ZCZC MIATCDAT3 ALL TTAA00 KNHC DDHHMM HURRICANE ISABEL DISCUSSION NUMBER 26 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 11 AM EDT FRI SEP 12 2003 MORNING SATELLITE IMAGERY AND MICROWAVE DATA FROM LAST NIGHT SHOW THAT ISABEL HAS COMPLETED A CONCENTRIC EYEWALL CYCLE. IT NOW HAS A WELL-DEFINED 35 NM WIDE EYE WELL EMBEDDED IN THE CENTRAL DENSE OVERCAST. THE CIRRUS OUTFLOW IS STILL GOOD IN THE SOUTHEAST AND NORTHWEST QUADRANTS...BUT IS ONLY FAIR TO POOR ELSEWHERE AND ON THE WHOLE LOOKS LESS GOOD THAN YESTERDAY. SATELLITE INTENSITY ESTIMATES ARE ALL 140 KT...SO THAT IS THE INITIAL INTENSITY. HOWEVER...SINCE THE TIME OF THE ESTIMATES THERE HAS BEEN SOME WARMING OF THE EYEWALL CONVECTIVE TOPS. NOAA AND AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT WILL INVESTIGATE THE HURRICANE THIS AFTERNOON. THE INITIAL MOTION IS 270/8. ISABEL IS CURRENTLY SOUTH OF A LOW/MID-LEVEL RIDGE...WITH AN ANTICYCLONE CENTER IN WATER VAPOR IMAGERY NEAR 29N55W...AND BETWEEN LARGE UPPER-LEVEL TROUGHS ALONG 68W AND 41W. IN THE SHORT TERM...THIS SHOULD PRODUCE A WESTWARD TO POSSIBLY WEST-NORTHWESTWARD MOTION. FURTHER UPSTREAM...A DEEP LAYER CYCLONIC CIRCULATION IS OVER THE SOUTHEASTERN UNITED STATES WITH A MID/UPPER-LEVEL RIDGE BUILDING OVER SOUTHEASTERN CANADA. THE EVOLUTION OF THESE FEATURES...ALONG WITH THE WESTERLIES OVER THE CENTRAL UNITED STATES...WILL DETERMINE THE LONG-RANGE COURSE OF ISABEL. NHC TRACK GUIDANCE GENERALLY AGREES ON A WESTWARD TO WEST-NORTHWESTWARD MOTION THROUGH 72 HR...AND THE OFFICIAL FORECAST TRACK FOLLOWS ALONG THE SOUTH EDGE OF THE GUIDANCE. CONSIDERABLE DIVERGENCE OCCURS AFTER 72 HR. THE UKMET CALLS FOR A GRADUAL TURN TOWARD THE NORTHWEST BY 120 HR TOWARD A WEAKNESS IN RIDGE LEFT BY THE DEEP-LAYER LOW AS IT LIFTS NORTHWARD. ON THE OTHER HAND...THE GFS...NOGAPS...GFDL... CANADIAN...AND ECMWF MODELS ALL CALL FOR AN EARLIER AND MORE NORTHWARD MOTION. SINCE ALL MODELS KEEP SOME DEEP LAYER RIDGING OVER THE NORTHWEST ATLANTIC NORTH AND NORTHEAST OF ISABEL...SO SHARP OF A TURN LOOKS A LITTLE DUBIOUS. THUS...THE 3-5 DAY OFFICIAL FORECAST WILL LEAN TOWARD THE UKMET AT A SLOWER FORWARD SPEED. ISABEL IS FORECAST TO REMAIN IN A WARM-WATER LOW-SHEAR ENVIRONMENT FOR ROUGHLY THE NEXT 72 HR. DURING THAT TIME...THE INTENSITY SHOULD BE CONTROLLED MAINLY BY HARD-TO-FORECAST EYEWALL REPLACEMENT CYCLES. THE INTENSITY FORECAST WILL THUS CALL FOR A SLOW WEAKENING THROUGH THAT TIME BASED ON CONTINUITY FROM THE PREVIOUS FORECASTS AND CLIMATOLOGY. AFTER 72 HR...THE GFS AND NOGAPS SUGGEST THAT ISABEL MAY MOVE NORTH OF A 200 MB ANTICYCLONE AND GET EXPOSED TO SOME SHEAR. THE SHIPS MODEL RESPONDS TO THIS BY FORECASTING 30 KT OF SOUTHWESTERLY SHEAR BY 120 HR. BASED ON THIS...THE INTENSITY FORECAST WILL CALL FOR A SOMEWHAT FASTER WEAKENING. IT SHOULD BE NOTED THAT THE LARGE-SCALE MODELS ALL FORECAST ISABEL TO GROW IN SIZE BEYOND 72 HR...WHICH MAY BE REFLECTED IN LATER WIND RADII FORECASTS. GIVEN THE DIVERGENCE OF THE MODELS AT 120 HR...IT IS STILL TO EARLY TO EVEN SPECULATE WHICH PARTS...IF ANY...OF EASTERN COAST OF THE UNITED STATES MAY GET AFFECTED BY ISABEL. FORECASTER BEVEN FORECAST POSITIONS AND MAX WINDS INITIAL 12/1500Z 21.6N 57.8W 140 KT 12HR VT 13/0000Z 21.8N 59.1W 135 KT 24HR VT 13/1200Z 22.2N 61.2W 135 KT 36HR VT 14/0000Z 22.8N 63.4W 135 KT 48HR VT 14/1200Z 23.4N 65.5W 130 KT 72HR VT 15/1200Z 24.5N 69.5W 125 KT 96HR VT 16/1200Z 25.5N 72.0W 110 KT 120HR VT 17/1200Z 27.5N 75.0W 105 KT NNNN
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Page last modified: Monday, 07-Feb-2005 16:49:58 UTC