Mesoscale Discussion

Mesoscale Discussion
Valid: Fri 17 Nov 2017 18:00 to Sat 18 Nov 2017 03:00 UTC
Issued: Fri 17 Nov 2017 18:35
Forecaster: TUSCHY

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

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

The latest WV loop (17UTC) indicates a gradually improving structure of the vortex over the N-Ionian Sea. During the afternoon hours, a prominent spiral band was present along the northern periphery, which was still pulsating in strength.
However during the last 1-2 hours, another and more persistent strong burst of convection occurred to the north
and west and also in proximity to the center of the vortex. In addition transient banding structure was evident in latest remote sensing data. This evolution and the wrapping spiral
band probably support a ST 2.5 on the Hebert-Poteat intensity scale and a subtropical system with a shallow warm core structure is present (fostered by phase diagrams). This rating would support persistent winds in the 35-40 kt range which is verified by ASCAT data (roughly 16 UTC) with a few uncontaminated 35kt flags in the eastern part of the vortex. Therefore this subtropical depression seems to be already strengthening into a subtropical storm.

The 00 and 12 UTC soundings of Brindisi sampled the stratisfication of the deep cold-core vortex very well with cold mid-/upper-level air, moderate lapse rates and an overall rather humid environment. Combined with the marine BL air mass, weak CAPE is realized with higher values offshore, where BL moisture further increases. Also, shear magnitude at all levels is weak with a slight enhancement in the lowest 2 km, indicating the offshore warm-core structure of the vortex (tightest gradient at lowest levels). During the following hours,
not much change of the quasi-barotropic environment is anticipated.

The vortex currently meanders over SSTs of 19-20 C with little
vertical gradient in the lowest 30 to 50 m (expected mixed layer depth for the Ionian Sea). Hence, despite ongoing upwelling due to augmented BL flow, no significant SST cooling is forecast at least for the following few hours.
The main question (also in fine-mesh and global models remains the depression's motion. Most models agree in a quasi-stationary mode until the evening hours, before either a loop to the south (over somewhat higher SSTs) or a movement to the east (towards cooler SSTs) begins. This determines, if further organization of the vortex occurs during the night. With
amplification of the upper trough aloft and its gradual eastward shift ahead of another amplifying and approaching upper-level trough further upstream, both scenarios seem plausible right now. In addition, DMC development over and next to the center
of the vortex also adds additional input to the final motion of the vortex.

To summarize, a vortex with distinct subtropical characteristica produces gusty winds over the N-Ionian Sea, which may approach severe criteria especially beneath DMC activity.
Large uncertainties exist with intensity and path forecast, but a further increase in strength seems likely during the following hours just offshore off SE-Italy. Excessive rain (training
with speed maximum in lowest 1-2 km AGL) and a low-end tornado risk (locally enhanced directional shear within a low LCL environment) are possible beneath organized banding features next to the center, which may impact far SE-Italy and W-Greece during the night.

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