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Hurricane HELENE


ZCZC MIATCDAT3 ALL
TTAA00 KNHC DDHHMM
HURRICANE HELENE DISCUSSION NUMBER  31
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082006
1100 PM EDT TUE SEP 19 2006
 
SATELLITE INTENSITY ESTIMATES AVERAGE NEAR 100 KT FROM TAFB...SAB
AND AFWA. HOWEVER...A 2039 UTC TRMM OVERPASS INDICATED THAT HELENE
WAS IN THE MIDDLE OF AN EYEWALL REPLACEMENT CYCLE WHICH SUGGESTS
THE SLIGHT FILLING OF THE CYCLONE IS NOT YET FINISHED. IN ADDITION
RECENT INFRARED SATELLITE IMAGERY SHOWS SOME ASYMMETRIES IN THE
DEEP CONVECTION WITH A FAIRLY RAGGED EYE AS WELL. THE INITIAL
INTENSITY IS KEPT AT 95 KT FOR THIS ADVISORY.
 
THE INITIAL MOTION IS 275 DEGREES AT 8 KT...HOWEVER A TURN TOWARD
THE WEST-NORTHWEST AND NORTHWEST IS EXPECTED SOON. THE TRACK MODEL
GUIDANCE IS TIGHTLY CLUSTERED THROUGH 48-72 HOURS AND VARIES WITH
FORWARD SPEED THEREAFTER. THE GFDL MODEL AND GUNA CONSENSUS WERE
FOLLOWED CLOSELY FOR THIS PACKAGE SINCE THESE MODELS HAVE EXHIBITED
THE BEST PERFORMANCE THUS FAR WITH HELENE. THE OFFICIAL FORECAST IS
SIMILAR TO THE PREVIOUS ADVISORY AND INDICATES A QUICK RECURVATURE
TO THE NORTH IN THE NEXT 12-24 HOURS...AND ACCELERATION TO THE
NORTHEAST AFTER 48 HOURS ON THE SOUTHEAST SIDE OF A LARGE 500 MB 
TROUGH MOVING OFF THE U.S. EAST COAST. 
 
HELENE REMAINS IN A LOW SHEAR ENVIRONMENT FOR NOW WITH STRONG UPPER
TROPOSPHERIC OUTFLOW OVER ALL BUT THE SOUTHWEST PORTION OF THE
CIRCULATION. ONCE THE EYEWALL REPLACEMENT IS COMPLETED THERE IS THE
POTENTIAL FOR SOME STRENGTHENING AND HELENE COULD REGAIN MAJOR
HURRICANE STATUS IN THE NEXT 12-24 HOURS. BEYOND 36 HOURS
INCREASING SOUTHWESTERLY SHEAR IS LIKELY TO BRING ABOUT GRADUAL
WEAKENING. BY 96 HOURS...GLOBAL MODELS...SUCH AS THE GFS...SHOW
CONSIDERABLE COLD AIR ADVECTION OVER THE SOUTHWESTERN SEMICIRCLE OF
THE CYCLONE...JUSTIFYING A FORECAST OF TRANSITION TO A STRONG
EXTRATROPICAL STORM AROUND THAT TIME. 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      20/0300Z 24.6N  54.8W    95 KT
 12HR VT     20/1200Z 25.5N  55.7W   100 KT
 24HR VT     21/0000Z 27.5N  56.8W   100 KT
 36HR VT     21/1200Z 30.0N  56.8W    95 KT
 48HR VT     22/0000Z 32.7N  56.0W    95 KT
 72HR VT     23/0000Z 39.0N  50.0W    85 KT
 96HR VT     24/0000Z 45.0N  40.0W    75 KT...EXTRATROPICAL
120HR VT     25/0000Z 49.0N  30.0W    60 KT...EXTRATROPICAL
 
$$
FORECASTER COBB/PASCH
 
NNNN


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Page last modified: Wednesday, 20-Sep-2006 02:50:01 UTC