000
WTNT42 KNHC 160903
TCDAT2
HURRICANE JULIA DISCUSSION NUMBER  17
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL122010
500 AM AST THU SEP 16 2010
 
THE SATELLITE PRESENTATION OF JULIA HAS CONTINUED TO DETERIORATE
OVER THE PAST FEW HOURS.  MOST OF THE DEEP CONVECTION IS DISPLACED
NORTH OF THE ESTIMATED CENTER POSITION...AND CLOUD TOPS HAVE
CONTINUED TO WARM.  JULIA IS CURRENTLY SITUATED OVER WATERS OF
26-27C...AND THE MID- TO UPPER-LEVEL TROUGH/LOW BETWEEN JULIA AND
HURRICANE IGOR IS IMPARTING 20 TO 30 KT OF SOUTHWESTERLY SHEAR OVER
JULIA ACCORDING TO THE UW-CIMSS SHEAR ANALYSIS.  BASED ON A BLEND
OF SUBJECTIVE AND OBJECTIVE DVORAK INTENSITY ESTIMATES THE INITIAL
INTENSITY IS LOWERED TO 90 KT FOR THIS ADVISORY.  WHILE THE SHIPS
MODEL OUTPUT SHOWS THE SHEAR DECREASING TO LESS THAN 20 KT FROM 12
TO 24 HOURS...JULIA WILL REMAIN OVER MARGINAL SSTS THROUGH THAT
TIME.  THEN BY 36 HOURS OR SO...WHEN JULIA TEMPORARILY MOVES BACK
OVER WARMER WATERS...THE SHEAR INCREASES TO AROUND 30 KT AND
REMAINS HIGH FOR THE REST OF THE PERIOD.  THIS SCENARIO SHOULD
RESULT IN A GRADUAL WEAKENING FOR THE FIRST 48 HOURS...WITH MORE
RAPID WEAKENING TOWARD THE END OF THE PERIOD.  THE OFFICIAL FORECAST
REFLECT THIS AND FOLLOWS A BLEND OF THE LGEM AND SHIPS MODELS
THROUGH 48 HOURS...AND IS CLOSEST TO THE LGEM THEREAFTER. 
 
WITH A LACK OF MICROWAVE IMAGERY OVER THE PAST FEW HOURS...THE
INITIAL MOTION ESTIMATE OF 310/16 IS BASED LARGELY ON CONTINUITY
AND THE ASSUMPTION THAT THE LOW-LEVEL CENTER REMAINS DISPLACED SOUTH
AND EAST OF THE DEEP CONVECTION.  THE TRACK FORECAST REASONING
REMAINS UNCHANGED FROM THE PREVIOUS PACKAGE.  JULIA IS CURRENTLY
BEING STEERED NORTHWESTWARD BY THE UPPER-LEVEL LOW TO ITS SOUTHWEST
AND A LARGE MID-LEVEL RIDGE TO ITS EAST...AND THIS WILL CONTINUE
FOR THE NEXT DAY OR SO. THEN...AS JULIA MOVES AWAY FROM THE
UPPER-LOW...THE CYCLONE WILL MOVE AROUND THE WESTERN PERIPHERY OF
THE RIDGE AND TURN EAST-NORTHEASTWARD BY THE END OF THE PERIOD AS
IT COMES UNDER THE INFLUENCE OF THE MID-LATITUDE WESTERLIES.  SOME
DIFFERENCES REMAIN IN HOW SHARPLY JULIA MOVES AROUND THE RIDGE...
WITH THE UKMET SHOWING A SHARPER TURN...WHILE THE ECMWF AND HWRF
SHOW A WIDER TURN.  THE OFFICIAL FORECAST HAS BEEN ADJUSTED TOWARD
THE TVCN MULTI-MODEL CONSENSUS...AND IS CLOSE TO THE GFS FOR
THE FIRST 72 HOURS...WHICH REPRESENTS A COMPROMISE BETWEEN THE
ECMWF AND UKMET. THE NEW NHC FORECAST IS ALSO FASTER THAN THE
PREVIOUS PACKAGE DURING THE FIRST 24 HOURS...ACCOUNTING FOR THE
OBSERVED ACCELERATION OF JULIA.   
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      16/0900Z 21.2N  36.2W    90 KT
 12HR VT     16/1800Z 22.8N  38.8W    85 KT
 24HR VT     17/0600Z 24.6N  42.4W    80 KT
 36HR VT     17/1800Z 26.4N  45.5W    75 KT
 48HR VT     18/0600Z 28.3N  47.7W    70 KT
 72HR VT     19/0600Z 32.5N  49.5W    65 KT
 96HR VT     20/0600Z 35.0N  46.5W    50 KT
120HR VT     21/0600Z 36.0N  42.5W    40 KT
 
$$
FORECASTER BRENNAN