ZCZC MIATCDAT5 ALL TTAA00 KNHC DDHHMM TROPICAL STORM ZETA DISCUSSION NUMBER 5 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 10 AM EST SAT DEC 31 2005 ZETA CONTINUES TO GENERATE DEEP CONVECTION...BUT WESTERLY SHEAR IS RESTRICTING THIS CONVECTION TO THE EASTERN SEMICIRCLE. VISIBLE AND MICROWAVE IMAGERY GIVE THE IMPRESSION THAT THE CENTER COULD BECOME EXPOSED AT ANY TIME. OVERALL THE SYSTEM DOES NOT LOOK AS WELL ORGANIZED AS IT DID AT THIS TIME YESTERDAY...WHEN AN EYE-LIKE FEATURE WAS BRIEFLY PRESENT. DVORAK INTENSITY ESTIMATES FROM TAFB AND SAB ARE 55 AND 45 KT...RESPECTIVELY...AND THE ADVISORY INTENSITY WILL REMAIN 50 KT AS A COMPROMISE BETWEEN THESE TWO ESTIMATES. THIS MORNING'S QUIKSCAT PASS MISSED THE CENTER BUT WAS CLOSE ENOUGH TO INDICATE THAT THE 34 KT WIND RADII HAVE CONTRACTED. A SEQUENCE OF OBSERVATIONS FROM SHIP WCBP...WHICH PASSED WITHIN 40 NMI OF THE CENTER BUT NEVER REPORTED WINDS HIGHER THAN 34 KT... ALSO SUGGEST THAT ZETA PRESENTLY HAS A FAIRLY SMALL WIND FIELD. ZETA HAS FOUND A RELATIVELY SOFT SPOT BETWEEN STRONG SHEAR TO THE NORTH AND SOUTH OF THE CYCLONE...BUT STRONGER WESTERLIES LIE AHEAD AND THIS FLOW WILL CONTINUE TO POUND AWAY AT ZETA. GRADUAL WEAKENING IS THEREFORE ANTICIPATED BUT AT A SLIGHTLY SLOWER RATE THAN INDICATED BY THE SHIPS GUIDANCE. THE INITIAL MOTION IS ESTIMATED TO BE 280/4...WHICH IS TO THE LEFT OF THE PREVIOUS TRACK...AND IN FACT THE CENTER COULD BE A LITTLE SOUTH OF MY ADVISORY POSITION. WEAK RIDGING IS PRESENT IN THE LOWER- TO MIDDLE TROPOSPHERE TO THE NORTH OF ZETA...AND THIS STEERING SHOULD PREDOMINATE FOR THE NEXT DAY OR TWO AS ZETA BECOMES INCREASINGLY SHEARED. AFTER THAT...A FRONTAL SYSTEM CURRENTLY ABOUT 700 NMI TO THE WEST-NORTHWEST OF ZETA COULD TURN THE SYSTEM NORTHWARD...PARTICULARLY IF ZETA HOLDS TOGETHER LONGER THAN EXPECTED. MODEL GUIDANCE IS IN DECENT AGREEMENT FOR THE NEXT COUPLE OF DAYS...BUT AFTER THAT THERE ARE DIFFERENCES IN HOW ZETA WILL RESPOND TO THE APPROACHING FRONTAL TROUGH. THE GFDL...WHICH STRENGTHENS ZETA TO A HURRICANE IN THE FACE OF PERSISTENT WESTERLY SHEAR...TURNS THE SYSTEM SHARPLY NORTHWARD AHEAD OF THE TROUGH...WHILE THE GFS AND NOGAPS KEEP A VERY WEAK SYSTEM ON A WESTERLY OR WEST-SOUTHWESTERLY TRACK. THE OFFICIAL FORECAST FAVORS THE LATTER SCENARIO...WITH JUST A SLIGHT BEND TO THE RIGHT AS THE REMNANTS OF ZETA BECOME ABSORBED INTO THE FRONTAL SYSTEM IN 3 DAYS OR SO. FORECASTER FRANKLIN FORECAST POSITIONS AND MAX WINDS INITIAL 31/1500Z 25.9N 38.2W 50 KT 12HR VT 01/0000Z 26.0N 38.9W 45 KT 24HR VT 01/1200Z 26.0N 40.0W 40 KT 36HR VT 02/0000Z 26.0N 41.1W 30 KT 48HR VT 02/1200Z 26.3N 42.0W 25 KT...DISSIPATING 72HR VT 03/1200Z 27.0N 42.5W 25 KT...REMNANT LOW 96HR VT 04/1200Z...DISSIPATED $$ NNNN
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Page last modified: Saturday, 31-Dec-2005 14:40:19 UTC