Tropical Storm SERGIO

ZCZC MIATCDEP1 ALL
TTAA00 KNHC DDHHMM
TROPICAL STORM SERGIO DISCUSSION NUMBER  15
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   EP212006
100 AM PST FRI NOV 17 2006
 
SERGIO APPEARS TO BE SHEARING APART THIS MORNING...WITH THE
LOW-LEVEL CENTER AT LEAST PARTLY EXPOSED ON THE WEST SIDE OF THE
REMAINING DEEP CONVECTION.  ANALYSES FROM CIMSS AT THE UNIVERSITY
OF WISCONSIN SHOW ABOUT 20 KT OF SOUTHWESTERLY SHEAR AT THE PRESENT
TIME.  WHILE SATELLITE INTENSITY ESTIMATES AT 06Z WERE STILL AT
HURRICANE STRENGTH...THE SATELLITE SIGNATURE HAS DECREASED
SIGNIFICANTLY SINCE THEN...AND THUS SERGIO IS DOWNGRADED TO A
TROPICAL STORM.

THE INITIAL MOTION IS A SOMEWHAT UNCERTAIN 350/3.  WATER VAPOR
IMAGERY INDICATES A BROAD MID/UPPER-LEVEL LOW NEAR 20N112W...WITH
BROAD MID/UPPER-LEVEL RIDGING DEVELOPING NORTH OF THE LOW NEAR
NORTHERN BAJA CALIFORNIA.  THE LARGE-SCALE MODELS FORECAST THE LOW
TO MOVE WESTWARD UNDER THE RIDGE...WITH THE RIDGE BUILDING EASTWARD
AND SOUTHEASTWARD INTO CENTRAL MEXICO.  THE TRACK GUIDANCE IS SPLIT
ROUGHLY INTO TWO CLUSTERS.  THE FIRST... WHICH INCLUDES THE CONU...
THE FSU SUPERENSEMBLE...THE NOGAPS...AND THE GFDL...CALL FOR A TURN
TOWARD THE NORTHWEST DURING THE NEXT 24 HOURS...WITH THAT MOTION
CONTINUING THROUGH THE FORECAST PERIOD.  THE SECOND...WHICH
INCLUDES THE GFS...THE GFS ENSEMBLE MEAN...THE UKMET...AND THE BAM
SHALLOW...CALLS FOR A GENERAL WEST-NORTHWESTWARD MOTION.  THE
OFFICIAL FORECAST IS AN UPDATE OF THE PREVIOUS PACKAGE AND IS DOWN
THE MIDDLE OF THE FIRST CLUSTER.  HOWEVER...IF SERGIO TOTALLY
SHEARS APART LOW-LEVEL STEERING WILL BECOME PREDOMINANT LEADING TO
A TRACK SIMILAR TO THE SECOND CLUSTER.  SOME ADJUSTMENT OF THE
FORECAST TRACK MAY BE NECESSARY LATER TODAY DEPENDING ON HOW THE
STRUCTURE OF SERGIO EVOLVES.

WHILE SERGIO IS CURRENTLY IN A RATHER UNFAVORABLE ENVIRONMENT...THE
LARGE-SCALE MODELS CONTINUE TO SUGGEST THAT THE SHEAR WILL DECREASE
SLIGHTLY DURING THE NEXT 48 HR...ACCOMPANIED BY AN INCREASE IN
UPPER-LEVEL DIVERGENCE.  BASED ON THIS...THE INTENSITY FORECAST
WILL CALL FOR THE CURRENT WEAKENING TO STOP IN 12-24 HR.  THIS PART
OF THE INTENSITY FORECAST IS SIMILAR TO...BUT WEAKER THAN...THE
SHIPS AND SUPERENSEMBLE MODELS.  AFTER 48 HR...THE LARGE-SCALE
MODELS AGREE THAT THE SHEAR WILL INCREASE SIGNIFICANTLY...WHICH
SHOULD CAUSE WEAKENING AND EVENTUAL DISSIPATION BY 120 HR.  IF THE
CURRENT SHEAR NOT DECREASE...SERGIO MAY DISSIPATE EARLIER THAN
CURRENTLY FORECAST.
 
THE WIND RADII WERE ADJUSTED IN THE BASIS OF A 0100Z QUIKSCAT
OVERPASS.  RAINBANDS CONTINUE TO AFFECT THE MEXICAN COAST...AND
INTERESTS IN SOUTHWESTERN MEXICO SHOULD CONTINUE TO MONITOR THE
PROGRESS OF SERGIO.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      17/0900Z 14.0N 102.8W    60 KT
 12HR VT     17/1800Z 14.6N 103.0W    55 KT
 24HR VT     18/0600Z 15.2N 103.5W    50 KT
 36HR VT     18/1800Z 15.9N 104.4W    50 KT
 48HR VT     19/0600Z 16.7N 105.4W    50 KT
 72HR VT     20/0600Z 18.0N 107.0W    40 KT
 96HR VT     21/0600Z 19.5N 109.5W    30 KT
120HR VT     22/0600Z 21.0N 112.0W    25 KT...REMNANT LOW
 
$$
FORECASTER BEVEN
 
NNNN