ZCZC MIATCDEP2 ALL TTAA00 KNHC DDHHMM TROPICAL STORM NORA DISCUSSION NUMBER 6 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL EP172009 200 AM PDT THU SEP 24 2009 THE COVERAGE AND INTENSITY OF THE CENTRAL CONVECTION ASSOCIATED WITH NORA DECREASED IN THE HOURS PRIOR TO THE GOES-WEST ECLIPSE PERIOD. THIS LIKELY OCCURRED DUE TO INCREASING SOUTHWESTERLY WIND SHEAR ANALYZED BY UW-CIMSS AND THE SHIPS MODEL...ALONG WITH A DRYING TREND IN THE MID-TROPOSPHERE AND A DECREASE IN UPPER-LEVEL DIVERGENCE INDICATED BY SHIPS. SUBJECTIVE AND OBJECTIVE FINAL-T NUMBERS WERE A UNANIMOUS 3.0 AT 0600 UTC...AND ON THIS BASIS THE INITIAL INTENSITY IS LOWERED TO 45 KT. THE INTENSITY GUIDANCE IS IN UNANIMOUS AGREEMENT THAT NORA HAS PEAKED. THE SHIPS MODEL FORECASTS AT LEAST 15-20 KT OF SHEAR THROUGH THE PERIOD...WHICH IN COMBINATION WITH THE MARGINAL ATMOSPHERIC ENVIRONMENT SHOULD RESULT IN SLOW WEAKENING. THE INTENSITY FORECAST HAS BEEN ADJUSTED DOWNWARD IN CLOSE AGREEMENT WITH THE ICON INTENSITY CONSENSUS. NORA SHOULD BECOME A REMNANT LOW IN ABOUT 4 DAYS AND BE DISSIPATING BY DAY 5. BASED ON SEVERAL MICROWAVE FIXES AND GEOSTATIONARY IMAGERY PRIOR TO THE ECLIPSE...NORA APPEARS TO HAVE BEEN MOVING VERY LITTLE OVER THE PAST FEW HOURS. A 12-HOUR INITIAL MOTION ESTIMATE OF 300/04 WAS USED FOR THIS ADVISORY. THE TRACK MODEL GUIDANCE IS IN REASONABLY GOOD AGREEMENT ON THE OVERALL FORECAST SCENARIO. AS NORA WEAKENS... THE CYCLONE WILL TURN WESTWARD TO THE SOUTH OF THE LOW-LEVEL RIDGE THROUGH ABOUT 72 HOURS. BEYOND THAT TIME...MOST OF THE GUIDANCE SHOWS A TURN TOWARD THE WEST-NORTHWEST IN 3 OR 4 DAYS AS THE LOW-LEVEL RIDGE WEAKENS...ALLOWING WHAT IS LEFT OF NORA TO GAIN SOME LATITUDE. THE OFFICIAL FORECAST IS ADJUSTED SLIGHTLY NORTH THROUGH 24 HOURS TO ACCOUNT FOR THE INITIAL MOTION...AND THEN IS SIMILAR TO THE PREVIOUS PACKAGE THROUGH 72 HOURS. AT DAYS 4 AND 5 THE FORECAST IS SHIFTED SLIGHTLY SOUTHWARD TOWARD THE TVCN MULTI-MODEL CONSENSUS AND LIES NEAR THE MIDDLE OF THE GUIDANCE ENVELOPE. THE INITIAL 34-KT WIND RADII WERE ADJUSTED BASED ON A QUIKSCAT PASS AT 0150 UTC. FORECAST POSITIONS AND MAX WINDS INITIAL 24/0900Z 17.2N 119.2W 45 KT 12HR VT 24/1800Z 17.4N 120.0W 45 KT 24HR VT 25/0600Z 17.5N 121.2W 40 KT 36HR VT 25/1800Z 17.3N 122.5W 35 KT 48HR VT 26/0600Z 17.3N 124.0W 30 KT 72HR VT 27/0600Z 18.0N 127.0W 25 KT 96HR VT 28/0600Z 19.0N 129.5W 25 KT...REMNANT LOW 120HR VT 29/0600Z 19.6N 131.5W 20 KT...DISSIPATING $$ FORECASTER BRENNAN NNNN
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Page last modified: Thursday, 31-Dec-2009 12:09:28 UTC