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TROPICAL STORM HANNA DISCUSSION NUMBER 30

Published on September 4, 2008 7:21 AM | Comments

TROPICAL STORM HANNA DISCUSSION NUMBER 30 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082008 500 AM EDT THU SEP 04 2008

LOOKS CAN CERTAINLY BE DECEIVING. HANNA HAS NOT BEEN PRODUCING VERY MUCH DEEP CONVECTION SINCE AN EARLIER STRONG BURST DIED AFTER ABOUT 04Z...AS THE UPPER-LEVEL LOW TO ITS WEST CONTINUES TO IMPOSE STRONG SOUTHERLY SHEAR. NEVERTHELESS...HANNA HAS A LARGE AND ROBUST CIRCULATION...AND A NOAA RECONNAISSANCE AIRCRAFT HAS RECENTLY REPORTED A MINIMUM CENTRAL PRESSURE OF 990 MB. FLIGHT-LEVEL WINDS AT 850 MB HAVE BEEN AS STRONG AS 65 KT...AND NUMEROUS SFMR VALUES OF 55-60 KT HAVE BEEN RETRIEVED. I HAVE NO GOOD REASON TO DISCOUNT THE SFMR WINDS...SO THE INITIAL INTENSITY IS INCREASED TO 60 KT. GIVEN THAT THE SOUTHERLY SHEAR IS FORECAST BY THE DYNAMICAL MODELS TO PERSIST OVER HANNA EVEN AS IT GAINS LATITUDE DURING THE NEXT COUPLE OF DAYS...STRENGTHENING WOULD AT FIRST GLANCE SEEM UNLIKELY. CONSISTENT WITH THAT THINKING...THE GFDL AND HWRF SHOW LITTLE CHANGE IN STRENGTH OR EVEN SOME WEAKENING...EXCEPT MUCH LATER WHEN HANNA BECOMES AN EXTRATROPICAL CYCLONE. ON THE OTHER HAND...THE SHIPS AND LGEM...DESPITE DIAGNOSING LITTLE CHANGE IN SHEAR...FORECAST A LITTLE STRENGTHENING TO HURRICANE STATUS. A CONSENSUS OF THESE VARIOUS MODELS WOULD YIELD A FLAT 60 KT INTENSITY FORECAST UNTIL HANNA PASSES NEAR OR OVER THE CAROLINAS...BUT FOR CONTINUITY AND SINCE HANNA IS ALREADY PRODUCING WINDS JUST SHY OF HURRICANE STRENGTH....65 KT IS STILL SHOWN EXPLICITLY IN THE FORECAST. GRADUAL WEAKENING IS THEN FORECAST AS HANNA ACCELERATES NORTHEASTWARD AND BECOMES EXTRATROPICAL.

DESPITE HAVING AIRCRAFT FIXES...THE INITIAL MOTION IS HARD TO GAGE...PROBABLY DUE TO THE SPRAWLING NATURE OF THE CIRCULATION...BUT THE INITIAL MOTION ESTIMATE IS 320/10. HANNA SHOULD ROUND THE BUILDING WESTERN ATLANTIC RIDGE AND ROUGHLY HUG THE EAST COAST OF THE UNITED STATES...BUT PASSING NEAR OR OVER THE COAST OF THE CAROLINAS ALONG THE WAY. PERHAPS DUE IN PART TO THE BENEFIT OF DATA FROM THE NOAA G-IV JET MISSION LAST NIGHT...THE LATEST MODEL TRACKS HAVE COME INTO A LITTLE TIGHTER AGREEMENT...AND IN GENERAL THEY HAVE SHIFTED SLIGHTLY TO THE LEFT. THE NEW TRACK FORECAST IS SHIFTED JUST SLIGHTLY LEFT OF THE PREVIOUS ONE...TO BE CLOSER TO THE CONSENSUS. AS ALWAYS...ONE SHOULD NOT FOCUS ON THE EXACT FORECAST OF THE CENTER...ESPECIALLY SINCE MUCH OF THE WEATHER ASSOCIATED WITH HANNA IS WELL-REMOVED FROM THE CENTER.



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