ZCZC MIATCDEP5 ALL TTAA00 KNHC DDHHMM HURRICANE NORBERT DISCUSSION NUMBER 20 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL EP152008 200 PM PDT WED OCT 08 2008 NORBERT IS A VERY IMPRESSIVE-LOOKING HURRICANE WITH VERY COLD CLOUD TOPS SURROUNDING A RATHER SYMMETRIC 30-35 NM WIDE EYE. A 1709 UTC AMSU OVERPASS SUGGEST THAT NORBERT MAY BE ATTEMPTING TO DEVELOP CONCENTRIC EYEWALLS...BUT THERE HAVE NOT BEEN ANY ADDITIONAL MICROWAVE IMAGERY TO CORROBORATE THIS. SUBJECTIVE DVORAK INTENSITY ESTIMATES FROM TAFB AND SAB ARE A CONSENSUS T6.0 OR 115 KT...WHICH IS USED FOR THE INITIAL INTENSITY. ENVIRONMENTAL CONDITIONS ARE FORECAST TO REMAIN FAVORABLE FOR ANOTHER DAY OR SO...AND INTENSITY CHANGES DURING THIS TIME WILL LIKELY BE DETERMINED BY INNER-CORE STRUCTURE...WHICH IS TYPICALLY DIFFICULT TO PREDICT. THE INTENSITY FORECAST ALLOWS FOR SOME SLIGHT ADDITIONAL INTENSIFICATION IN THE NEAR TERM. THEREAFTER NORBERT WILL BE MOVING OVER SLIGHTLY COOLER WATERS...WHICH IS EXPECTED TO INITIATE GRADUAL WEAKENING. IT IS MUCH TOO EARLY TO DETERMINE HOW STRONG NORBERT WILL BE WHEN IT APPROACHES THE BAJA PENINSULA. IF NORBERT MOVE A LITTLE SLOWER AND TRACKS WEST OF THE CURRENT FORECAST IT WOULD SPEND MORE TIME OVER COOLER WATERS... THAN A FASTER AND MORE EASTWARD TRACK. IN ADDITION TO THE TRACK FORECAST DILEMMA...SOUTHWESTERLY SHEAR IS PREDICTED TO INCREASE AROUND THE TIME NORBERT APPROACHES BAJA CALIFORNIA. GIVEN THESE UNCERTAINTIES...THE OFFICIAL FORECAST IS CLOSE THE INTENSITY CONSENSUS AND IS NOT AS STRONG AS THE MORE AGGRESSIVE HWRF MODEL WHICH KEEPS NORBERT RATHER STRONG UNTIL FINAL LANDFALL IN MAINLAND MEXICO. THE INITIAL MOTION REMAINS THE SAME AS EARLIER TODAY...290/8 KT. THE TRACK GUIDANCE IS IN GENERALLY GOOD AGREEMENT THAT A NORTHWESTWARD MOTION SHOULD COMMENCE SOON...AS NORBERT BEGINS TO ROUND THE SOUTHWESTERN PERIPHERY OF A MID-LEVEL RIDGE LOCATED OVER NORTHERN MEXICO. THEREAFTER...NORBERT SHOULD TURN NORTHWARD...THEN NORTHEASTWARD AHEAD OF A DEEP-LAYER TROUGH MOVING INTO THE WESTERN UNITED STATES IN A COUPLE OF DAYS. WHILE THE GUIDANCE AGREES ON THIS SCENARIO...THERE CONTINUES TO BE A VERY LARGE SPREAD ON THE FORWARD MOTION OF THE HURRICANE IN 2-3 DAYS. FOR EXAMPLE...AT 72 HOURS...THE GFS...ECMWF...GFDN...AND UKMET ALL SHOW NORBERT NEAR OR SOUTHWEST OF THE BAJA COAST...WHILE THE GFDL...HWRF AND NOGAPS PREDICT THAT NORBERT WILL BE OVER MAINLAND MEXICO AT THAT TIME. SOME OF THESE DIFFERENCE ARE LIKELY IN RESPONSE TO HOW STRONG OR DEEP THE VARIOUS MODELS KEEP NORBERT. GIVEN THE LARGE SPREAD...THE NHC FORECAST REMAINS NEAR THE MODEL CONSENSUS UNTIL IT IS MORE CLEAR AS TO WHICH ONE OF THESE SOLUTIONS WILL PREVAIL. THE NEW FORECAST IS NEARLY IDENTICAL TO THE PREVIOUS ADVISORY. THE WIND RADII HAVE BEEN EXPANDED SLIGHTLY ON THE BASIS OF DATA FROM A QUIKSCAT OVERPASS. FORECAST POSITIONS AND MAX WINDS INITIAL 08/2100Z 16.6N 111.2W 115 KT 12HR VT 09/0600Z 17.2N 112.1W 120 KT 24HR VT 09/1800Z 18.3N 113.0W 115 KT 36HR VT 10/0600Z 19.7N 113.5W 105 KT 48HR VT 10/1800Z 21.5N 113.3W 95 KT 72HR VT 11/1800Z 26.0N 110.5W 70 KT 96HR VT 12/1800Z 32.0N 106.0W 25 KT...REMNANT LOW INLAND 120HR VT 13/1800Z...DISSIPATED $$ FORECASTER BROWN NNNN
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