TROPICAL STORM HANNA DISCUSSION NUMBER 17 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082008 500 AM EDT MON SEP 01 2008

DURING THE PAST 6 HOURS...A LARGE MASS OF INTENSE CONVECTION WITH CLOUD TOP TEMPERATURES AS COLD AS -87C HAS DEVELOPED IN THE SOUTHEASTERN SEMICIRCLE OF HANNA. NOAA BUOY 41046 JUST EAST OF THE ALLEGED CENTER HAS REPORTED 1-MINUTE SUSTAINED WINDS OF 43 KT... WHICH EASILY SUPPORTS AN INITIAL INTENSITY OF 45 KT...AND HANNA COULD EVEN BE A LITTLE STRONGER BASED ON AN EARLIER PRESSURE OF 997.1 MB FROM THE SAME BUOY. A RECON AIRCRAFT WILL INVESTIGATE HANNA LATER TODAY AND PROVIDE A BETTER ESTIMATE OF INTENSITY.

THE INITIAL MOTION IS AN UNCERTAIN 260/02...DUE MAINLY TO SOME UNCERTAINTY IN THE EXACT LOCATION OF THE CENTER. A 01/0224Z ASCAT OVERPASS SUGGESTED THE CENTER OF HANNA WAS FARTHER SOUTHEAST AND EMBEDDED IN THE DEEP CONVECTION. IF THAT IS THE CASE...HANNA WOULD EASILY BE A 55-KT OR STRONGER TROPICAL STORM. HOWEVER...EARLIER PASSIVE MICROWAVE IMAGERY...THE MOST RECENT NIGHTTIME VISIBLE IMAGERY...AND THE CURRENT NORTHWESTERLY SHEAR PATTERN ALL SUGGEST THAT THE CENTER SHOULD BE LOCATED NEAR THE NORTHWESTERN EDGE OF THE DEEP CONVECTION RATHER THAN EMBEDDED WITHIN IT. IN CONTRAST TO THE UNCERTAINTY ASSOCIATED WITH THE LOCATION OF HANNA...ALL OF THE GLOBAL MODELS HAVE NEARLY IDENTICAL FORECAST SCENARIOS OF THE MID- TO UPPER-LEVEL FLOW PATTERNS. THE BERMUDA HIGH IS FORECAST TO EXTEND WESTWARD ACROSS THE ATLANTIC AND INTO THE TENNESSEE VALLEY BY DAYS 3-5...WHILE AN UPPER-LEVEL TROUGH DIGS SOUTHWARD OFF THE U.S. EAST COAST AND SPLITS...WITH THE SOUTHERN PORTION DIGGING SOUTH OF HANNA AND THE NORTHERN PORTION LIFTING OUT AND ALLOWING RIDGING TO FILL IN BEHIND THE TROUGH TO THE NORTH OF HANNA. THIS COMBINED FLOW PATTERN IS EXPECTED TO ACCELERATE HANNA TOWARD THE SOUTHEASTERN U.S. COAST BY 72-96 HOURS/DAYS 3-4. THE PRIMARY DIFFERENCE IN THE MODELS IS HOW THEY HANDLE THE SIZE OF HANNA BETWEEN NOW AND 72 HOURS. THE ECMWF AND UKMET KEEP HANNA ABOUT 100 NMI IN DIAMETER SMALLER AT 500 MB THAN THE OTHER MODELS DO...WHICH RESULTS IN MORE RIDGING TO EXIST ON THE EAST SIDE OF HANNA...AND WHICH FORCES THE CYCLONE CLOSER TO THE FLORIDA EAST COAST. THE OFFICIAL FORECAST TRACK IS ALMOST IDENTICAL TO THE PREVIOUS TRACK AND IS VERY CLOSE TO THE MODEL CONSENSUS AND LEANS A LITTLE TOWARD THE ECMWF-UKMET SCENARIOS BASED ON HANNA'S CURRENT SIZE.

LITTLE...IF ANY...STRENGTHENING IS EXPECTED DURING THE NEXT 48 HOURS DUE TO HOSTILE NORTHERLY SHEAR CONDITIONS. HOWEVER...BY 96 HOURS... ALL OF THE MODELS HAVE ALMOST IDENTICAL 200 MB FLOW PATTERNS INDICATING THE DEVELOPMENT OF BOTH POLEWARD AND EQUATORWARD OUTFLOW REGIMES SURROUNDING HANNA. THIS TYPE OF DUAL JET PATTERN WOULD FAVOR SIGNIFICANT INTENSIFICATION...AND HANNA COULD EASILY BE A HURRICANE BEFORE IT MAKES LANDFALL ALONG THE SOUTHEASTERN U.S. COAST. THE OFFICIAL INTENSITY IS SIMILAR TO THE SHIPS MODEL...BUT REMAINS BELOW ALL OF THE AVAILABLE NHC INTENSITY GUIDANCE.

ALL INTERESTS IN FLORIDA SHOULD CLOSELY MONITOR THE FORECASTS FOR HANNA SINCE ONLY A SLIGHT DEVIATION TO THE WEST OF THE OFFICIAL TRACK COULD BRING THE CENTER OF HANNA MUCH CLOSER TO THE FLORIDA EAST COAST AND THE BAHAMAS THAN CURRENTLY INDICATED.