Mesoscale Discussion

Mesoscale Discussion
Valid: Sat 18 Nov 2017 11:00 to Sat 18 Nov 2017 18:00 UTC
Issued: Sat 18 Nov 2017 11:08
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.

Latest HVIS imagery places a vortex roughly 60-80 km SW of Corfu with numerous smaller swirls embedded in the main vortex. During the second half of the night a healthy looking and long-lived curved spiral band with DMC activity next to the center and an overall well organized banding structure (also next to the LLCC) pushed the intensity rating probably to a ST3.0 on the Hebert Poteat intensity scale. No adjustmenet was needed due to a very slow forward motion of the vortex. This equals a 45-50 kt subtropical storm, which was also verified by past few ASCAT scans. Numerous uncontaminated 45 kt flags were noted especially in the western quadrant (probably enhanced by channeling between the vortex and SE-Italy but also where the prominent curved band was present). In addition latest phase diagrams indicate a moderate and rather symmetric warm core structure, so it seems like this was and still is a strong but confined subtropical storm.

Brindisi 00 UTC already came in a bit more stable compared to yesterday and this is in line with a weakening and eastward departing upper trough, well seen on IPV maps. A tender warming trend in the 600-500 hPa level occurred, which could be one reason for the morning weakening trend of DMC next to the center of the vortex. Another reason should be a gradual decreasing trend in SSTs due to some upwelling with ongoing enhanced BL flow (SSTs of 18-19 C and decreasing mixed layer depth) and due to the general diurnal cycle of such a subtropical storm. Else, not much change of the quasi-barotropic setup is forecast until landfall.

These mixed signals are also reflected in general model guidance, which keeps this vortex in a quasi-steady mode of intensity and motion, showing a slow east to southeastward
drift with no substantial deepening. We would not be surprised to see pulsating convection next to the center during the day (especially where embedded smaller-scale vortices induce enhanced convergence), which could result in another temporal burst of intensity, but current idea is that a subtropical storm with gale-force wind gusts approaches W-Greece during the evening and overnight hours. Severe gusts may occur with bands with enhanced convection. Beside the wind gust threat, isolated tornadoes are well possible, especially beneath bands with organized DMC, where LL shear is augmented in a low LCL environment. Training convection with wind speed maximum in the lowest 3 km AGL also points to an excessive rainfall and flash flood risk.

The current MD covers the daytime hours and may be re-evaluated during the evening for the overnight hours.

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