WDPN31 PGTW 181500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 09W (WIPHA)        
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.3N 122.5E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 499 NM EAST-SOUTHEAST OF HONG KONG
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 11 KTS
   SIGNIFICANT WAVE HEIGHT: 15 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL STORM (TS) 09W HAS TRANSITIONED FROM A MONSOON DEPRESSION
TO A WARM-CORE TROPICAL SYSTEM AS INDICATED IN ANIMATED ENHANCED
INFRARED SATELLITE IMAGERY, WITH IMPROVED, TIGHTER DEEP CONVECTIVE
BANDING WRAPPING INTO THE LOW-LEVEL CIRCULATION CENTER (LLCC) AND
CONSOLIDATING CORE STRUCTURE. AN 181255Z ASCAT-B PARTIAL IMAGE 
INDICATES 30-35 KNOT SOUTHERLY WINDS ALONG THE EASTERN SEMICIRCLE, 
WITH A RADIUS OF MAXIMUM WINDS OF APPROXIMATELY 65 NM, MORE TYPICAL OF 
TROPICAL CYCLONES. A 181000Z RCM-1 SAR PASS REVEALS A BROAD CENTER, 
WITH 35-45 KNOT WINDS ALONG THE EASTERN PERIPHERY. HOWEVER, INCIDENCE 
ANGLES ARE RATHER HIGH (45-55 DEGREES) AND THE DATA APPEARS TO BE 
EXCESSIVELY HIGH. ENVIRONMENTAL CONDITIONS ARE FAVORABLE, WITH LOW 
VERTICAL WIND SHEAR, WARM SST VALUES AND RADIAL OUTFLOW ALOFT. THE 
INITIAL INTENSITY OF 35 KNOTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED 
ON THE CONCURRENT AGENCY DVORAK CURRENT INTENSITY ESTIMATES OF 2.5 (35 
KNOTS) FROM ALL AGENCIES.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: DEEP-LAYERED SUBTROPICAL RIDGE
POSITIONED TO THE NORTH. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.5 - 35 KTS
   RCTP: T2.5 - 35 KTS
   KNES: T2.5 - 35 KTS
   CIMSS D-PRINT: 24 KTS AT 181200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE RADIAL

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: TS 09W IS FORECAST TO TRACK WEST-NORTHWESTWARD
ALONG THE SOUTHWEST PERIPHERY OF THE SUBTROPICAL RIDGE (STR) TOWARD
A BREAK IN THE STR ASSOCIATED WITH A MIDLATITUDE SHORTWAVE TROUGH
EXPECTED TO DIG OVER CENTRAL CHINA FROM TAU 24 TO TAU 36.
ENVIRONMENTAL CONDITIONS ARE FAVORABLE FOR FURTHER DEVELOPMENT,
WITH RADIAL OUTFLOW, LOW VERTICAL WIND SHEAR AND WARM SST VALUES.
HOWEVER, TS 09W HAS A LIMITED PERIOD OF TIME OF ABOUT 36 HOURS TO
DEVELOP BEFORE APPROACHING SOUTHERN CHINA AND THE HONG KONG REGION,
WITH INCREASED FRICTIONAL EFFECTS LIMITING THE PEAK INTENSITY.
THERE IS SOME UNCERTAINTY IN THE PEAK INTENSITY, WITH 60-65 KNOTS
POSSIBLE IF THE SYSTEM IS ABLE TO CONSOLIDATE QUICKLY AND DEVELOP A
MORE CONTRACTED LOW-LEVEL CIRCULATION CENTER. AFTER TAU 36, THE
SHORTWAVE TROUGH WILL PROPAGATE NORTHEASTWARD ALLOWING THE STR TO
RE-BUILD TO THE NORTH AND NORTHWEST. THIS WILL DRIVE 09W (WIPHA) ON
A MORE WESTWARD TO WEST-SOUTHWESTWARD TRACK TOWARD AND OVER
NORTHERN VIETNAM. AS THE SYSTEM SKIRTS THE SOUTHERN COAST OF CHINA,
INTERACTION WITH LAND WILL GRADUALLY WEAKEN THE LARGE SYSTEM FROM
TAU 36 TO TAU 120, WITH DISSIPATION LIKELY TOWARD THE END OF THE
FORECAST PERIOD.        

MODEL DISCUSSION: WITH THE EXCEPTION OF AFUM, THE DETERMINISTIC
MODEL GUIDANCE IS IN GOOD AGREEMENT LENDING MEDIUM CONFIDENCE IN
THE JTWC TRACK FORECAST. HOWEVER, GFS AS WELL AS THE 180600Z GEFS
AND ECENS ENSEMBLE RUNS DEPICT A LOW PROBABILITY SCENARIO OVER
THE GULF OF TONKIN. THEY SUGGEST THE REMNANT SYSTEM MAY STALL OR
TURN EASTWARD AS IT INTERACTS WITH A DEVELOPING TROPICAL CYCLONE
FORECAST TO DEVELOP EAST OF LUZON IN THE 3-4 DAY PERIOD. THIS
SYSTEM TO THE EAST IS OVER 1000NM EAST, WITH EXTENSIVE WESTERLY
FLOW OVER THE SOUTH CHINA SEA FEEDING INTO THE SOUTHWESTERN FLANK.
THE LATEST ENSEMBLE RUNS SHOW THE VAST MAJORITY OF SOLUTIONS
TRACKING THE SYSTEM INTO VIETNAM AND SUPPORT A DISSIPATION
SCENARIO. RELIABLE INTENSITY GUIDANCE INDICATES A WIDE RANGE OF
PEAK INTENSITY VALUES RANGING FROM 40 TO 70 KNOTS (COAMPS-TC GFS)
FROM TAU 24 TO TAU 36. HAFS-A REVEALS A MODEST INTENSIFICATION TO A
PEAK OF 45 KNOTS AT TAU 24. ENSEMBLE GUIDANCE INDICATES PEAK VALUES
IN THE 40-60 KNOT RANGE, WITH SOME ISOLATED SOLUTIONS IN THE 60-70
KNOT RANGE.          

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