Tropical Storm ZETA

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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
 
 
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