000
WTNT42 KNHC 130837
TCDAT2
TROPICAL STORM JULIA DISCUSSION NUMBER   4
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL122010
500 AM AST MON SEP 13 2010
 
THERE HAS NOT BEEN MUCH CHANGE TO THE OVERALL ORGANIZATION OF THE
CLOUD PATTERN OF JULIA.  MOST OF THE BANDING FEATURES ARE OVER THE
WESTERN SEMICIRCLE OF THE CIRCULATION WITH PERHAPS A SLIGHT
INCREASE IN DEEP CONVECTION NEAR THE CENTER.  DVORAK INTENSITY
ESTIMATES FROM BOTH SAB AND TAFB ARE 35 KT...SO THE WIND SPEED IS
UNCHANGED FROM THE PREVIOUS ADVISORY.  VERTICAL SHEAR IS EXPECTED
TO BE WEAK FOR THE NEXT DAY OR TWO AS JULIA MOVES AWAY FROM AN AREA
OF STRONG UPPER-LEVEL EASTERLIES ASSOCIATED WITH A RIDGE OVER WEST
AFRICA.  LATER IN THE FORECAST PERIOD THE TROPICAL CYCLONE COULD
EXPERIENCE A SIGNIFICANT INCREASE IN SHEAR DUE TO THE INFLUENCE OF
AN UPPER-LEVEL TROUGH OVER THE EASTERN SUBTROPICAL ATLANTIC...WHICH
WOULD LIMIT INTENSIFICATION.  THERE IS THE POSSIBILITY THAT A
CYCLONE WILL CUT OFF FROM THE TROUGH AND BECOME SITUATED TO THE
SOUTH OR SOUTHWEST OF JULIA.  THIS SCENARIO COULD RESULT IN LESS
SHEAR AND ALLOW MORE STRENGTHENING NEAR THE END OF THE PERIOD.
HOWEVER...THE GFS INPUT TO THE SHIPS AND LGEM MODELS SHOWS 25 TO 35
KT OF SHEAR AT 96-120 HOURS.  GIVEN THE UNCERTAINTIES THE OFFICIAL
FORECAST...LIKE THE PREVIOUS ONE...SHOWS NO CHANGE IN STRENGTH
BEYOND 48 HOURS.  THIS IS CLOSE TO THE INTENSITY MODEL CONSENSUS
THROUGH 48 HOURS AND IS SIMILAR TO THE LGEM GUIDANCE FOR DAYS 3 TO
5.
 
LATEST CENTER FIXES ARE A BIT TO THE NORTH OF THE PREVIOUS TRACK.
THIS MAY BE THE RESULT OF SOME REFORMATION OR MERELY BECAUSE THE
CENTER HAS BEEN DIFFICULT TO LOCATE.  AFTER SOME ADJUSTMENTS...THE
INITIAL MOTION IS ESTIMATED TO BE 300/11.  NOTWITHSTANDING THE
SLIGHT NORTHWARD ADJUSTMENT...THE TRACK FORECAST REASONING REMAINS
ESSENTIALLY THE SAME AS IN THE PREVIOUS ADVISORY.  JULIA IS EXPECTED
TO MOVE WEST-NORTHWESTWARD AROUND THE SOUTHWESTERN SIDE OF A
MID-LEVEL RIDGE THAT EXTENDS FROM AFRICA WESTWARD INTO THE EASTERN
ATLANTIC.  IN ABOUT 48 HOURS...A WEAKNESS IN THE RIDGE SHOULD CAUSE
THE TROPICAL CYCLONE TO TURN NORTHWESTWARD.  THERE IS A FAIR AMOUNT
OF SPREAD IN THE MODEL FORECAST TRACKS FOR DAYS 3 TO 5....AND THE
OFFICIAL FORECAST IS BASICALLY DOWN THE MIDDLE OF THE SUITE.  THIS
IS CLOSE TO BUT A TAD SLOWER THAN THE MODEL CONSENSUS DURING THE
LATTER HALF OF THE FORECAST PERIOD...AND ALSO A LITTLE NORTH OF THE
PREVIOUS NHC FORECAST TRACK DUE TO THE NORTHWARD RELOCATION.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      13/0900Z 14.4N  24.6W    35 KT
 12HR VT     13/1800Z 15.1N  26.2W    45 KT
 24HR VT     14/0600Z 16.2N  28.2W    50 KT
 36HR VT     14/1800Z 17.5N  30.1W    55 KT
 48HR VT     15/0600Z 19.0N  31.9W    65 KT
 72HR VT     16/0600Z 22.5N  36.0W    65 KT
 96HR VT     17/0600Z 25.5N  41.0W    65 KT
120HR VT     18/0600Z 28.5N  46.5W    65 KT
 
$$
FORECASTER PASCH