Tropical Storm ZETA

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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
 
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