Sun Sep 02 2007 12:23 PM
Subtropical Gabriel, no longer 99L; still some concern for East Coast

We have what appears to be a book-end vortex attempting to develop in the vicinity of the NC/SC Coast. Both satellite and radar are beginning to show a cyclonic turning amid the on-going convection that has been lingering there for the past 18 to 24 hours.

Radar: http://radar.weather.gov/radar.php?rid=CLX&product=N0Z&overlay=11101111&loop=yes
Satellite: ...See below

This feature was originally triggered by a stalling frontal zone. Originally a baroclinic system altogether, this now appears to be entering a phase-transition because the air mass north to south through the axis of weak rotation is becoming less differentiable. This, while convection persists, needs to be monitored so long as we have a general southwest wind field beneath the axis of frontalysis and east-northeast wind above, which provides a natural cyclonic convergence in the low levels.

Two key factors:
  • Shear remains low in the deep layer analysis. Moreover, the upper level winds are somewhat divergent while being light in that area. This can be seen using
    http://www.ssd.noaa.gov/goes/east/nwatl/loop-avn.html ..and clicking the radial button that states, HDW-High, which overlays the wind field on the image. There is no floater assigned to this feature but NHC has given it a mention in the 11am so perhaps an Invest is in the works. Given to some cyclonic presentation emerging a circulation is likely in the process of forming and an Invest would have more success at that time.

  • Oceanic heat content; ample availability in the area as is suggested via:
    http://www.erh.noaa.gov/box/maps/sst/2007090200_sst_AB.png Additionally, we all know the Gulf Stream is in that area and that supplies an artery pumping rich fuel directly into any fledging system. Sometimes if there is an offshore component, cooler shelf water can upwell along the immediate Coast in that vicinity. That does not appear to be the case now as buoy data indicates temperatures ranging between 81 and almost 90F!

Whatever develops there it is likely to move very slowly and be there for awhile. The larger scale synoptic evolution does not provide for very many steering signals. There is a ridge tending to develop surface and aloft, moving off the Mid Atlantic and New England states and into the NW Atlantic, from day-3 through day-6. That should help pin whatever is there in place, or perhaps even push it SE or SW. Some of the models actually do suggest that occurring, showing a slow but gradual development only drifting around in that area.

First step...get a system going.

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