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| #63825 (Received by flhurricane at: 4:17 AM 01.Jan.2006) TCDAT5 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 |
