000
WTNT42 KNHC 301433
TCDAT2
TROPICAL STORM LAURA DISCUSSION NUMBER   6
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL122008
1100 AM EDT TUE SEP 30 2008
 
WHILE SOMEWHAT OF A PARADOX...LAURA CAN NOW BE DESIGNATED AS A
TROPICAL CYCLONE DESPITE THE FACT THAT IT CONTINUES TO MOVE
NORTHWARD OVER PROGRESSIVELY COLDER WATERS.  THE ONE ASPECT WHICH
WAS HOLDING US BACK FROM DECLARING IT TROPICAL WAS THE UPPER LOW
COLLOCATED WITH THE SYSTEM.  WATER VAPOR IMAGERY INDICATES THAT
THIS FEATURE HAS BECOME OFFSET AND STRETCHED TO THE SOUTHWEST OF
LAURA...AND UPPER LEVEL OUTFLOW IS NOW EXPANDING TO THE NORTH OF
THE CYCLONE.  IN ADDITION...SCATTEROMETER DATA INDICATE THAT THE
RADIUS OF MAXIMUM WINDS HAS CONTINUED TO CONTRACT TO ABOUT 60 NM. 
CLOUD TOP TEMPERATURES ARE NOT ALL THAT COLD...BUT THAT IS QUITE
NORMAL FOR HIGH-LATITUDE TROPICAL CYCLONES DUE TO THE LOWER
TROPOPAUSE.  FRANKLY...LAURA NOW LOOKS LIKE A TROPICAL STORM.

THE INITIAL INTENSITY IS HELD AT 50 KT BASED ON AN 0827 UTC QUIKSCAT
PASS ALTHOUGH THE AREA OF STRONGEST WINDS HAS BEEN SHRINKING. 
LAURA SHOULD BE ABLE TO HOLD THAT INTENSITY FOR ONLY ANOTHER 12
HOURS OR SO BEFORE MOVING OVER SEA SURFACE TEMPERATURES LESS THAN
18C.  GLOBAL MODEL FIELDS INDICATE THAT THERMAL ADVECTION AROUND
THE SYSTEM BECOMES MORE PROMINENT LATER TONIGHT...AND LAURA IS
EXPECTED TO BE EXTRATROPICAL IN 24 HOURS OR SO AS IT MIGRATES INTO
THE COLDER AIR MASS.  SOME GRADUAL WEAKENING THROUGH 48 HOURS IS
EXPECTED DUE TO THE COLDER OCEAN WATERS AND MARGINAL BAROCLINIC
FORCING.  THEREAFTER...SOME RE-STRENGTHENING AND AN EXPANSION OF
THE WIND FIELD COULD OCCUR AS THE EXTRATROPICAL LOW RACES EAST
WITHIN THE MID-LATITUDE WESTERLIES TOWARDS THE NORTHERN BRITISH
ISLES.  EXCEPT FOR A SLIGHT WESTWARD SHIFT IN THE TRACK GUIDANCE AT
36-48 HOURS...THERE HAS BEEN NO SIGNIFICANT CHANGE IN THE GLOBAL
MODELS FOR THIS CYCLE AND THE OFFICIAL FORECAST LIES VERY CLOSE TO
THE MODEL CONSENSUS.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      30/1500Z 41.2N  48.8W    50 KT
 12HR VT     01/0000Z 42.8N  48.4W    50 KT
 24HR VT     01/1200Z 45.5N  47.4W    45 KT...EXTRATROPICAL
 36HR VT     02/0000Z 48.7N  45.6W    40 KT...EXTRATROPICAL
 48HR VT     02/1200Z 51.7N  43.6W    40 KT...EXTRATROPICAL
 72HR VT     03/1200Z 56.0N  35.0W    45 KT...EXTRATROPICAL
 96HR VT     04/1200Z 57.0N  19.0W    50 KT...EXTRATROPICAL
120HR VT     05/1200Z 58.0N   3.5W    50 KT...EXTRATROPICAL
 
$$
FORECASTER BERG/PASCH