21 September 2010

Igor racing by Newfoundland, Lisa forms in eastern Atlantic, disturbance crossing Windward Islands...

Igor is still officially a hurricane, though is in the midst of its extra-tropical transition.  Storms like this that are over very cold SSTs (15F) and in 50kts of vertical shear can still be very powerful thanks to baroclinic enhancement.  The same trough that's steering and shearing it is providing an energy transfer to the dying [formerly-barotropic] hurricane.  Here's one model's (HWRF) representation of an along-shear vertical cross-section of the storm showing a barely barotropic structure: http://einstein.atmos.colostate.edu/~mcnoldy/tropics/hwrf/2010/plots/AL11/2010AL11_HWRFXSEC_201009210600_F000.PNG

At 15Z, Igor's intensity was 65kts, 952mb, and cruising by Newfoundland at 40kts.  It probably has another 6-12 hours remaining as a system that NHC tracks.  It has been a hurricane for nearly 10 days and using NTC as a measure of seasonal activity, has provided ~40% of a typical season's entire activity by itself (NTC is Net Tropical Cyclone activity which combines the numbers and longevities of the various storm intensities --TS, H, MH-- and compares them to climatology)!

At 03Z today, the easterly wave in the far eastern Atlantic was upgraded to TD14, then six hours later, to TS Lisa, the 12th named storm of the season.  It's located about 530 miles WNW of the Cape Verde islands, and drifting to the north.  It is forecast to continue drifting to the north, and only intensify slightly, probably never reaching hurricane status and never affecting any land.

Back on the 16th, I mentioned a "suspicious surge of southwesterlies" about 1700 miles east of the Lesser Antilles.  Well, it has indeed festered for a few days, and is now a fairly impressive disturbance located just north of the central Venezuelan coast.  Conditions are favorable for further (and significant) development, so this will be watched very closely.  The next name on the list is Matthew.

Please visit my tropical Atlantic headquarters.

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