915 
 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