000 WTPZ45 KNHC 090231 TCDEP5 HURRICANE NORBERT DISCUSSION NUMBER 21 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL EP152008 800 PM PDT WED OCT 08 2008 SATELLITE IMAGES CONTINUE TO DISPLAY A RATHER IMPRESSIVE TROPICAL CYCLONE WITH A WELL-DEFINED EYE AND CIRRUS OUTFLOW FANNING AWAY FROM THE CENTER. ALTHOUGH CLOUD-TOP TEMPERATURES HAVE WARMED A LITTLE IN THE EYEWALL RECENTLY...DVORAK SATELLITE CLASSIFICATIONS HAVEN'T CHANGED...SO THE INITIAL INTENSITY IS LEFT AT 115 KT. THE HURRICANE SHOULD CONTINUE TO EXPERIENCE LIGHT SHEAR CONDITIONS AND WARM WATERS FOR THE NEXT DAY OR SO. THE SHEAR IS THEN FORECAST TO INCREASE DUE TO AN APPROACHING UPPER-TROUGH AND...COMBINED WITH SOME COOLER WATERS...THE HURRICANE SHOULD WEAKEN AS IT APPROACHES LAND. THE BIGGEST QUESTION IS HOW QUICKLY IT WILL LOSE STRENGTH. THE DYNAMICAL MODELS ARE SHOWING A MUCH STRONGER SYSTEM APPROACHING BAJA CALIFORNIA WHILE THE STATISTICAL GUIDANCE SHOW A RATHER RAPID DROP OFF IN THE WINDS. THE INTENSITY FORECAST WILL BE SHADED A LITTLE HIGHER THAN THE INTENSITY CONSENSUS AND IS SIMILAR TO THE PREVIOUS FORECAST. THE HURRICANE HAS NOW TURNED TOWARD THE NORTHWEST AND IS MOVING AT ABOUT 8 KT. NORBERT IS ROUNDING THE SOUTHWESTERN PERIPHERY OF RIDGING OVER MEXICO. LATE IN THE DAY TOMORROW...THE TROPICAL CYCLONE SHOULD TURN TOWARD THE NORTH INTO A BREAK IN THE SUBTROPICAL RIDGE. ALL MODELS FORECAST A RATHER DEEP TROUGH DIVING INTO THE WESTERN USA AND NORTHWESTERN MEXICO BY THE WEEKEND... HOWEVER THERE IS A BIG DISCREPANCY ON HOW THE MODELS HANDLE THE TROPICAL CYCLONE RESPONDING TO THE TROUGH. THE CORE OF THE DIFFERENCES REALLY REVOLVES AROUND IF THE NORBERT STAYS INTACT UNDER INCREASING SHEAR. AN INTACT SYSTEM WOULD LIKELY JUST ACCELERATE NORTHEASTWARD AHEAD OF THE TROUGH...BUT A RAPIDLY WEAKENING ONE MIGHT SHEAR APART AND LEAVE A WEAKER SYSTEM BEHIND BEFORE A MEXICAN LANDFALL...LIKE THE GFS/ECMWF MODELS. GIVEN HOW STRONG THE SYSTEM IS NOW AND THE ENVIRONMENTAL CONDITIONS AHEAD OF IT...NORBERT SEEMS UNLIKELY TO WEAKEN TO THAT DEGREE BEFORE LANDFALL. THUS...THE OFFICIAL FORECAST WILL LEAN ON THE FASTER GUIDANCE AND GIVES LESS WEIGHT TO THE SLOWER SOLUTIONS OF THE GFS/ ECMWF MODELS. THE NEW NHC FORECAST IS A LITTLE FASTER THAN THE PREVIOUS ONE AND IS AHEAD OF THE DYNAMICAL MODEL CONSENSUS. FORECAST POSITIONS AND MAX WINDS INITIAL 09/0300Z 17.1N 111.8W 115 KT 12HR VT 09/1200Z 17.8N 112.6W 115 KT 24HR VT 10/0000Z 18.9N 113.4W 110 KT 36HR VT 10/1200Z 20.5N 113.6W 100 KT 48HR VT 11/0000Z 22.5N 113.2W 90 KT 72HR VT 12/0000Z 28.0N 109.0W 45 KT...INLAND 96HR VT 13/0000Z 35.0N 104.0W 20 KT...REMNANT LOW INLAND 120HR VT 14/0000Z...DISSIPATED $$ FORECASTER BLAKE