterspouts are possible along the W-facing coasts of Greece and Albania.

SYNOPSIS

A pronounced upper trough over E-Europe continues to amplify and it affects the central Mediterreanean area during the forecast. Ridging takes place further west and results in stable conditions over W/NW-Europe.

At lower levels, an high pressure area over central Europe intensifies and pushes the cold air further to the south. A LL depression forms somewhere over the S-CNTRL Mediterranean and moves slowly to the east into the Ionian Sea until 06Z. An active baroclinic zone evolves between both features and this one will be the focus for scattered shower and thunderstorm development.

DISCUSSION

... South-central Mediterranean ...

Geopotential height falls overspread the area of interest as an upper trough builds SW-wards. Accompanied cold mid-level temperatures overspread the 19-24°C warm Mediterranean and widespread, nearly uncapped 500-1000 J/kg SBCAPE are forecast with peak values evolving offshore. Main concern will be a southward shifting boundary, which evolves between a constantly strengthening/southward expanding high pressure area over central Europe and an E-W aligned and somewhat diffuse surface depression over Sicily and adjacent areas. This constellation may assist in repeatedly development of showers and thunderstorms over the same area with an augmented excessive rainfall/flash flood risk.
Depending on the exact placement of the surface depression, ageostrophic deflection of the BL wind by this feature may also result in a somewhat augmentend directional shear magnitude especially over S-Italy/Sicily and the W-Ionian Sea. Multicells and an isolated supercell are well possible with severe wind gusts, large hail and an ioslated tornado event (the latter risk mainly due to a deepening BL moisture plume south of the boundary with increasing 0-3 km CAPE and somewhat augmented shear). Accordingly to the slow southward progression of the boundary, the main thunderstorm risk also gradually translates southwards during the night with an ongoing risk of mainly heavy rainfall.

...Ionian Sea after 21 Z ...

A complex pattern evolves with a W-E aligned boundary present after sunset. Models like GFS and EZMWF agree in a gradually strengthening depression, which moves along this boundary eastwards and affects the Ionian Sea after sunset. Final strength of this depression is very uncertain right now with GFS indicating a weak system whereas some higher resolution models try to blow this feature up to a depression with a shallow warm-core structure (mainly end of the forecast period onwards). Despite those uncertainties, increasing backing of the wind field below 3 km is expected ahead of this feature, which results in increasing moisture advection and a sharpening of the boundary. Favorable placement beneath right entrance region of a strong mid-/upper level jet causes constant high-level air mass evacuation and hence rapid clustering of thunderstorms is expected over the central/E-Ionian Sea after 21 Z. This constellation may result in a more widespread heavy rainfall risk over Greece, Albania and Macedonia. We therefore expanded the level 1 far to the east and also out of any thunderstorm area. A few waterspout events along the coastal regions are possible, given rich BL instability/moisture and strong mid-level updrafts atop. This risk keeps going after 06Z.