WDPN33 PGTW 010300
MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W WARNING NR 08//
RMKS/
1. FOR METEOROLOGISTS.
2. 12 HOUR SUMMARY AND ANALYSIS.
   A. TROPICAL STORM (TS) 21W HAS WEAKENED AS IT HAS ENCOUNTERED THE
PHILIPPINE ISLANDS. THE PAST MOTION IS UNCERTAIN SINCE THE STORM WAS 
EXTREMELY DIFFICULT TO LOCATE IN INFRARED AND MICROWAVE IMAGERY. 
NOW THAT MULTISPECTRAL IMAGERY IS AVAILABLE, TS 21W HAS BEEN RE-
LOCATED FURTHER EASTWARD. THE PAST 12 HOUR MOTION IS NOW ESTIMATED TO
BE APPROXIMATELY 08 KTS.
   B. THE CURRENT POSITION IS BASED ON THE PGTW MULTISPECTRAL FIX.
FIX AGREEMENT BETWEEN RJTD, PGTW, AND KNES HAS BEEN POOR, 
EXACERBATING THE UNCERTAINTY IN POSITION AND TRACK. THE INITIAL
INTENSITY IS SET AT 35 KTS BASED ON SURROUNDING QUIKSCAT DATA.
DVORAK ESTIMATES HAVE BEEN UNREPRESENTATIVE DUE TO LAND INTERACTION.
3. FORECAST REASONING
   A. THE PAST TWO TRACK FORECASTS PREDICT THE STORM TO SLOW
SOUTH OF HONG KONG INSTEAD OF MAKING LANDFALL WITHIN THE FORECAST
PERIOD.
   B. TS 21W WILL TRACK INTO THE SOUTH CHINA SEA UNDER THE INFLUENCE
OF THE SUBTROPICAL RIDGE EXTENDING FROM SOUTHERN CHINA ACROSS TAIWAN.
THE NUMERICAL GUIDANCE IS IN GENERAL AGREEMENT WITH THIS MOTION IN 
THE SHORT TERM. THE SOUTH CHINA SEA IS CURRENTLY DOMINATED BY MODERATE 
EASTERLY VERTICAL WIND SHEAR, BUT THE MODELS INDICATE A DEVELOPING 
ANTICYCLONE AROUND TAU 48, WHICH WILL ALLOW TS 21W TO INTENSIFY. THE 
SYSTEM IS CURRENTLY FORECAST TO REACH TYPHOON INTENSITY BY TAU 72.
   C. THE EXTENDED FORECAST IS SOMEWHAT UNCERTAIN. THE FORECAST
MODELS INDICATE A SHORTWAVE TROUGH DEEPENING OVER EASTERN CHINA AND
CREATING A BREAK IN THE SUBTROPICAL RIDGE NORTH OF HONG KONG. THE
MODELS ADDITIONALLY PREDICT RIDGING TO BUILD SOUTH OF TS 21W. 
IN RESPONSE TO THESE FEATURES, MOST OF THE FORECAST MODELS ARE 
PREDICTING THE SYSTEM TO TURN NORTHEAST BEFORE REACHING THE CHINA 
COAST. THE OFFICIAL FORECAST CALLS FOR THE STORM TO SLOW TO 03 KTS 
NEAR THE CHINA COAST. GIVEN THE POTENTIALLY WEAK STEERING ENVIRONMENT,
ERRATIC MOVEMENT IS POSSIBLE AFTER TAU 72.
FORECAST TEAM: BRAVO//
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