WDPN31 PGTW 200900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 09W (WIPHA) WARNING NR 008//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 21.9N 113.2E
   INITIAL INTENSITY: 65 KTS
   GEOGRAPHIC REFERENCE: 61 NM WEST-SOUTHWEST OF HONG KONG
   MOVEMENT PAST 6 HOURS: WESTWARD AT 13 KTS
   SIGNIFICANT WAVE HEIGHT: 29 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A FULLY
OBSCURED LOW-LEVEL CIRCULATION CENTER OF TYPHOON (TY) 09W (WIPHA),
AS IT PASSES APPROXIMATELY 25 NM SOUTH OF MACAU, CHINA, WHILE ON A
WEST-NORTHWESTWARD TRACK TOWARD THE COAST. SPIRAL BANDS OF DEEP
CONVECTION DEVELOPED INTO A TIGHTLY WRAPPED SYSTEM OVER THE LAST 12
HOURS, RESULTING IN PEAK INTENSITY OF 65 KTS. THE INITIAL POSITION
IS PLACED WITH HIGH CONFIDENCE BASED ON A COMBINATION OF RADAR
AND VISIBLE SATELLITE IMAGERY, AS WELL AS WIND DIRECTION SHIFT
OBSERVED AT MACAU INTERNATIONAL AIRPORT. THE INITIAL INTENSITY OF
65 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON A PERFECT T4.0
AGREEMENT FROM ALL FIXING AGENCIES, AS WELL AS THE OBJECTIVE
ESTIMATES LISTED BELOW. ENVIRONMENTAL CONDITIONS REMAIN FAVORABLE,
AS INDICATED BY WARM (29-30 C) SEA SURFACE TEMPERATURES (SST),
SUFFICIENT MID- TO UPPER-TROPOSPHERIC MOISTURE CONTENT, AND
MODERATE EQUATORWARD OUTFLOW ALOFT. THESE FAVORABLE CONDITIONS ARE
BEING PARTIALLY OFFSET BY LOW TO MODERATE (15-20 KTS) NORTHEASTERLY
VERTICAL WIND SHEAR (VWS) IMPINGING ON THE CONVECTIVE STRUCTURE.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: EXTENSIONS OF TWO SUBTROPICAL RIDGES
(STR) TO THE NORTHEAST AND NORTHWEST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.0 - 65 KTS
   RJTD: T4.0 - 65 KTS
   RCTP: T4.0 - 65 KTS
   KNES: T4.0 - 65 KTS
   DEMS: T4.0 - 65 KTS
   CIMSS SATCON: 60 KTS AT 200510Z
   CIMSS ADT: 55 KTS AT 200530Z
   CIMSS AIDT: 65 KTS AT 200600Z
   CIMSS D-PRINT: 65 KTS AT 200600Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TY 09W IS EXPECTED TO CONTINUE ON A GENERAL
WESTWARD TRAJECTORY, HOWEVER THE STEERING PATTERN IS WEAKENING
THROUGH THE RETROGRADING EXTENSION OF A STR TO THE NORTHEAST.
MEANWHILE, ANOTHER STR POSITIONED TO THE NORTHEAST IS SLOWLY
BUILDING AND IS EXPECTED TO ASSUME THE PRIMARY STEERING. AS A
RESULT, TY 09W WILL TRANSIT OVER OR NEAR THE SOUTHERN COASTLINE OF
CHINA, HEADING TOWARD NORTHERN VIETNAM ON A WEST-SOUTHWESTWARD
TRACK. DURING THIS TIMEFRAME, TYPHOON WIPHA WILL REMAIN WITHIN A
SUPPORTIVE ENVIRONMENT, OFFSET BY THE EFFECTS OF LAND INTERACTION,
PARTICULARLY DURING PERIODS OF OVER LAND VORTEX LOCATION. BETWEEN
TAUS 12 AND 24 THE SYSTEM IS EXPECTED TO REEMERGE OVER THE THE GULF
OF TONKIN. TY 09W WILL THEN TRAVERSE VERY WARM (30-31 C) SEA
WATERS, WHICH MAY SUPPORT TEMPORARY REINTENSIFICATION OR
STABILIZATION OF WIND SPEEDS. CONVERSELY, NORTHEASTERLY VWS IS EXPECTED 
TO INCREASE TO MORE THAN 30 KTS AROUND TAU 36, PUTTING A CAP ON 
INTENSIFICATION. SUBSEQUENTLY, AS THE TYPHOON MAKES LANDFALL EAST OF 
HANOI, RAPID WEAKENING IS EXPECTED DUE TO STRONG FRICTIONAL AND 
OROGRAPHIC INTERACTION, WITH COMPLETE DISSIPATION ANTICIPATED BY TAU 
96.

MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE IS IN GOOD
CROSS-TRACK AGREEMENT, AS INDICATED BY A MAXIMUM OF 45 NM WIDTH OF
CONSENSUS GUIDANCE ENVELOPE THROUGH LANDFALL OVER NORTHERN VIETNAM,
EXPANDING TO 85 NM AT THE END OF THE FORECAST PERIOD. UNCERTAINTY
ASSOCIATED WITH THE TRACK INCREASES SIGNIFICANTLY WHEN IT COMES TO 
ALONG-TRACK SPREAD. FRICTIONAL, LAND-INDUCED SLOW DOWN IS ASSESSED 
DIFFERENTLY AMONG THE MODELS, WITH NAVGEM BEING THE SLOWEST AND 
DETERMINISTIC GFS OFFERING THE FASTEST SOLUTION. THE TWO MODELS CREATE 
A 115 NM TRACK GAP AT TAU 24. THE EXACT TIMELINE OF TY 09W REEMERGENCE 
OVER THE GULF OF TONKIN IS THEREFORE ASSESSED WITH MEDIUM CONFIDENCE, 
WHICH CARRIES OVER INTO THE TIMELINE OF THE WHOLE FORECAST TRACK. IN 
TERMS OF INTENSITY ALL MODELS INDICATE INITIAL WEAKENING OVER THE NEXT 
24 HOURS, AS THE SYSTEM SKIRTS THE COASTLINE. AFTERWARD, ALL GUIDANCE 
INDICATES POTENTIAL FOR INTENSIFICATION, HOWEVER THE DEGREE OF THEREOF 
DIFFERS DRASTICALLY. DETERMINISTIC GFS SUGGEST A MINOR (5 KTS) 
INCREASE, WHILE NAVGEM-DRIVEN COAMPS-TC AND HAFS INDICATE SPIKES 
RESULTING IN MAXIMUM INTENSITY FOR THE TYPHOON AT THE LEVELS OF 65-70 
KTS. AFTER TAU 48, AROUND LANDFALL, MAJORITY OF THE AVAILABLE GUIDANCE 
SUGGESTS RAPID WEAKENING. JTWC OVERALL INTENSITY FORECAST CONFIDENCE IS 
LOW DUE TO UNCERTAINTY IN SYSTEM FORWARD SPEED, AS WELL AS LAND 
INTERACTION TIMING.

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   TRACK 72-120 HR: MEDIUM
   INTENSITY 0 - 72 HR: LOW
   INTENSITY 72-120 HR: LOW////
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