WDPN31 PGTW 060300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 13W (THIRTEEN) WARNING 
NR 06//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM 13W (THIRTEEN), LOCATED APPROXIMATELY 460 NM NORTH-
NORTHWEST OF WAKE ISLAND, HAS TRACKED NORTH-NORTHWESTWARD AT 05 
KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE 
IMAGERY (MSI) SHOWS THE DEEPEST CONVECTION REMAINS ALONG THE 
NORTHERN HALF OF THE SYSTEM WITH FLARING CENTRAL CONVECTION OVER THE 
PARTIALLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC). THE TRACK HAS 
BEEN STEADILY NORTHWARD UNDER THE INFLUENCE OF A NEAR EQUATORIAL 
RIDGE TO THE SOUTHEAST OF THE LLCC. THE TRACK SPEED HAS DECREASED AS 
THE SYSTEM IS MOVING TOWARD AN AREA OF WEAK STEERING INFLUENCE. MSI 
INDICATES THE POLEWARD OUTFLOW CHANNEL CONTINUES TO IMPROVE IN 
ASSOCIATION WITH THE HIGH AMPLITUDE TROUGH CURRENTLY LOCATED TO THE 
NORTH OF THE SYSTEM. UPPER LEVEL ANALYSIS INDICATES THERE REMAINS A 
WEAKENING OUTFLOW CHANNEL TO THE SOUTHEAST INTO AN EASTWARD TRACKING 
TROPICAL UPPER TROPOSPHERIC TROUGH. ADDITIONALLY, THE LLCC IS 
LOCATED WITHIN A REGION OF MODERATE VERTICAL WIND SHEAR (VWS) (10 TO 
20 KNOTS).   
3. FORECAST REASONING.
   A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY FROM THE PREVIOUS
PROGNOSTIC REASONING BULLETIN. 
   B. TS 13W IS EXPECTED TO CONTINUE TRACKING NORTHWARD OVER THE 
NEXT 24 HOURS, WITH A BUILDING SUBTROPICAL RIDGE TO THE NORTHEAST 
SLOWLY SHIFTING THE TRACK TO THE NORTHWEST THROUGH TAU 72. IMPROVING 
OUTFLOW OVER THE NEXT 36 HOURS WILL ALLOW FOR MODEST 
INTENSIFICATION. CURRENTLY FAVORABLE SEA SURFACE TEMPERATURES (SST) 
WILL SUPPORT THIS INCREASE. HOWEVER, AROUND TAU 36, THE LLCC WILL 
MOVE INTO A REGION OF UNFAVORABLE SST'S. INCREASING VWS WILL BEGIN 
TO HINDER DEVELOPMENT BEYOND TAU 36 WITH A WEAKENING TREND EXPECTED 
BEYOND TAU 48.
   C. BEYOND TAU 72, DECREASING SST'S AND PERSISTENTLY HIGH VWS WILL 
ERODE THE SYSTEM DOWN TO 30 KNOTS BY THE END OF THE FORECAST PERIOD. 
THERE IS HIGH CONFIDENCE IN THE TRACK FORECAST THROUGH THE ENTIRE 
PERIOD. MODEL GUIDANCE OVER THE PAST SIX HOURS HAS SHOWN MARKED 
IMPROVEMENT, WITH TIGHT GROUPING THROUGH TAU 120. THERE ARE SOME 
SPEED DIFFERENCES IN THE LATER TAUS, BUT FOR THE MOST PART, THERE IS 
GOOD AGREEMENT WITHIN THE MODEL GUIDANCE. THIS FORECAST IS CLOSER IN 
LINE WITH NGPS BASED ON THE FORECASTED TRACK SPEEDS REMAINING SLOWER 
IN THE EXTENDED TAUS AROUND THE STR.//
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