Subtropical Storm LAURA

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SUBTROPICAL STORM LAURA DISCUSSION NUMBER   1
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL122008
500 AM EDT MON SEP 29 2008
 
DURING THE PAST 6 HOURS...CONVENTIONAL SATELLITE IMAGERY INDICATES
DEEP CONVECTION WITH CLOUD TOPS TO NEAR -70C HAS CONTINUED TO
INCREASE AND HAS STARTED WRAPPING AROUND AND VERY NEAR THE CENTER
OF THE LARGE NON-TROPICAL LOW PRESSURE SYSTEM LOCATED ABOUT 775 NMI
WEST OF THE WESTERNMOST AZORES ISLANDS. THERE IS ENOUGH CONVECTION
NEAR THE CENTER NOW FOR THIS SYSTEM TO BE CLASSIFIED AS A
SUBTROPICAL STORM. THE 06Z SATELLITE CLASSIFICATION FROM TAFB WAS
ST3.0/45 KT...BUT USING THE DVORAK TECHNIQUE FOR A TROPICAL SYSTEM
YIELDS NEARLY EIGHT TENTHS BANDING...OR T3.5/55 KT. SO THE INITIAL
INTENSITY IS BEING SET AT 50 KT...WHICH IS CONSISTENT WITH A FEW
50-KT WIND VECTORS NOTED IN THE NORTHWEST QUADRANT IN A 28/2133Z
QUIKSCAT HI-RES OVERPASS. HOWEVER...SINCE THE TIME OF THAT
OVERPASS... CONVECTION HAS INCREASED MARKEDLY AND IS NOW THE SOLID
BAND OF CONVECTION IN THE SOUTHERN SEMICIRCLE. THEREFORE...IT IS
POSSIBLE THAT THE INITIAL INTENSITY COULD BE A LITTLE HIGHER THAN
CURRENTLY INDICATED.
 
THE INITIAL MOTION ESTIMATE IS 285/7. ALL OF THE NHC MODEL GUIDANCE
IS IN GOOD AGREEMENT ON SUBTROPICAL STORM LAURA MOVING TO THE
WEST-NORTHWEST FOR THE NEXT 12 HOURS OR SO...AND THEN TURNING
TOWARD THE NORTHWEST AND NORTH AND ACCELERATING AS THE INCIPIENT
MID-LEVEL CIRCULATION WEAKENS AND OPENS UP INTO A TROUGH AND GETS
LIFTED NORTHWARD AROUND THE WESTERN PERIPHERY OF A NORTH-SOUTH
ORIENTED MID-LEVEL RIDGE ANCHORED OVER THE AZORES ISLANDS. AFTER
REACHING THE HIGHER LATITUDES IN ABOUT 72 HOURS...LAURA IS FORECAST
TO GET PICKED BY THE FAST WESTERLIES AND MOVE QUICKLY EASTWARD AS
AN EXTRATROPICAL STORM SYSTEM.
 
LAURA IS EXPECTED TO REMAIN OVER 25C-26C SSTS FOR THE NEXT 24 HOURS
AND ALSO BE UNDER LIGHT TO MODERATE SHEAR CONDITIONS. THEREFORE...
SOME ADDITIONAL STRENGTHENING IS FORECAST. HOWEVER...IF THE CURRENT
TREND OF CONVECTION WRAPPING AROUND THE CENTER CONTINUES...THEN
SUBTROPICAL STORM LAURA COULD QUICKLY TRANSITION OVER TO A TROPICAL
CYCLONE AND BECOME A HURRICANE WITH A BANDING EYE FEATURE LATER
TODAY. BY 36 HOURS...LAURA IS EXPECTED TO BE MOVING OVER MUCH
COOLER WATER...WHICH SHOULD INDUCE WEAKENING ALONG WITH A GRADUAL
TRANSITION TO EXTRATROPICAL STATUS. THE OFFICIAL INTENSITY FORECAST
IS A BLEND OF THE SHIPS...GFDL...HWRF...AND ICON INTENSITY MODELS.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      29/0900Z 37.2N  47.3W    50 KT
 12HR VT     29/1800Z 38.0N  48.3W    55 KT
 24HR VT     30/0600Z 39.8N  48.9W    60 KT
 36HR VT     30/1800Z 41.7N  48.8W    60 KT...BECOMING EXTRATROPICAL
 48HR VT     01/0600Z 44.0N  48.0W    55 KT...EXTRATROPICAL
 72HR VT     02/0600Z 51.5N  44.0W    50 KT...EXTRATROPICAL
 96HR VT     03/0600Z 58.0N  34.5W    45 KT...EXTRATROPICAL
120HR VT     04/0600Z 57.0N  21.5W    40 KT...EXTRATROPICAL
 
$$
FORECASTER STEWART
 
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