WDPN33 PGTW 181500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 23W (NESAT) WARNING NR 016//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.5N 112.7E
   INITIAL INTENSITY: 75 KTS
   GEOGRAPHIC REFERENCE: 268 NM EAST-NORTHEAST OF DA NANG, VIETNAM
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 29 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A RAPIDLY 
DECAYING SYSTEM WITH DIMINISHED CORE CONVECTION DUE TO STRONG 
SOUTHERLY VERTICAL WIND SHEAR AND DRY AIR ENTRAINMENT. AN 180911Z 
SSMIS 91GHZ COLOR COMPOSITE MICROWAVE IMAGE REVEALS A FULLY-EXPOSED, 
WELL-DEFINED LOW-LEVEL CIRCULATION CENTER WITH DEEP CONVECTION 
CONFINED TO THE NORTHWEST QUADRANT, WHICH SUPPORTS THE INITIAL 
POSITION WITH MEDIUM CONFIDENCE. EIR INDICATES THAT THE EXTENSIVE 
STRATOCUMULUS FIELD HAS WRAPPED AROUND THE SOUTHERN SEMICIRCLE OF THE 
SYSTEM AND IS ENTRAINING COOLER, DRIER INTO THE CORE. AN 181034Z SMAP 
IMAGE, HOWEVER, SHOWS 74-77 KNOT WINDS (ONE-MINUTE AVERAGE) OVER THE 
NORTHWEST QUADRANT WHICH SUPPORTS THE INITIAL INTENSITY OF 75 KNOTS. 
THE DVORAK ESTIMATES FROM PGTW AND RJTD ALSO SUPPORT THIS ASSESSMENT.
BASED ON SURFACE OBSERVATIONS, EXTENSIVE GALE-FORCE WINDS REMAIN 
ENTRENCHED TO THE SOUTH AND SOUTHEAST OF HONG KONG AS INDICATED IN 
THE UNUSUALLY LARGE 34-KNOT WIND RADII OVER THE NORTHEAST QUADRANT OF 
THE SYSTEM. 

INITIAL WIND RADII BASIS: SMAP DATA AND SURFACE OBSERVATIONS

CURRENT STEERING MECHANISM: WEAK EXTENSION OF THE SUBTROPICAL
RIDGE (STR) TO THE NORTH WITH WESTERLIES OVER SOUTHERN AND
SOUTHEASTERN CHINA.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.5 - 77 KTS
   RJTD: T4.5 - 77 KTS
   KNES: T5.0 - 90 KTS
   CIMSS SATCON: 73 KTS AT 181130Z
   CIMSS ADT: 67 KTS AT 181140Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 25-30 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE POLEWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TY 23W IS RAPIDLY DECAYING AND IS EXPECTED TO
CONTINUE ITS SHARP DECLINE AS THE DRY, COOL AIR OVER THE NORTHERN
SEMICIRCLE ENTRAINS INTO THE SYSTEM. DUE TO THE WEAK STEERING
INFLUENCE, TY 23W WILL TRACK SLOWLY WESTWARD THROUGH TAU 24. AFTER
TAU 24, A BROAD UPPER-LEVEL TROUGH IS FORECAST TO DIG OVER
SOUTHERN CHINA WHICH SHOULD ALLOW THE SYSTEM TO TURN WEST-
NORTHWESTWARD AND SLOW WHILE DISSIPATING. AFTER TAU 48, THE TRACK 
MOTION IS MORE UNCERTAIN WITH QUASI-STATIONARY MOTION LIKELY EITHER 
OVER VIETNAM OR OFF THE COAST OF VIETNAM. TY 23W WILL DISSIPATE BY 
TAU 72 OR PERHAPS EARLIER AS ENVIRONMENTAL CONDITIONS QUICKLY 
DEGRADE.    

MODEL DISCUSSION: WITH THE EXCEPTION OF NAVGEM, NUMERICAL MODEL 
GUIDANCE IS IN CLOSE AGREEMENT WITH A 40NM SPREAD IN SOLUTIONS AT TAU 
36. AFTER TAU 36, NUMERICAL GUIDANCE DIVERGES WITH SOME MODELS 
INDICATING A SLOW WESTWARD OR QUASI-STATIONARY MOTION WHILE THE 
REMAINING MODELS DISSIPATE THE SYSTEM. RELIABLE INTENSITY GUIDANCE 
SUPPORTS THE RAPID WEAKENING SCENARIO INDICATED IN THE JTWC FORECAST 
WITH MEDIUM CONFIDENCE.    

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