000 WTNT44 KNHC 150834 TCDAT4 TROPICAL STORM NICHOLAS DISCUSSION NUMBER 7 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 5 AM EDT WED OCT 15 2003 NICHOLAS HAS BECOME MUCH BETTER ORGANIZED DURING THE PAST 6 HOURS AND CLOUD TOPS HAVE CONTINUED TO COOL TO AS LOW -85C. THE LOW-LEVEL CENTER HAS BEEN DIFFICULT TO PLACE AND THIS HAS HAD AN IMPACT ON BOTH THE CURRENT INTENSITY AND THE FORECAST TRACK. A 15/0218Z TRMM OVERPASS CLEARLY INDICATED THAT THE LOW-LEVEL CENTER WAS A LITTLE SOUTH OF THE PREVIOUS POSITION AND FORECAST TRACK. USING THE TRMM POSITION AND EXTRAPOLATION OF THE PREVIOUS MOTION WOULD PLACE THE CENTER NEAR THE SOUTHWESTERN EDGE OF THE COLD CLOUD SHIELD. BOTH TAFB AND SAB HAVE A DVORAK INTENSITY ESTIMATE OF T3.5/55 KT BASED ON A SHEAR PATTERN WITH THE CENTER FARTHER EAST INTO THE DEEP CONVECTION. IF THE LOW-LEVEL CENTER IS NEAR THE ADVISORY POSITION ...THEN THAT WOULD YIELD A LOWER DATA T-NUMBER OF T3.0/45 KT... WHICH IS WHAT WAS USED FOR THIS ADVISORY. THE INITIAL MOTION ESTIMATE REMAINS 295/9. THERE ARE SOME RECENT SATELLITE INDICATIONS THAT THE LOW-LEVEL CENTER MAY BE NEARLY HALF A DEGREE LATITUDE FARTHER SOUTH. HOWEVER...I DO NOT WANT TO MAKE ANY RADICAL CHANGES TO THE POSITION OR FORECAST TRACK AT NIGHT DUE TO THE UNCERTAINLY IN THE EXACT LOCATION OF THE LOW-LEVEL CENTER. HOWEVER...THE AFOREMENTIONED TRMM LOCATION AND EXTRAPOLATED INITIAL POSITION REQUIRES THAT THE FORECAST TRACK BE ADJUSTED TO THE LEFT OF THE PREVIOUS TRACK. THE GLOBAL MODELS REMAIN QUITE DIVERGENT FROM THE OUTSET. IT SEEMS THAT THE GLOBAL MODELS ARE NOT HANDLING VERY WELL THE UPPER-LEVEL CONVERGENCE ZONE THAT LIES TO THE NORTH AND NORTHWEST OF THE CYCLONE AS SEEN IN WATER VAPOR IMAGERY. THIS ZONE OF SUBSIDENCE HAS BEEN SLIFTING SLOWLY SOUTHEASTWARD AND HAS LIKELY BEEN KEEPING THE SUBTROPICAL RIDGE BENEATH IT STRONGER THAN THE MODELS HAVE BEEN FORECASTING. BASED ON THAT ASSUMPTION...THE NOGAPS MODEL WAS IGNORED SINCE IT TAKES NICHOLAS ALMOST DUE NORTH FROM THE OUTSET. THE GFS AND GFDL MODELS SIMILARLY TAKE THE CYCLONE NORTH-NORTHWESTWARD FROM ITS INITIAL POSITION...WHICH ALSO SEEMS UNLIKELY GIVEN THE PAST 24 HOURS OF WEST-NORTHWESTWARD MOTION AND THE SHARP RIGHT-OF-TRACK BIAS THAT THOSE MODELS HAVE HAD. THE UKMET SEEMS TO HAVE THE BETTER HANDLE ON THE OVERALL PATTERN...ALTHOUGH IT INITIALIZED NICHOLAS AT LEAST 60 NMI TOO FAR TO THE SOUTHEAST AT 00Z. THE BAM MODELS...ON THE OTHER HAND...HAVE BEEN REMARKABLY CONSISTENT THE PAST 36 HOURS AND HAVE THE BETTER ERROR STATISTICS SO FAR. SURPRISINGLY...ALL OF THE GLOBAL AND REGIONAL MODELS AGREE THAT...ONCE THE NEXT UPSTREAM SHORTWAVE TROUGH MOVES NORTHEAST OF NICHOLAS IN 36-48 HOURS...A MUCH STRONGER EAST-WEST ORIENTED RIDGE WILL BUILD TO THE NORTH OF THE CYCLONE AND TURN IT BACK WESTWARD. THE BIG QUESTION IS HOW FAR NORTH WILL NICHOLAS MOVE BEFORE THE WESTWARD TURN BEGINS. THE OFFICIAL FORECAST TRACK IS A LITTLE TO THE LEFT OF THE PREVIOUS TRACK AND IS A BLEND BETWEEN THE GLOBAL MODEL CONSENSUS AND THE BAM MODELS CONSENSUS...WITH A LITTLE MORE WEIGHT PLACED ON THE UKMET MODEL SOLUTION. NICHOLAS IS UNDERNEATH AN UPPER-LEVEL DIFLUENT AXIS...WHICH HAS ALLOWED THE OUTFLOW AND OVERALL CONVECTIVE PATTERN TO IMPROVE SIGNIFICANTLY DURING THE PAST 12 HOURS. WITH NICHOLAS FORECAST TO REMAIN AT A LOWER LATITUDE AND UNDER FAVORABLE SHEAR CONDITIONS FOR THE NEXT 24-36 HOURS...MORE STRENGTHENING IS EXPECTED THAN HAS BEEN INDICATED IN PREVIOUS FORECASTS. HOWEVER...IF THE LOW-LEVEL CENTER IS ACTUALLY FARTHER SOUTHEAST INTO THE DEEPER CONVECTION...THEN NICHOLAS COULD UNDERGO A PERIOD OF RAPID INTENSIFICATION AND EVEN BECOME A HURRICANE WITHIN THE NEXT 24 HOURS. THE OFFICIAL INTENSITY FORECAST IS A BLEND BETWEEN THE WEAKER SHIPS INTENSITY MODEL AND THE STRONGER GFDL MODEL. FORECASTER STEWART FORECAST POSITIONS AND MAX WINDS INITIAL 15/0900Z 11.9N 43.7W 45 KT 12HR VT 15/1800Z 12.5N 44.7W 55 KT 24HR VT 16/0600Z 13.3N 45.7W 60 KT 36HR VT 16/1800Z 14.5N 46.7W 65 KT 48HR VT 17/0600Z 15.7N 47.2W 70 KT 72HR VT 18/0600Z 16.9N 48.2W 75 KT 96HR VT 19/0600Z 17.5N 49.5W 75 KT 120HR VT 20/0600Z 18.0N 51.0W 75 KT