WDPN33 PGTW 180900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 23W (NESAT) WARNING NR 015//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 18.1N 113.1E
   INITIAL INTENSITY: 80 KTS
   GEOGRAPHIC REFERENCE: 259 NM SOUTH-SOUTHWEST OF HONG KONG
   MOVEMENT PAST 6 HOURS: WESTWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 35 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS AN
INCREASINGLY EXPOSED LOW-LEVEL CIRCULATION WITH RAPIDLY DIMINISHING
CORE CONVECTION. AN 180612Z ATMS 88.2GHZ MICROWAVE IMAGE AND
ANIMATED RADAR IMAGERY INDICATE LIMITED DEEP CONVECTIVE BANDING
CONFINED TO THE WESTERN AND NORTHWESTERN QUADRANTS. MSI ALSO
REVEALS A LARGE REGION OF COOL-AIR STRATOCUMULUS ADVECTING FROM
SOUTH OF HONG KONG INTO THE NORTHERN PERIPHERY OF THE SYSTEM. THE 
INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON THE MSI. 
THE INITIAL INTENSITY IS ASSESSED AT 80 KNOTS HEDGED BELOW THE DVORAK 
ESTIMATES BASED ON THE WEAKENED STRUCTURE AND LOWER DATA-T ESTIMATES 
OF T4.5 (77 KNOTS). ADDITIONALLY, EXTENSIVE GALE-FORCE WINDS 
ASSOCIATED WITH THE NORTHEAST SURGE EVENT AND CHANNELING THROUGH THE 
TAIWAN STRAIT HAVE LINKED WITH THE SYSTEM CREATING AN UNUSUALLY LARGE 
34-KNOT WIND RADII OVER THE NORTHEAST QUADRANT OF THE SYSTEM. SURFACE
OBSERVATIONS CURRENTLY SHOW 35-45 KNOT WINDS SOUTH AND SOUTHEAST OF
HONG KONG. SURFACE OBSERVATIONS WITHIN THE STRATOCUMULUS FIELD
INDICATE UNSEASONABLY COOL TEMPERATURES WITH VERY LOW DEW POINT
TEMPERATURES (LESS THAN 15C) INDICATIVE OF THE DRY AIR AROUND THE
NORTHERN PERIPHERY OF THE SYSTEM. IN ADDITION TO THE COOL, DRY AIR,
UPPER-LEVEL CONDITIONS ARE MARGINAL WITH STRONG SOUTHERLY VERTICAL
WIND SHEAR.

INITIAL WIND RADII BASIS: PERSISTENCE 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: T5.0 - 90 KTS
   RJTD: T5.0 - 90 KTS
   KNES: T5.0 - 90 KTS
   CIMSS SATCON: 88 KTS AT 180243Z
   CIMSS ADT: 84 KTS AT 180540Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 20-25 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: MEDIUM
   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 36. AFTER
TAU 36, 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 72, THE TRACK MOTION IS MORE 
UNCERTAIN WITH QUASISTATIONARY MOTION LIKELY EITHER OVER VIETNAM OR 
OFF THE COAST OF VIETNAM. TY 23W WILL DISSIPATE BY TAU 96 OR PERHAPS
EARLIER AS ENVIRONMENTAL CONDITIONS QUICKLY DEGRADE.    

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN FAIR AGREEMENT
WITH A 125NM SPREAD IN SOLUTIONS AT TAU 48. AFTER TAU 48, 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
   TRACK 72-120 HR: MEDIUM
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
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