Tropical Storm HUMBERTO

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TROPICAL STORM HUMBERTO DISCUSSION NUMBER  29
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092013
1100 AM AST TUE SEP 17 2013
 
HUMBERTO HAS BEEN WELL OBSERVED BY DROPSONDE DATA FROM THE NASA
GLOBAL HAWK AND ASCAT DATA...WHICH SHOW THAT THE STORM HAS
UNDERGONE AN INTERESTING EVOLUTION DURING THE PAST 12 HOURS...AND
IS ARGUABLY MORE SUBTROPICAL THAN TROPICAL.  OVERNIGHT...THE SYSTEM
WAS CHARACTERIZED BY AN EXPOSED CIRCULATION WITH A LARGE RADIUS OF
MAXIMUM WIND...ABOUT 130 N MI.  THE SURFACE CENTER OF CIRCULATION
BEING TRACKED OVERNIGHT DISSIPATED THIS MORNING AND A NEW CENTER
FORMED WELL TO THE NORTH...CLOSER TO THE DEEP CONVECTION BUT ALSO
DIRECTLY UNDERNEATH THE UPPER LOW.  WITH THE UPPER LOW FORECAST TO
MOVE SOUTHEASTWARD AWAY FROM HUMBERTO WITHIN THE NEXT 12 TO 24
HOURS...HOWEVER...THIS VERTICAL STRUCTURE IS NOT EXPECTED TO
PERSIST. CONSEQUENTLY WE HAVE CHOSEN TO MAINTAIN THE TROPICAL
CLASSIFICATION FOR CONTINUITY AND TO MINIMIZE CONFUSION...AT LEAST
FOR NOW.  

THE ASCAT DATA INDICATE THAT THE CURRENT INTENSITY IS 40 KT.
THE VERTICAL SHEAR IS FORECAST TO REMAIN LOW TO MODERATE DURING THE
NEXT DAY OR SO.  AFTER THAT TIME...THE SHEAR IS FORECAST TO BECOME
NORTHWESTERLY AND INCREASE SOME...THEN INCREASE FURTHER IN 3 TO 4
DAYS AS AN UPPER-LEVEL TROUGH APPROACHES THE SYSTEM FROM THE WEST. 
AS A RESULT...ONLY SLIGHTLY STRENGTHENING IS PREDICTED DURING THE
NEXT FEW DAYS.  LATER IN THE PERIOD...SOME ADDITIONAL
INTENSIFICATION IS POSSIBLE DUE TO BAROCLINIC PROCESSES.  

BECAUSE OF THE CENTER REFORMATION...A CONFIDENT INITIAL MOTION IS
IMPOSSIBLE TO DETERMINE.  THE DYNAMICAL MODEL GUIDANCE SUGGESTS
THAT HUMBERTO WILL MOVE NORTHWESTWARD AS IT MOVES AWAY FROM THE
UPPER LOW.  IN 24 TO 36 HOURS...HUMBERTO SHOULD TURN
NORTHWARD...THEN BEGIN TO ACCELERATE NORTHEASTWARD IN ABOUT 48
HOURS AHEAD OF THE AFOREMENTIONED MID-LATITUDE TROUGH.  SOME OF THE
GLOBAL MODELS SHOW THAT HUMBERTO COULD BECOME AN OPEN TROUGH IN
ABOUT 72 HOURS...WITH A SEPARATE EXTRATROPICAL DEVELOPMENT
OCCURRING ALONG A FRONT OVER THE EASTERN ATLANTIC TO THE NORTH OF
HUMBERTO.  FOR NOW...THE NHC FORECAST WILL KEEP CONTINUITY WITH THE
PREVIOUS ADVISORIES BY INDICATING THAT HUMBERTO WILL BECOME THE
MAIN EXTRATROPICAL LOW OVER THE NORTH ATLANTIC IN 4 TO 5 DAYS.  

FORECAST POSITIONS AND MAX WINDS
 
INIT  17/1500Z 29.4N  42.5W   40 KT  45 MPH
 12H  18/0000Z 29.7N  43.2W   40 KT  45 MPH
 24H  18/1200Z 30.6N  43.5W   45 KT  50 MPH
 36H  19/0000Z 31.6N  43.3W   45 KT  50 MPH
 48H  19/1200Z 33.0N  42.9W   45 KT  50 MPH
 72H  20/1200Z 36.7N  39.5W   45 KT  50 MPH
 96H  21/1200Z 47.5N  30.5W   55 KT  65 MPH...POST-TROP/EXTRATROP
120H  22/1200Z 60.0N  22.0W   50 KT  60 MPH...POST-TROP/EXTRATROP
 
$$
FORECASTER BROWN
 
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