27 September 2007

Karen weakens, Lorenzo forms...

The much-anticipated vertical shear has finally met Karen head-on.  The low-level center is now displaced 3 degrees to the west of the deep convection and completely exposed, thanks to 25kts+ of westerly vertical wind shear (defined as the vector difference between the 850mb winds and the 200mb winds over the system).  The intensity has fallen to 55kts and 1004mb.  It's tracking WNW at 10kts, and the forecast is for heading NW and weakening/maintaining intensity.  However, after this confrontation with the trough (5+ days) Karen could warrant a much more cautious watch as a ridge moves over the storm and decreases the shear and steers it more toward the west.

At 18Z today, TD13 was upgraded to TS Lorenzo based on aircraft recon into the storm.  This is the twelfth named storm of the season, already above the long-term average of 10 for the season, and there are still 2 more months in the Atlantic hurricane season.  Lorenzo has become much more compact, with a very vigorous central core and CDO with cloud tops hitting -80C. 
The circulation is still visible from the Alvarado radar at http://smn.cna.gob.mx/radares/radares.html
The latest intensity is 50kts and 1004mb and it still appears to be intensifying.  The forecast brings it up to 55kts prior to landfall on Friday night.  The primary problem with this storm is the motion: almost none.  It is drifting at 2-3kts toward the Mexican coast, and in the meantime will begin to spread very heavy rain throughout central Veracruz and even further inland.

There's a new healthy tropical wave near the Cape Verde Islands (~25W) which exited the African coast on Tuesday.  Perhaps of more note is a wave still over land that should exit the coast tomorrow... that one is favored to develop quickly by several global models.

Please visit my tropical Atlantic headquarters.

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