WDPN31 PGTW 211500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 04W (PRAPIROON) WARNING 
NR 008//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.9N 110.9E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 188 NM NORTHEAST OF DA NANG, VIETNAM
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 15 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED SATELLITE IMAGERY DEPICTS TROPICAL
STORM (TS) 04W (PRAPIROON) TRACKING QUICKLY NORTH-NORTHWESTWARD OVER 
THE PAST 6 HOURS. THE CONVECTION HAS STARTED TO BUILD OVER THE CENTER 
AND THE SYSTEM HAS BECOME COMPACT IN SIZE AS IT TRACKS TOWARD HAINAN. 
ENVIRONMENTAL ANALYSIS INDICATES THAT 04W IS IN A MARGINALLY 
FAVORABLE ENVIRONMENT FOR FURTHER INTENSIFICATION CHARACTERIZED BY 
GOOD EQUATORWARD OUTFLOW ALOFT, LOW (10-15 KTS) VERTICAL WIND SHEAR, 
AND WARM (29-30 C) SEA SURFACE TEMPERATURES. AS THE SYSTEM NEARS 
HAINAN THE ENVIRONMENT WILL QUICKLY WORSEN DUE TO LAND INTERACTION. 
THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON THE 
ANIMATED EIR AND USING LOW-LEVEL CLOUD LINE TRACING. THE INITIAL 
INTENSITY OF 40 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE 
OBJECTIVE AND SUBJECTIVE DVORAK ESTIMATES LISTED BELOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A SUBTROPICAL
RIDGE (STR) TO THE NORTHEAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.0 - 30 KTS
   RCTP: T2.5 - 35 KTS
   KNES: T2.5 - 35 KTS
   CIMSS ADT: 45 KTS AT 211200Z
   CIMSS AIDT: 39 KTS AT 211200Z
   CIMSS DPRINT: 41 KTS AT 211200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG EQUATORWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: LOW

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TS O4W IS FORECAST TO CONTINUE ON A 
NORTHWESTWARD TRACK, ALONG THE WESTERN PERIPHERY OF THE STR 
THROUGHOUT THE REMAINDER OF THE FORECAST PERIOD. THE PEAK INTENSITY 
REMAINS AT 40 KTS DUE TO ITS PROXIMITY TO HAINAN. AFTER TAU 12, 04W 
WILL BEGIN A WEAKENING TREND AND DROP TO AROUND 25 KTS DUE TO VERY 
HIGH VERTICAL WIND SHEAR VALUES (30 KTS) IN THE GULF OF TONKIN. 
RIDGING TO THE NORTH IS ANTICIPATED TO START DRIVING THE SYSTEM 
WESTWARD AFTER MAKING LANDFALL NEAR TAU 48. AFTER MAKING LANDFALL 
THOUGH, 04W WILL QUICKLY DISSIPATE.

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN GOOD AGREEMENT
WITH A TIGHT CROSS-TRACK SPREAD OF AROUND 70 NM AT TAU 36. AFTER
TAU 36, GUIDANCE BECOMES VERY ERRATIC AND SPREADS SIGNIFICANTLY.
MODELS SEEM TO LOSE THE WEAKENED VORTEX OF 04W IN THE GULF OF TONKIN 
AROUND TAU 48 AS IT MAKES LANDFALL IN SOUTHERN CHINA. MOST TRACKERS 
DO SUGGEST A WESTWARD TURN OF HIGHLY VARYING DEGREES AFTER TAU 48 
THOUGH. UKMET, NAVGEM, AND GALWEM DEPICT ANOTHER SCENRIO, TRACKING 
THE VORTEX BACK TO THE GULF OF TONKIN AFTER TAU 48. AS A RESULT, THE 
JTWC TRACK FORECAST IS PLACED WITH MEDIUM CONFIDENCE. INTENSITY 
GUIDANCE AGREES THAT 04W WILL REACH A PEAK OF 35-40 KTS NEAR TAU 12 
BEFORE WEAKENING FOR THE REMAINDER OF THE FORECAST. THIS LENDS HIGH 
CONFIDENCE TO THE JTWC INTENSITY FORECAST. 

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   INTENSITY 0 - 72 HR: HIGH//
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