Tropical Storm SERGIO

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TROPICAL STORM SERGIO DISCUSSION NUMBER   6
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   EP212006
100 AM PST WED NOV 15 2006
 
MICROWAVE DATA FROM WINDSAT AT 0057Z AND SSM/I AT 0305Z SHOW THAT
THE CENTRAL CONVECTION OF SERGIO IS ASYMMETRIC...CURRENTLY CONFINED
TO THE EASTERN QUADRANT.  THE SHIPS MODEL AND ANALYSES FROM CIMMS
AT THE UNIVERSITY OF WISCONSIN INDICATE NORTHWESTERLY SHEAR OF LESS
THAN 10 KT...WHICH SEEMS INSUFFICIENT TO EXPLAIN THE CURRENT
STRUCTURE.  HOWEVER...IT MAY BE SUFFICIENT TO AFFECT THE CIRRUS
OUTFLOW...WHICH IS CURRENTLY POOR TO THE NORTHWEST AND FAIR TO GOOD
ELSEWHERE.  SATELLITE INTENSITY ESTIMATES REMAIN 55 KT FROM TAFB
AND SAB...AND THAT REMAINS THE INITIAL INTENSITY.

THE INITIAL MOTION IS 135/1.  SERGIO IS CURRENTLY IN A REGION OF
LIGHT STEERING CURRENTS BETWEEN WEAK MID-LEVEL RIDGES TO THE EAST
AND WEST...AND SOUTH OF A DEEP LAYER TROUGH OVER THE CENTRAL UNITED
STATES.  LARGE-SCALE MODELS FORECAST A MID/UPPER-LEVEL SHORTWAVE
TROUGH TO MOVE SOUTHWARD FROM THE WESTERN UNITED STATES AND BECOME
A CUTOFF LOW WEST OR SOUTHWEST OF SOUTHERN BAJA CALIFORNIA IN 24-48
HR.  THIS SHOULD ALLOW SERGIO TO TURN MORE TOWARD THE NORTH AT A
SLIGHTLY FASTER FORWARD SPEED.  AFTER 48 HR...THE MODELS DIVERGE ON
THE STRENGTH AND TIMING OF AN EAST-WEST RIDGE FORECAST TO FORM OVER
NORTHERN MEXICO.  THE NOGAPS TAKES SERGIO INLAND OVER MEXICO INTO
THE WESTERLIES BEFORE THE RIDGE FORMS.  THE UKMET TRAPS THE SYSTEM
IN WEAK STEERING CURRENTS SOUTHWEST OF MANZANILLO THROUGH 120 HR...
WITH THE OTHER MODELS BETWEEN THESE EXTREMES IN CALLING FOR A
NORTH-NORTHWESTWARD MOTION.  THE NEW FORECAST TRACK AGREES BEST
WITH THE CONU CONSENSUS AND FLORIDA STATE SUPERENSEMBLE MODELS...
CALLING FOR A SLOW NORTH-NORTHWESTWARD MOTION FROM 48-120 HR.  THE
NEW TRACK HAS ONLY MINOR CHANGES FROM THE PREVIOUS FORECAST TRACK.

IN THE SHORT TERM...THE LACK OF A CLOSED EYEWALL AND A LESS
FAVORABLE OUTFLOW PATTERN SUGGEST A DECREASED CHANCE OF RAPID
INTENSIFICATION IN THE NEXT 24 HR OR SO.  INDEED...THE SHIPS MODEL
NO LONGER CALLS FOR SERGIO TO BECOME A HURRICANE DUE TO INCREASING
SHEAR.  HOWEVER...THE SHIPS SHEAR APPEARS TOO STRONG COMPARED TO
THE FORECAST 200 MB WINDS OF THE GFS...AND THE GFDL AND FSU
SUPERENSEMBLE STILL FORECAST SERGIO TO REACH HURRICANE STRENGTH. 
IN THE LONGER TERM...THE LARGE-SCALE MODELS ALL FORECAST A
DIFFLUENT UPPER-LEVEL RIDGE TO FORM NEAR SERGIO FROM 24-72 HR. 
WHAT THEY DO NOT AGREE ON IS HOW MUCH SHEAR THIS RIDGE WILL
PRODUCE.  AFTER 72 HR...THE MODELS AGREE THAT SERGIO SHOULD
ENCOUNTER INCREASING SHEAR...IF IT IS AS FAR NORTH AS FORECAST. 
GIVEN THE UNCERTAINTIES...THERE WILL BE NO CHANGE TO THE INTENSITY
FORECAST FROM THE PREVIOUS ADVISORY.  HOWEVER...THIS IS A LOW
CONFIDENCE INTENSITY FORECAST.

 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      15/0900Z 12.6N 104.2W    55 KT
 12HR VT     15/1800Z 12.5N 104.0W    65 KT
 24HR VT     16/0600Z 13.1N 103.7W    75 KT
 36HR VT     16/1800Z 13.9N 103.7W    80 KT
 48HR VT     17/0600Z 15.1N 104.0W    80 KT
 72HR VT     18/0600Z 16.5N 104.5W    75 KT
 96HR VT     19/0600Z 18.0N 105.5W    65 KT
120HR VT     20/0600Z 19.0N 106.5W    55 KT
 
$$
FORECASTER BEVEN
 
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