Tropical Storm NICHOLAS

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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
 
 
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