Storm Forecast

Storm Forecast
Valid: Fri 23 Jan 2015 06:00 to Sat 24 Jan 2015 06:00 UTC
Issued: Thu 22 Jan 2015 17:03
Forecaster: TUSCHY

A level 1 was issued for parts of Italy, the S-Adriatic, Ionian and Aegean Sea mainly for excessive rain, strong to severe wind gusts, isolated large hail and a few tornado reports.

SYNOPSIS

A cyclonic vortex, almost cut off of the westerlies, bobs up and down over the CNTRL Mediterranean. Downstream, deep SW-erlies advect a moist and unstable air mass towards Greece and Turkey, whereas upstream advection consists of a dry continental air mass from CNTRL Europe towards the W-Mediterranean. Wave train of progressive troughs continues over N-Europe.

Most interesting feature -synoptic-wise- is an ongoing small-scale low pressure area over the SE-Tyrrhenian Sea. SSTs of 14-16 C beneath cold mid-levels support widespread modest CAPE of 300-700 J/kg (SBCAPE) over the Tyrrhenian Sea. Phase diagrams show a pronounced kink towards a transient shallow warm-core structure in lowest 3-4 km AGL. Run-to-run consistency is good with peak mean sea level pressure forecasts of 986-990 hPa.
During the start of the forecast, this vortex should be in full strength either just offshore of or just onshore over Campania (SW-Italy). Thereafter, models agree in a cyclonic turn of this feature towards the SW and later-on towards the S. Despite slightly warmer mid-levels, SSTs also increase a bit just north of Sicily, so probabilities remain augmented that a strong vortex comes ashore either over Sicily or over W-Calabria. Main threat during the vortex's passage will be strong winds and heavy rain. Channeling of the wind between the vortex and the Madonie/Nebrodi and Peloritani Ranges (N/NE Sicily) could locally push winds to severe strength. Locally excessive rain is also possible over N-Sicily and SW Italy (spots of 100 l/qm/24h).

In case of landfall over Campania during the start of the forecast, a much weaker system could be the result or even a dissolving system could occur (shown by some 12Z runs). Its path has to be monitored closely.

DISCUSSION

... CNTRL Mediterranean ...

Aside from the interesting vortex, widespread thunderstorm activity occurs beneath the broad pool of cold mid-layer air. Enhanced SST-low-tropospheric temperature gradients with deep updrafts (fostered by ~ 500 J/kg MLCAPE) support the idea of local vortex stretching atop mesoscale convergence zones. A few waterspout reports are possible in the level 1 region.

Dominant cyclonic flow also causes prolonged period of onshore moving showers/thunderstorms. Orographic enhancement could result in excessive rain over the mountainous regions of Abruzzo, Umbria and Sicily.

... Aegean Sea ...

25 m/s DLS and ~ 700 J/kg MLCAPE overlap during the day results in organized convection with large hail and isolated severe wind gusts. An isolated tornado is possible, too. The rain risk is a bit diffuse and depends on the final geometry/placement of the trough and timing/strength of passing short-waves. In case of a deep S-erly flow, shown by GFS, some temporal training along the N-coast of the Aegean Sea is possible. Latest QPF remains in a level 1 criterion. Main thunderstorm activity occurs along the N-Aegean Sean.

During the night, shear weakens but CAPE increases a bit with decreasing CIN, so expect scattered DMC over the Aegean Sea with heavy rain, marginal hail and an isolated waterspout event.

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