Mesoscale Discussion
Valid: Sat 18 Nov 2017 06:00 to Sun 19 Nov 2017 06:00 UTC
Issued: Sat 18 Nov 2017 19:15
Forecaster: TUSCHY / DAFIS

Mesoscale Discussion (MD) for the Ionian Sea and surroundings.

This MD was issued to update on latest development/data for the vortex over the N-Ionian Sea.

There is not much change in the reasoning of how the event unfolds during the night. Latest HVIS imagery indicates an LLCC mostly avoid of DMC despite pulsating convection along the northern periphery. Since then, DMC seems to have worked its way closer to the center of the vortex as seen in WV/IR loops. In fact at 16/17 UTC onwards, a radially expanding thunderstorm cluster evolved over the N/W periphery of the vortex which resembles a weak central dense overcast (CDO) pattern.Overshooting tops within this cluster suggest vigorous updrafts and downdrafts in NW rain bands. Although not yet covering the whole vortex, this probably once again results in further intensification of the subtropical cyclone. This burst of convection may be assisted by gradually increasing upper divergence ahead of an approaching upper trough further upstream (highlighted by drying over the N-Tyrrhenian Sea in WV imagery). This trough sharpens during the following hours while shifting east with constantly increasing upper divergence over the Ionian Sea/Greece.

Model guidance converged towards a landfall between Leucas and Zante (Greece) during the night and extrapolation of latest vortex displacement would support that idea. Not much intensity change is present in most models (at least those which don't suffer of convective feed back problems). Latest convective trends may keep the vortex at least in a steady state regarding strength although further intensification with improving upper divergence can't be ruled out before landfall. Marginally increasing SSTs just offshore off the west coast of Greece would also support further intensification.

Latest ASCAT data shows 45 kt winds along the western periphery of the vortex with 30-35 kt elsewhere. Surface observations report wind gusts up to 17 m/s (Paxoi Kefalonia) and high rain rate values are mostly found in Corfu and NW Peloponnese. Rating is a bit tough with pulsating convection but latest curved band and radially expanding convection over parts of the LLCC probably still support a rating of ST3.0 on the Hebert-Poteat intensity scale. Banding structure remains good in most quadrants.

To summarize, a moderate to strong subtropical storm approaches the W-coast of Greece during the night with landfall around midnight onwards. Gale-force winds spread ashore and even gusts matching our severe level are possible if latest convective trends continue. Strongest winds remain confined to the western quadrant which weakens during landfall and hence it remains questionable if indeed hurricane-force gusts occur. Nevertheless, this will be an high-impact event with a low-end tornado risk (augmented LL shear in a low LCL environment and excessive rain due to training convection.

Beyond landfall, a gradual and constant weakening trend is forecast as latent heating driven support by DMC activity weakens onshore. Strengthening upper divergence may offset the weakening trend and hence excessive rain and gusty winds may spread well inland, reflected by a broad MD area.