WDPN31 PGTW 280900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 20W (TWENTY) WARNING 
NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 14.9N 115.9E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 444 NM EAST OF DA NANG, VIETNAM
   MOVEMENT PAST 6 HOURS: WESTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 10 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL DEPRESSION (TD) 20W HAS COMPLETED ITS QUICK TRANSITION FROM A 
MONSOON DEPRESSION, WITH DEFINITE TROPICAL CYCLONE CHARACTERISTICS. 
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A BROAD, EXPOSED 
LOW-LEVEL CIRCULATION CENTER (LLCC), WITH TWO SMALL MESOVORTICES 
ROTATING CYCLONICALLY AROUND THE EASTERN AND NORTHERN QUADRANTS. THE 
INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON THE CENTROID 
APPROACH USING MSI AND RECENT ASCAT DATA. IMPROVED DEEP CONVECTIVE 
BANDING IS EVIDENT OVER THE WESTERN SEMICIRCLE WRAPPING INTO THE 
SOUTHWEST QUADRANT OF THE LLCC, WITH FLARING, ISOLATED CONVECTION OVER 
THE NORTHERN QUADRANT. THE 280059Z ASCAT-B IMAGE SHOWS AN ELONGATED 
CIRCULATION, HOWEVER, THE RADIUS OF MAXIMUM WIND (RMW) HAS CONTRACTED 
SIGNIFICANTLY OVER THE PAST 12 HOURS, WITH A RMW OF 70-75NM AND A SWATH 
OF 25 TO 30 KNOT WINDS OVER THE SOUTHERN SEMICIRCLE. MAXIMUM WINDS WERE
VERIFIED BY RECENT SHIP OBSERVATIONS AND SURFACE OBSERVATIONS FROM 
NANSHA DAO (59997) NEAR 30 KNOTS. THE INITIAL INTENSITY OF 30 KTS IS 
ASSESSED WITH HIGH CONFIDENCE BASED ON ASCAT DATA.

INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).

CURRENT STEERING MECHANISM: DEEP-LAYERED SUBTROPICAL RIDGE (STR)
ENTRENCHED TO THE NORTH.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS
   KNES: T1.5 - 25 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 20-25 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE WESTWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: HIGH
   INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TROPICAL DEPRESSION (TD) 20W WILL TRACK WEST-
NORTHWESTWARD THROUGH TAU 36 ALONG THE SOUTHWESTERN PERIPHERY OF THE 
STR, WITH SLOW INTENSIFICATION DUE TO THE MARGINALLY FAVORABLE 
CONDITIONS, BROAD NATURE OF THE LLCC AND LACK OF A VERTICALLY ALIGNED 
VORTEX. PEAK INTENSITY IS EXPECTED TO REMAIN IN THE 40-45 KNOT RANGE AT 
BEST. AFTER TAU 36, THE SYSTEM WILL ENCOUNTER INCREASING NORTHEASTERLIES 
AND INCREASING VERTICAL WIND SHEAR, WHICH WILL LEAD TO A STEADY 
WEAKENING TREND. AFTER LANDFALL NEAR TAU 54, THE SYSTEM WILL WEAKEN 
RAPIDLY AND DISSIPATE NO LATER THAN TAU 72. 

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE IS IN TIGHT AGREEMENT, 
WITH A 40NM CROSS-TRACK SPREAD AT TAU 48. THE 280000Z GEFS AND ECENS 
ENSEMBLES ARE ALSO IN GOOD AGREEMENT LENDING HIGH CONFIDENCE TO THE JTWC 
TRACK FORECAST. RELIABLE INTENSITY GUIDANCE TO INCLUDE HAFS-A, COAMPS-TC 
(GFS) AND HWRF ARE IN FAIR AGREEMENT, WITH A PEAK INTENSITY RANGING FROM 
32 TO 43 KNOTS AT TAU 24. 

FORECAST CONFIDENCE:
   TRACK 00-72 HR: HIGH
   INTENSITY 00-72 HR: MEDIUM//
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