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Tropical Storm LISA (Text)

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TROPICAL STORM LISA DISCUSSION NUMBER   6
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL142010
500 AM EDT WED SEP 22 2010

INFRARED SATELLITE IMAGES INDICATE THAT THERE HAS BEEN LITTLE
SIGNIFICANT CHANGE ASSOCIATED WITH THE CLOUD PATTERN OF LISA.  THE
CYCLONE CONSISTS OF A CENTRAL MASS OF PULSATING DEEP CONVECTION...
ALTHOUGH WITH FEWER BANDING FEATURES THAN YESTERDAY.  SEVERAL RECENT
MICROWAVE OVERPASSES SUGGEST THAT THE LOW- AND MID-LEVEL CENTERS
ARE NOT CO-LOCATED...A SIGN THAT THE CYCLONE IS BEING IMPACTED BY
VERTICAL WIND SHEAR.  DVORAK CI NUMBERS AT 0600 UTC ARE AT 2.5 AND
3.0 FROM SAB AND TAFB...RESPECTIVELY.  A BLEND OF THESE ESTIMATES
IS THE BASIS FOR KEEPING THE INITIAL INTENSITY AT 40 KT FOR THIS
ADVISORY.  

THE CENTER HAS BEEN DIFFICULT TO LOCATE BUT RECENT MICROWAVE AND
SATELLITE FIXES SUGGEST THAT IT BE A LITTLE FARTHER SOUTH THAN
PREVIOUSLY INDICATED.  A SHORT-TERM INITIAL MOTION FROM THESE FIXES
YIELDS AN ESTIMATE OF 060/4.  THE TRACK GUIDANCE CONTINUES TO BE
FAIRLY DIVERGENT BUT GENERALLY SHOWS LISA DRIFTING SOUTH OR EAST IN
A REGION OF WEAK STEERING DURING THE 48 HOURS.  AFTER THAT...A TURN
TOWARD WEST-NORTHWEST IS EXPECTED WITH A FASTER FORWARD SPEED AS A
WEAK LOW- TO MID-LEVEL RIDGE BUILDS NORTH OF THE STORM.  GIVEN THE
LARGE SPREAD IN THE GLOBAL MODELS...THE NEW NHC TRACK FORECAST
REMAINS OF LOW CONFIDENCE AND IS SLOWED FURTHER UNTIL A MORE
ESTABLISHED TREND IS EVIDENT IN THE GUIDANCE.

WATER VAPOR IMAGERY SHOWS A SHARP UPPER-LEVEL TROUGH NORTH OF LISA
THAT IS FORECAST TO LIFT OUT IN 24-48 HOURS.  THIS FEATURE HAS BEEN
IMPARTING A NORTHWESTERLY SHEAR OVER THE STORM...BUT SHIPS MODEL
OUTPUT INDICATES THAT THIS SHEAR SHOULD NOT BE PARTICULARLY STRONG
DURING THIS TIME.  AFTER ABOUT 60-72 HOURS...HOWEVER...GLOBAL
MODELS FORECAST A STRENGTHENING OF UPPER-LEVEL WESTERLY FLOW OVER
LISA...AND THE INTENSITY GUIDANCE SHOWS WEAKENING LATER IN THE
PERIOD.  OF COURSE...THE INTENSITY GUIDANCE IS HIGHLY DEPENDENT ON
WHERE LISA IS LOCATED AT THAT TIME...AND THE GREATER THAN NORMAL
UNCERTAINTY REGARDING ITS FORECAST TRACK MAKES THE CURRENT
INTENSITY FORECAST DIFFICULT.  THE NEW NHC INTENSITY FORECAST HAS
BEEN ADJUSTED A BIT DOWNWARD BEFORE 72 HOURS BUT REMAINS AT OR
ABOVE THE STATISTICAL-DYNAMICAL OUTPUT.

 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      22/0900Z 18.0N  30.5W    40 KT
 12HR VT     22/1800Z 18.0N  30.5W    45 KT
 24HR VT     23/0600Z 17.9N  30.5W    50 KT
 36HR VT     23/1800Z 17.8N  30.5W    50 KT
 48HR VT     24/0600Z 17.8N  30.7W    55 KT
 72HR VT     25/0600Z 18.5N  32.5W    50 KT
 96HR VT     26/0600Z 19.5N  35.0W    45 KT
120HR VT     27/0600Z 21.0N  38.0W    35 KT
 
$$
FORECASTER KIMBERLAIN
 
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