ZCZC MIATCDAT1 ALL TTAA00 KNHC DDHHMM HURRICANE IGOR DISCUSSION NUMBER 26 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL112010 1100 AM AST TUE SEP 14 2010 THERE HAS BEEN LITTLE CHANGE IN THE STRUCTURE OF IGOR DURING THE PAST SEVERAL HOURS...WITH THE 20 NM WIDE EYE REMAINING WELL DEFINED AND EYEWALL CONVECTIVE TOPS REMAINING COLDER THAN -70C. SATELLITE INTENSITY ESTIMATES ARE UNCHANGED FROM SIX HRS AGO...SO THE INITIAL INTENSITY REMAINS 115 KT. RECENT MICROWAVE OVERPASSES DO NOT YET SHOW AN OUTER EYEWALL...BUT DO SHOW THAT A LARGE RAIN-FREE MOAT HAS FORMED BETWEEN THE EYEWALL AND THE OUTER CONVECTIVE BANDS. THE CIRRUS OUTFLOW IS GOOD IN ALL DIRECTIONS. THE INITIAL MOTION IS NOW 290/6. IGOR IS MOVING TOWARD A WEAKNESS IN THE SUBTROPICAL RIDGED CAUSED BY A BROAD DEEP-LAYER TROUGH OVER THE NORTHEASTERN UNITED STATES AND THE NORTHWESTERN ATLANTIC. THE DYNAMICAL MODELS FORECAST THIS WEAKNESS TO PERSIST FOR THE NEXT SEVERAL DAYS AS A SERIES OF SHORTWAVE TROUGHS MOVE THROUGH THE BROADER TROUGH. IN ADDITION...THE DYNAMICAL MODELS FORECAST AN UPPER-LEVEL TROUGH TO DEVELOP NEAR THE SOUTHEASTERN COAST OF THE UNITED STATES BY 96-120 HR...WITH A LARGE UPPER-LEVEL HIGH FORMING OVER THE CENTRAL ATLANTIC. THIS EVOLUTION SHOULD STEER IGOR WEST-NORTHWESTWARD TO NORTHWESTWARD FOR THE NEXT 48-72 HR... FOLLOWED BY A MORE NORTHWARD MOTION. THE TRACK GUIDANCE IS IN GOOD AGREEMENT WITH THIS SCENARIO...ALTHOUGH THERE REMAINS SOME SPREAD BETWEEN THE MORE WESTWARD UKMET/NOGAPS AND THE MORE EASTWARD GFDL. THE NEW FORECAST TRACK IS SHIFTED A LITTLE TO THE WEST OF THE PREVIOUS TRACK AND LIES NEAR THE CENTER OF THE GUIDANCE ENVELOPE. IGOR REMAINS IN AN ENVIRONMENT OF LIGHT VERTICAL WIND SHEAR AND WARM SEA SURFACE TEMPERATURES...AND THESE CONDITIONS ARE FORECAST TO PERSIST FOR THE NEXT 48-72 HR. THUS...THE INTENSITY IS LIKELY TO BE CONTROLLED BY HARD-TO-TIME EYEWALL REPLACEMENT CYCLES DURING THIS TIME. THE INTENSITY FORECAST CALLS FOR SOME INTENSIFICATION BY 24 HR ON THE PREMISE THAT THE CURRENT INTERNAL CYCLE WILL BE COMPLETE. AFTER 72 HR...THE SEA SURFACE TEMPERATURES GRADUALLY COOL ALONG THE FORECAST TRACK...AND SOUTHERLY UPPER-LEVEL WINDS BETWEEN THE PREVIOUSLY MENTIONED TROUGH AND HIGH ARE EXPECTED TO CAUSE INCREASED SHEAR. THESE FACTORS SHOULD CAUSE IGOR TO GRADUALLY WEAKEN...AND THE NEW INTENSITY FORECAST IS THE SAME AS THE PREVIOUS FORECAST AT 96-120 HR. INTERESTS IN BERMUDA SHOULD NOT FOCUS ON THE 5-DAY POINT...SINCE TRACK ERRORS COULD EASILY BE A FEW HUNDRED MILES AT THAT FORECAST INTERVAL. IT IS STILL TOO EARLY TO SAY WITH CERTAINTY WHETHER IGOR WILL HAVE A SIGNIFICANT IMPACT ON THAT ISLAND. FORECAST POSITIONS AND MAX WINDS INITIAL 14/1500Z 18.3N 52.3W 115 KT 12HR VT 15/0000Z 18.8N 53.3W 115 KT 24HR VT 15/1200Z 19.8N 54.8W 120 KT 36HR VT 16/0000Z 20.8N 56.3W 120 KT 48HR VT 16/1200Z 21.9N 57.8W 120 KT 72HR VT 17/1200Z 24.5N 61.0W 115 KT 96HR VT 18/1200Z 28.0N 64.0W 100 KT 120HR VT 19/1200Z 32.0N 65.5W 90 KT $$ FORECASTER BEVEN NNNN
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