Tropical Storm HANNA

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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.
  
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      01/0900Z 23.6N  72.4W    45 KT
 12HR VT     01/1800Z 23.4N  73.0W    50 KT
 24HR VT     02/0600Z 23.4N  73.5W    50 KT
 36HR VT     02/1800Z 23.7N  74.0W    50 KT
 48HR VT     03/0600Z 24.2N  74.5W    55 KT
 72HR VT     04/0600Z 26.2N  76.5W    60 KT
 96HR VT     05/0600Z 30.0N  80.0W    65 KT
120HR VT     06/0600Z 35.0N  82.5W    30 KT...INLAND
 
$$
FORECASTER STEWART
 
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