Caribbean-On-Line's Caribbean Hurricane Blog



TROPICAL STORM HANNA DISCUSSION NUMBER 5

Published on August 29, 2008 7:33 AM | Comments

The 5:00 AM EST updates are out from the NHC. Hanna looks like a interesting storm. The models are not in agreement about the track or intensity at the moment:

TROPICAL STORM HANNA DISCUSSION NUMBER 5 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082008 500 AM EDT FRI AUG 29 2008

ALTHOUGH THE UPPER-LOW TO THE NORTHWEST OF HANNA HAS BEGUN TO MOVE AWAY...IT IS STILL PRODUCING SHEAR OVER THE TROPICAL CYCLONE. ONCE AGAIN THE CENTER HAS MOVED AWAY FROM THE DEEP CONVECTION WHICH IS NOW CONFINED TO A CYCLONICALLY CURVED BAND IN THE EASTERN SEMICIRCLE. DVORAK T-NUMBERS STILL SUPPORT AN INITIAL INTENSITY OF 45 KNOTS...BUT WITH THE CURRENT SHEARED PATTERN...THIS ESTIMATE COULD BE GENEROUS. THE EVOLUTION OF THE UPPER-LEVEL LOW APPEARS TO BE THE KEY PLAYER FOR BOTH THE INTENSITY AND TRACK OF HANNA FOR THE NEXT 24 TO 48 HOURS. IF THE LOW WEAKENS AND MOVES AWAY AS INDICATED BY GLOBAL MODELS...THE ENVIRONMENT BECOMES MORE FAVORABLE FOR STRENGTHENING. THE OFFICIAL FORECAST ASSUMES A GOOD SOLUTION FROM GLOBAL MODELS AND CALLS FOR A GRADUAL INTENSIFICATION FROM 24 TO 72 HOURS. THE FORECAST IS SIMILAR TO THE SHIPS MODEL BUT DOES NOT SHOWS AS MUCH STRENGTHENING AS SUGGESTED BY THE GFDL AND HWRF.

HANNA HAS BEEN MOVING AROUND THE EAST SIDE OF THE UPPER-LOW AND THE BEST ESTIMATE OF THE INITIAL MOTION IS 310 DEGREES AT 12 KNOTS. A GENERAL NORTHWEST TO WEST-NORTHWEST MOTION IS EXPECTED DURING THE NEXT 2 TO 3 DAYS WHILE THE CYCLONE IS EMBEDDED WITHIN THE FLOW BETWEEN THE UPPER LOW AND THE SUBTROPICAL RIDGE. BEYOND 3 DAYS...MOST OF THE GLOBAL MODELS ARE CARVING A LARGE TROUGH OVER THE CENTRAL ATLANTIC AND DEVELOPING A LARGE RIDGE OVER THE EASTERN UNITED STATES. THIS PATTERN SHOULD FORCE HANNA ON A SLOW SOUTHWESTERLY TRACK BEYOND THREE DAYS...BUT AT THE SAME TIME THE UPPER-LEVEL WINDS SHOULD WEAKEN THE CYCLONE. THE OFFICIAL FORECAST SHOWS A LITTLE BIT OF THE SOUTHWESTERLY MOTION BUT DOES NOT GO AS FAR SOUTH AS SHOWN BY MOST OF THE MODELS. IN FACT...THE GFS HAS JOINED THE REST OF THE GLOBAL MODELS IN BRINGING THE CYCLONE SOUTHWESTWARD. I PREFER TO WAIT FOR MORE MODEL RUNS BEFORE FORECAST SUCH A RARE BUT NOT UNIQUE TRACK TOWARD THE SOUTHWEST.



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