000 WTNT42 KNHC 300234 TCDAT2 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