Subtropical Storm LAURA

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SUBTROPICAL STORM LAURA DISCUSSION NUMBER   4
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL122008
1100 PM EDT MON SEP 29 2008
 
OVERALL THE CONVECTIVE CLOUD TOPS HAVE WARMED A LITTLE OVER THE PAST
SEVERAL HOURS...AND THE SATELLITE CLASSIFICATIONS FROM BOTH TAFB
AND SAB ARE BELOW THEIR EARLIER PEAKS.  INDEED...THE DEEP
CONVECTION IS QUITE PALTRY AT THIS TIME.  HOWEVER...A QUIKSCAT PASS
FROM 21Z CAUGHT THE EASTERN HALF OF THE CIRCULATION...AND THESE
DATA STILL SUPPORT 45-50 KT...SO THE ADVISORY INTENSITY IS
UNCHANGED. EVEN THOUGH THE RADIUS OF MAXIMUM WINDS IS AROUND 70 N
MI...SMALLER THAN FOR MANY SUBTROPICAL CYCLONES... LAURA IS STILL
VERY MUCH INVOLVED WITH AN UPPER-LOW...WHICH IS A DEFINING
CHARACTERISTIC OF A SUBTROPICAL STORM. I'M NOT INCLINED TO CALL
THIS THING FULLY TROPICAL UNTIL IT DOES A BETTER JOB OF ERODING THE
UPPER LOW. OVER THE NEXT DAY OR SO THE UPPER-LOW IS EXPECTED TO
WEAKEN AS LAURA MOVES INTO THE WESTERLIES...AND LAURA STILL COULD
BECOME A TROPICAL CYCLONE BEFORE SEA SURFACE TEMPERATURES DROP
DRAMATICALLY ALONG THE FORECAST TRACK IN ABOUT 18 HOURS.  WEAKENING
IS EXPECTED ONCE THAT HAPPENS...ALTHOUGH BOTH THE GFS AND THE ECMWF
SUGGEST THAT LAURA COULD INTENSIFY AND EXPAND AS AN EXTRATROPICAL
LOW.  BASED ON THIS LATTER GUIDANCE...THE FORECAST WIND RADII HAVE
BEEN EXPANDED AT 48 AND 72 HOURS. 

LAURA IS MOVING SLOWLY NORTHWARD...360/8...BETWEEN A LARGE AREA OF
HIGH PRESSURE OVER THE AZORES AND A MID- TO UPPER-LEVEL TROUGH OVER
EASTERN CANADA THAT CONTAINS THE REMNANTS OF KYLE.  MODEL GUIDANCE
IS IN GOOD AGREEMENT THAT LAURA WILL CONTINUE NORTHWARD BETWEEN
THESE FEATURES AND ENTER THE WESTERLIES IN ABOUT THREE DAYS.  THE
OFFICIAL FORECAST IS NEAR THE DYNAMICAL MODEL CONSENSUS AND A
LITTLE LEFT OF AND SLOWER THAN THE PREVIOUS ADVISORY.
  
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      30/0300Z 39.0N  48.4W    50 KT
 12HR VT     30/1200Z 40.3N  48.4W    55 KT
 24HR VT     01/0000Z 42.6N  47.9W    50 KT
 36HR VT     01/1200Z 45.3N  46.8W    45 KT...EXTRATROPICAL
 48HR VT     02/0000Z 48.5N  45.5W    40 KT...EXTRATROPICAL
 72HR VT     03/0000Z 53.5N  39.0W    45 KT...EXTRATROPICAL
 96HR VT     04/0000Z 55.0N  27.0W    50 KT...EXTRATROPICAL
120HR VT     05/0000Z 55.0N  13.0W    55 KT...EXTRATROPICAL
 
$$
FORECASTER FRANKLIN
 
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