000
WTNT44 KNHC 211445
TCDAT4
TROPICAL STORM LISA DISCUSSION NUMBER   3
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL142010
1100 AM EDT TUE SEP 21 2010
 
INTERMITTENT BURSTS OF DEEP CONVECTION HAVE BEEN OCCURRING NEAR THE
WELL-DEFINED CIRCULATION CENTER NOTED IN A 21/0932Z SSMIS OVERPASS. 
UPPER-LEVEL OUTFLOW HAS ALSO GRADUALLY BEEN IMPROVING TO THE NORTH
AND SOUTH OF LISA. THE INITIAL INTENSITY HAS BEEN INCREASED TO 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 ESTIMATES OF T3.5/45 KT.

AFTER A BRIEF JOG TO THE NORTHEAST A FEW HOURS AGO...LISA HAS
RESUMED A MOTION ESTIMATE OF 360/4. LISA IS EXPECTED TO MOVE SLOWLY
NORTHWARD FOR THE NEXT 24 HOURS OR SO DUE TO THE COMBINATION OF
WEAK STEERING CURRENTS ASSOCIATED WITH A BREAK IN THE SUBTROPICAL
RIDGE AND MODERATE NORTHWESTERLY FLOW ON THE WEST SIDE OF A
DEEP-LAYER TROUGH SITUATED TO THE NORTH OF THE CYCLONE. AS THE
AFOREMENTIONED TROUGH LIFTS OUT TO THE NORTHEAST BY 36-48 HOURS...
THE RIDGE TO THE NORTH OF LISA IS EXPECTED TO BUILD BACK IN AND
FORCE THE CYCLONE ON A SLOW WEST-NORTHWESTWARD TRACK. THE MODEL
GUIDANCE IS WIDELY DIVERGENT WITH THE HWRF/GFDL MODELS TAKING LISA
DUE NORTH THROUGHOUT THE FORECAST PERIOD...WHILE THE NOGAPS/GFDN
MODELS MOVE LISA DUE WEST. THE REST OF THE NHC MODEL GUIDANCE IS
SPREAD OUT EVENLY BETWEEN THESE TWO EXTREMES. SUCH A LARGE SPREAD
IN THE MODEL GUIDANCE TYPICALLY SUGGESTS THAT THE FORWARD MOTION
WILL BE RELATIVELY SLOW...AND THAT IS REFLECTED IN THIS FORECAST
PACKAGE. THE OFFICIAL FORECAST TRACK IS SIMILAR THE PREVIOUS
ADVISORY TRACK AND LIES A LITTLE NORTH OF THE MODEL CONSENSUS.
 
ALTHOUGH THE VERTICAL SHEAR IS EXPECTED TO REMAIN LOW FOR ABOUT THE
NEXT 72 HOURS...THE MID-LEVEL MOISTURE ENVIRONMENT SURROUNDING LISA
IS FORECAST TO DRY OUT CONSIDERABLY AND SSTS ARE EXPECTED TO DROP
BELOW 27C. THE DRIER AIR AND COOLER WATER MAY PUT THE BREAKS ON THE
INTENSIFICATION PROCESS AS THE CYCLONE NEARS HURRICANE STRENGTH IN
48-72 HOURS. AFTER 72 HOURS...LISA IS EXPECTED TO BEGIN FEELING THE
EFFECTS OF INCREASING WESTERLY SHEAR AS ANOTHER UPPER-LEVEL TROUGH
DIGS SOUTHWARD AND AMPLIFIES TO THE NORTHWEST OF THE CYCLONE. IT
SHOULD BE NOTED THAT THE ECMWF MODEL DOES NOT MOVE LISA NORTH OF
20N LATITUDE...WHICH KEEPS THE CYCLONE IN A LOWER SHEAR ENVIRONMENT
BENEATH AN UPPER-LEVEL ANTICYCLONE FROM 72-120 HOURS. THE OFFICIAL
INTENSITY FORECAST IS A BLEND OF THE SHIPS AND LGEM INTENSITY
MODELS...AND LEANS A LITTLE TOWARD THE STRONGER ECMWF SOLUTION.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      21/1500Z 18.1N  31.7W    40 KT
 12HR VT     22/0000Z 18.3N  31.7W    45 KT
 24HR VT     22/1200Z 18.7N  31.8W    50 KT
 36HR VT     23/0000Z 19.0N  32.2W    55 KT
 48HR VT     23/1200Z 19.4N  33.0W    60 KT
 72HR VT     24/1200Z 20.1N  34.5W    55 KT
 96HR VT     25/1200Z 20.9N  35.8W    50 KT
120HR VT     26/1200Z 21.0N  37.5W    45 KT
 
$$
FORECASTER STEWART