000
WTNT44 KNHC 212034
TCDAT4

TROPICAL STORM LISA DISCUSSION NUMBER   4
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL142010
500 PM EDT TUE SEP 21 2010
 
AFTER A SHORT-LIVED DECREASE IN THE CENTRAL CONVECTION...A BURST
OF STRONG CONVECTION HAS RECENTLY DEVELOPED OVER THE WELL-DEFINED
LOW-LEVEL CIRCULATION CENTER. RECENT TRMM AND AMSU MICROWAVE
OVERPASSES INDICATE THE LOW- AND MID-LEVEL CIRCULATIONS HAVE
TIGHTENED UP AND BECOME MORE VERTICALLY ALIGNED...SUGGESTING THAT
THE VERTICAL SHEAR AFFECTING THE SYSTEM HAS DECREASED. UPPER-LEVEL
OUTFLOW CONTINUES TO IMPROVE IN ALL QUADRANTS. THE INITIAL
INTENSITY REMAINS AT 40 KT BASED ON A BLEND OF DVORAK SATELLITE
INTENSITY ESTIMATES OF T2.5/35 KT FROM BOTH TAFB AND SAB...AND
UW-CIMSS ADT CURRENT INTENSITY ESTIMATE OF T3.4/53 KT.
 
THE INITIAL MOTION ESTIMATE IS 360/2. LISA HAS BEEN WOBBLING BETWEEN
A NORTH AND NORTHEAST DIRECTION...PROBABLY DUE TO RE-ORGANIZATION
OF THE SYSTEM CENTER. NOW THAT THE VERTICAL CIRCULATIONS APPEAR TO
BE MORE VERTICALLY ALIGNED...HOPEFULLY THIS WILL RESULT IN LESS
SCATTER IN THE SATELLITE FIX POSITIONS. LISA IS EXPECTED REMAIN IN
VERY WEAK STEERING FLOW FOR THE NEXT 48-72 HOURS. AFTERWARDS...A
MID-LEVEL TROUGH SITUATED JUST NORTH OF THE CYCLONE IS FORECAST TO
SLOWLY LIFT OUT TO THE NORTHEAST...ALLOWING THE SUBTROPICAL RIDGE
TO BUILD WESTWARD TO THE NORTH OF LISA...WHICH SHOULD NUDGE THE
CYCLONE ON A SLOW WESTWARD OR WEST-NORTHWESTWARD TRACK. THE NHC
MODEL GUIDANCE REMAINS WIDELY DIVERGENT BETWEEN THE NORTHWARD HWRF
AND THE WESTWARD GFDN MODEL SOLUTIONS. THE OFFICIAL FORECAST TRACK
IS SIMILAR THE PREVIOUS ADVISORY...AND REMAINS NORTH OF THE MODEL
CONSENSUS AND BETWEEN THE GFS AND ECMWF TRACKS.
 
THE VERTICAL SHEAR IS FORECAST TO REMAIN BELOW 10 KT FOR THE NEXT 
48 HOURS...WHICH USUALLY FAVORS SIGNIFICANT INTENSIFICATION.
HOWEVER...MID-LEVEL MOISTURE IS FORECAST TO DECREASE WITH HUMIDITY
LEVELS BELOW 40 PERCENT AFTER 24 HOURS. DESPITE THE PRESENCE OF DRY
AIR...JULIA IS FORECAST TO STRENGTHEN FAIRLY QUICKLY IN THE NEXT 24
HOURS DUE TO THE IMPROVED INNER-CORE STRUCTURE NOTED IN THE
AFOREMENTIONED MICROWAVE IMAGERY. GIVEN THAT A 75-PERCENT EYEWALL
FEATURE EXISTED 6 HRS AGO AND CLOUD TOPS HAVE RECENTLY COOLED TO
BELOW -80C NEAR THE CENTER...A PERIOD OF RAPID INTENSIFICATION CAN
NOT BE RULED OUT. THE ECMWF MODEL CONTINUES TO KEEP LISA SOUTH OF
20N LATITUDE AND BENEATH AN UPPER-LEVEL ANTICYCLONE. THE OFFICIAL
INTENSITY FORECAST IS ABOVE THE OBJECTIVE INTENSITY GUIDANCE THROUGH
48 HOURS AND CONTINUES TO LEAN TOWARD THE STRONGER ECMWF SOLUTION.

 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      21/2100Z 18.3N  31.6W    40 KT
 12HR VT     22/0600Z 18.6N  31.7W    50 KT
 24HR VT     22/1800Z 19.0N  31.9W    60 KT
 36HR VT     23/0600Z 19.3N  32.5W    65 KT
 48HR VT     23/1800Z 19.7N  33.3W    65 KT
 72HR VT     24/1800Z 20.4N  34.7W    55 KT
 96HR VT     25/1800Z 21.0N  36.0W    45 KT
120HR VT     26/1800Z 21.0N  38.0W    40 KT
 
$$
FORECASTER STEWART