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| #63643 (Received by flhurricane at: 4:17 AM 31.Dec.2005) TCDAT5 TROPICAL STORM ZETA DISCUSSION NUMBER 4 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 4 AM EST SAT DEC 31 2005 A BURST OF RELATIVELY DEEP CONVECTION...WITH CLOUD TOPS TO -70C... HAS DEVELOPED OVER THE LOW-LEVEL CENTER. IN FACT... IT APPEARS THAT A SMALL CDO FEATURE MAY BE FORMING OVER THE WELL-DEFINED CENTER DEPICTED IN RECENT TRMM AND AMSU MICROWAVE DATA. THE 50-KT INITIAL INTENSITY IS BASED ON A BLEND OF SATELLITE INTENSITY ESTIMATES OF 55 KT FROM TAFB AND 45 KT FROM SAB. DATA T-NUMBERS USING A CDO FEATURE SUGGEST THE INTENSITY COULD EVEN BE 55-60 KT... BUT I WOULD PREFER TO WAIT AND SEE IF THE CDO-LIKE FEATURE PERSISTS FOR AT LEAST ANOTHER 6 HOURS BEFORE INCREASING THE INTENSITY ANY FURTHER. THE INITIAL MOTION ESTIMATE IS 320/04 KT. MICROWAVE IMAGERY DURING THE PAST 9 HOURS INDICATES ZETA HAS SLOWED ITS NORTHWESTWARD MOTION. A TURN TOWARD THE WEST-NORTHWEST IS EXPECTED TO BEGIN DURING THE NEXT 6-12 HOURS AS ZETA COMES MORE UNDER THE INFLUENCE OF THE SUBTROPICAL RIDGE TO NORTH... AS THE MID- TO UPPER-LEVEL LOW TO THE SOUTH WEAKENS. THE PRIMARY DILEMMA WITH THE TRACK FORECAST IS WHEN WILL ZETA WEAKEN AND BECOME VERTICALLY SHALLOW... AND THEN BE STEERED MORE WESTWARD OR EVEN WEST-SOUTHWESTWARD BY THE BERMUDA RIDGE. SINCE ZETA IS CURRENTLY STRONGER THAN THE PREVIOUS THREE RUNS OF THE SHIPS MODEL HAS BEEN FORECASTING... AN ADDITIONAL 12-24 HOURS OF TROPICAL CYCLONE STATUS HAS BEEN ADDED TO THE FORECAST. SINCE THE 00Z GFS MODEL REAMINS POORLY INITIALIZED... THE OFFICIAL TRACK LEANS CLOSER TO THE GFDL-NOGAPS-UKMET CONSENSUS. LIKE THE FORECAST TRACK... THE INTENSITY FORECAST IS SOMEWHAT PROBLEMATIC. THE GFS-BASED SHIPS MODEL CURRENTLY SHOWS 45 KT OF VERTICAL SHEAR ACROSS THE SYSTEM... WHICH IS LIKELY MUCH TOO STRONG GIVEN THE IMPRESSIVE CONVECTIVE DEVELOPMENT THAT HAS OCCURRED OVER THE PAST 6 HOURS. SINCE ZETA IS A RELATIVELY SHALLOW TROPICAL CYCLONE LIKE ITS PREDECESSORS DELTA AND EPSILON... THE SHIPS SHEAR CALCULATIONS ARE ALSO PROBABLY TOO HIGH WITH ZETA. SINCE THE 300 MB FLOW IS FORECAST BY ALL THE GLOBAL MODELS TO BE WESTERLY AT ONLY 15-20 KT FOR AT LEAST THE NEXT 36 HOURS... THEN LESS SHEAR SHOULD RESULT IN LESS WEAKENING THAN FORECAST BY THE SHIPS MODEL. IT IS QUITE POSSIBLE THAT ZETA COULD EVEN SURVIVE BEYOND 72 HOURS. FORECASTER STEWART FORECAST POSITIONS AND MAX WINDS INITIAL 31/0900Z 26.0N 37.7W 50 KT 12HR VT 31/1800Z 26.3N 38.3W 50 KT 24HR VT 01/0600Z 26.5N 39.4W 45 KT 36HR VT 01/1800Z 26.6N 40.5W 35 KT 48HR VT 02/0600Z 26.6N 41.7W 30 KT 72HR VT 03/0600Z 27.0N 42.5W 25 KT...REMNANT LOW 96HR VT 04/0600Z...DISSIPATED |
