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TROPICAL STORM ARTHUR DISCUSSION NUMBER   5
NATIONAL WEATHER SERVICE MIAMI FL
5 PM EDT MON JUL 15 2002
 
TROPICAL STORM ARTHUR HAS CONTINUED TO GET BETTER ORGANIZED DURING 
THE DAY. CONVECTIVE CLOUD TOPS HAVE COOLED AND THE CENTER HAS NOW 
MOVED UNDER THE WESTERN EDGE OF THE CONVECTIVE CLOUD SHIELD...
DESPITE THE WESTERLY UPPER-LEVEL SHEAR. THE INITIAL INTENSITY OF 50 
KT IS BASED ON A DVORAK SATELLITE INTENSITY ESTIMATE OF 45 KT FROM 
BOTH TAFB AND SAB...AND THE IMPROVED CONVECTIVE CLOUD APPEARANCE 
DURING THE PAST 2 HOURS.
 
THE INITIAL MOTION IS 065/23.  THERE REMAINS NO SIGNIFICANT CHANGE
TO THE PREVIOUS FORECAST TRACK OR REASONING THROUGH 24 HOURS.  AFTER
THAT...THE FORECAST TRACK WAS SHIFTED SLIGHTLY WESTWARD TO ACCOUNT
FOR THE DEEP UPPER-LEVEL LOW THAT IS FORECAST TO DIG SOUTHWARD TO
THE WEST OF ARTHUR BY 36 HOURS.  ALL OF THE GLOBAL MODELS AGREE ON
THE GENERAL TREND OF THE DIGGING LOW CREATING BACKING...INCREASINGLY
DIFFLUENT...AND WEAKENING SOUTHERLY FLOW ACROSS ARTHUR IN 36 TO 48
HOURS. THE GOOD NEWS IS THAT THIS WILL OCCUR OVER SUB-20C SST
WATER. THE BAD NEWS IS THAT THE ATMOSPHERE ABOVE THE COOL BOUNDARY
LAYER WILL REMAIN WARM AND UNSTABLE.  THIS SHOULD KEEP THE LOW- AND
UPPER-LEVEL CIRCULATIONS MORE VERTICALLY ALIGNED AND TRACK NORTHWARD
AT A MUCH SLOWER SPEED AS THE DEEP-LAYER FLOW NORTHEAST OF THE
UPPER-LEVEL LOW WEAKENS.  BY 48 HOURS...ARTHUR IS EXPECTED TO BE AN
EXTRATROPICAL LOW PRESSURE SYSTEM HEADED FOR NEWFOUNDLAND.  THE
OFFICIAL FORECAST TRACK IS VERY CLOSE TO THE CONSENSUS OF THE
AVN...UKMET...GFDL...GFDN...AND NOGAPS MODELS.

SOME ADDITIONAL INTENSIFICATION SEEMS REASONABLE GIVEN THE CURRENT
IMPRESSIVE CONVECTIVE ORGANIZATION AND THE FACT THAT SST ANALYSES
INDICATE THAT ARTHUR WILL BE PASSING OVER A WARM POOL...AT LEAST
26C...FOR THE NEXT 6 TO 12 HOURS.  DURING THE NEXT 24 HOURS...
ARTHUR IS ALSO EXPECTED TO REMAIN SOUTH OF THE SHARP SST GRADIENT
THAT LIES BETWEEN 41N AND 43N LATITUDE...SO CONTINUED SLIGHT
STRENTHENING SEEMS IN ORDER.  AFTER THAT...MUCH COOLER SSTS TO THE
NORTH WOULD NORMALLY RESULT IN RAPID WEAKENING.  HOWEVER...
INCREASING BAROCLINIC EFFECTS COMBINED WITH THE WARM UNSTABLE AIR
AND CONVECTION COULD INSTEAD RESULT IN A STRONG HYBRID LOW PRESSURE
SYSTEM EXISTING AT THAT TIME.  COORDINATION WITH THE MARINE
PREDICTION CENTER AND THE CANADIAN HURRICANE CENTER CONCURS WITH
THIS POSSIBLE SCENARIO.  AS SUCH...THE PEAK FORECAST INTENSITIES
WERE INCREASED TO 60 KT...BUT IT WOULD NOT SURPRISE ME AT ALL IF A
BRIEF EYE-LIKE FEATURE APPEARED IN 36 TO 48 HOURS.

NOTE: INITIAL 34-KT AND 12 FT SEAS RADII WERE INCREASED BASED ON 
SURROUNDING 18Z SHIP REPORTS. 34-KT WIND RADII WERE ALSO INCREASED 
SIGNIFICANTLY AT AND BEYOND 24 HOURS BASED ON INPUT FROM MPC.
 
FORECASTER STEWART
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     15/2100Z 37.3N  67.0W    50 KTS
12HR VT     16/0600Z 38.8N  63.1W    55 KTS
24HR VT     16/1800Z 41.3N  59.1W    55 KTS...BECOMING EXTRATROPICAL
36HR VT     17/0600Z 43.5N  58.9W    60 KTS...EXTRATROPICAL
48HR VT     17/1800Z 45.3N  58.5W    60 KTS...EXTRATROPICAL
72HR VT     18/1800Z 49.5N  56.0W    55 KTS...EXTRATROPICAL/INLAND
 
 
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