WDPN31 PGTW 210900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 04W (PRAPIROON) WARNING 
NR 007//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.1N 111.0E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 169 NM EAST-NORTHEAST OF DA NANG, VIETNAM
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 15 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
STORM (TS) 04W (PRAPIROON) MAKING A SHARP NORTHWARD TURN OVER THE
PAST 6 HOURS. THE TRACK SPEED OF 04W IS MUCH FASTER THAN PREVIOUSLY
ANTICIPATED, CHARGING TOWARD HAINAN RATHER QUICKLY. THE CONVECTION
HAS NOT BEEN ABLE TO COMPLETELY BUILD OVER THE PARTIALLY EXPOSED
LOW-LEVEL CIRCULATION CENTER (LLCC) AND REMAINS SOMEWHAT
DISORGANIZED. RAGGED BANDING IS PRESENT ALONG THE WESTERN
SEMICIRCLE OF THE SYSTEM BUT IS FRAGMENTED. ENVIRONMENTAL ANALYSIS
INDICATES THAT 04W IS IN A MARGINALLY FAVORABLE ENVIRONMENT FOR
FURTHER INTENSIFICATION CHARACTERIZED BY GOOD EQUATORWARD OUTFLOW
ALOFT, LOW TO MODERATE (15-20 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.

INITIAL WIND RADII BASIS: PERSISTENCE 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T2.0 - 30 KTS
   RCTP: T2.0 - 30 KTS
   KNES: T2.5 - 35 KTS
   CIMSS SATCON: 40 KTS AT 210610Z
   CIMSS ADT: 37 KTS AT 210610Z
   CIMSS AIDT: 33 KTS AT 210610Z
   CIMSS DPRINT: 34 KTS AT 210600Z
   CIMSS DMINT: 33 KTS AT 210556Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG EQUATORWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE TRACK SPEED OF 04W IS FASTER THAN
PREVIOUSLY FORECAST. LANDFALL OVER HAINAN IS NOW EXPECTED AROUND
TAU 12.

FORECAST DISCUSSION: NOW THAT THE STEERING PATTERN HAS BECOME MORE
DEFINED, 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 HAS BEEN SLIGHTLY LOWERED
TO 40 KTS DUE TO LESS TIME FOR INTENSIFICATION BEFORE LANDFALL AT
TAU 12. 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. ONCE 04W MAKES LANDFALL ALONG THE SOUTHERN
COAST OF CHINA NEAR TAU 48, IT WILL QUICKLY DISSIPATE.  

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN GOOD AGREEMENT,
DEPICTING A VERY SIMILAR SCENARIO TO THE JTWC FORECAST TRACK. THE
CROSS-TRACK SPREAD IS FAIRLY TIGHT, AT AROUND 90 NM AT TAU 48.
HOWEVER, SOME MODELS SEEM TO LOSE THE VORTEX OF 04W IN THE GULF OF
TONKIN AROUND TAU 48 AND BECOME ERRATIC AFTERWARD, BEFORE MAKING
LANDFALL IN CHINA. 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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