Tropical Storm ZETA

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TROPICAL STORM ZETA DISCUSSION NUMBER   8
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
4 AM EST SUN JAN 01 2006
 
THE BURST OF DEEP CONVECTION THAT OCCURRED EARLIER HAS BEEN ON THE
WANE SOMEWHAT DURING THE PAST FEW HOURS. HOWEVER... SHEAR PATTERN
SATELLITE INTENSITY ESTIMATES REMAIN AT ABOUT 45 KT... AND THAT IS
THE INTENSITY USED FOR THIS ADVISORY. UPPER-LEVEL OUTFLOW REMAINS
CONFINED MAINLY TO THE NORTHEASTERN QUADRANT... ALTHOUGH THE CIRRUS
OUTFLOW OVER THE LOW-LEVEL CENTER HAS EXPANDED SOMEWHAT TO THE
SOUTHWEST... WHICH HAS MADE LOCATING THE CENTER A LITTLE DIFFICULT.

THE MOTION ESTIMATE IS 270/02 KT... BASED ON AN 18-HR MOTION. ZETA
IS EXPECTED TO MOVE SLOWLY WEST OR EVEN WEST-SOUTHWESTWARD AROUND
THE SOUTHWESTERN PERIPHERY OF A LOW- TO MID-LEVEL HIGH PRESSURE
RIDGE LOCATED EAST-WEST OVER THE AZORES ISLANDS. THE MID- TO
UPPER-LEVEL FLOW...HOWEVER...IS FORECAST TO REMAIN FROM A GENERAL
WESTERLY DIRECTION...SO THE OFFSETTING FLOW PATTERNS SHOULD RESULT
IN A SLOW FORWARD SPEED THROUGHOUT THE FORECAST PERIOD. THE GLOBAL
MODELS ARE IN GENERAL AGREEMENT ON THIS MOTION... WHEREAS THE GFDN
AND GFDL MODELS APPEAR TO BE OUTLIERS BY TAKING ZETA NORTHWARD
ALONG 44W-46W LONGITUDE AFTER 72 HOURS AS A VERTICALLY DEEP
HURRICANE... WHICH SEEMS UNLIKELY. THE OFFICIAL FORECAST TRACK IS
CLOSE TO THE PREVIOUS TRACK AND THE GLOBAL MODEL CONSENSUS.
 
ZETA REMAINS CAUGHT IN A SOMEWHAT FAVORABLE DIFLUENT FLOW REGION
BETWEEN THE SUBTROPICAL JET TO THE SOUTH AND THE POLAR JET TO THE
NORTH. HOWEVER... SHEAR ANALYSES FROM UW-CIMSS INDICATE 25-30 KT OF
VERTICAL SHEAR... WHICH IS ABOUT 20-25 KT LESS THAN THE SHIPS MODEL
SHEAR ASSESSMENT... IS STILL IMPINGING ON ZETA. THAT MAGNITUDE OF
SHEAR IS FORECAST TO CONTINUE FOR THE NEXT 24 HOURS OR SO... WITH A
SLIGHT DECREASE IN SHEAR EXPECTED IN THE 24-48 HOUR TIME PERIOD.
HOWEVER... WITH ZETA MOVING INTO A DRIER ENVIRONMENT THROUGHOUT THE
FORECAST PERIOD... ANY DECREASE IN THE VERTICAL SHEAR SHOULD BE
OFFSET BY THE VERY DRY MID-LEVEL AIR. THE RESULT IS THAT GRADUAL
WEAKENING IS EXPECTED... BUT NOT QUITE AS FAST AS THE SHIPS MODEL
WHICH IS FORECASTING ZETA TO DISSIPATE IN 36-48 HOURS.

FORECASTER STEWART
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      01/0900Z 25.7N  38.5W    45 KT
 12HR VT     01/1800Z 25.6N  39.2W    40 KT
 24HR VT     02/0600Z 25.5N  40.2W    35 KT
 36HR VT     02/1800Z 25.5N  41.2W    30 KT
 48HR VT     03/0600Z 25.8N  42.2W    25 KT...REMNANT LOW
 72HR VT     04/0600Z...DISSIPATED

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