Tropical Storm Humberto - 10am Tuesday Discussion

By: National Hurricane Center
By: National Hurricane Center

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.


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