From 11/03/11-11/08/11, an interesting subtropical development occurred in the Mediterranean Sea, south of France. Although these developments are not completely unusual, they are few and far between. This particular event started from a vigorous jet streak on the south side of a strong North Atlantic trough with a parent low of 961 mb on 11/03/11. The SEVIRI RGB Airmass product captured this “advection jet” (red coloring on top of the convection) west of the Iberian Peninsula and the associated vigorous convection that resulted. As the system moved towards the Mediterranean, it was cutoff from the main flow and slowed significantly over the western portion of the basin. Sea surface temperatures were 18C on average, but the synoptic environment (airmass) was much colder as evidenced by the bluish to purplish coloring that appears as the PV anomaly associated with the jet weakens (reds diminish). This allows for mesoscale processes to take over and the relative warmth of the Mediterranean allows for convection to become more focused around a middle to low level vortex, thus acquiring subtropical characteristics. The storm produced a max wind gust of 83 kts at Porquerolles Island, south of Toulon, France.
I have included two animations of this evolution using the SEVIRI RGB Airmass product. The first one is hourly showing the larger scale evolution. The second animation is in 15 minute increments and focused on the western Mediterranean Sea. The limited color selection of NAWIPS produces a pixelated image, so some of the clarity is lost. Note two additional small PV anomalies that are rotating around the eastern and southern peripheries of the subtropical storm.
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