WDPN31 PGTW 300900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 20W (NONGFA) WARNING NR 
009//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.9N 106.6E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 150 NM NORTHWEST OF DA NANG, VIETNAM
   MOVEMENT PAST 6 HOURS: WESTWARD AT 16 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A PARTIALLY 
EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC) JUST OFF THE COAST OF 
VIETNAM. TROPICAL STORM 20W WILL MAKE LANDFALL 30NM NORTH OF DONG 
HOI. THE CONVECTION HAS BECOME MORE DISORGANIZED IN THE PAST 6 HOURS, 
WITH FLARING CONVECTION OVER THE EASTERN QUADRANT. A 300635Z AMSR2 
37GZ MICROWAVE IMAGE REVEALS LOW-LEVEL BANDING PRIMARILY CONFINED TO 
THE NORTHERN SEMICIRCLE. ENVIRONMENTAL ANALYSIS INDICATES A MARGINAL 
ENVIRONMENT CHARACTERIZED BY WARM (28-29 C) SEA SURFACE TEMPERATURES 
(SST), MODERATE (15-20 KTS) VERTICAL WIND SHEAR (VWS), AND BY 
MODERATE EQUATORWARD OUTFLOW, GREATLY OFFSET BY TERRAIN INTERACTION. 
THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON THE 
PARTIALLY EXPOSED LLCC VISIBLE IN THE AFOREMENTIONED MSI. THE INITIAL 
INTENSITY OF 40 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE 
LISTED AGENCY DOVORAK FIXES AND OBJECTIVE INTENSITY ESTIMATES.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: SOUTHERN PERIPHERY OF A SUBTROPICAL
RIDGE CENTERED T0 THE NORTH OF THE SYSTEM.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   RJTD: T2.5 - 35 KTS
   RCTP: T3.0 - 45 KTS
   KNES: T2.0 - 30 KTS
   DEMS: T3.0 - 45 KTS
   CIMSS SATCON: 42 KTS AT 300520Z
   CIMSS ADT: 45 KTS AT 300540Z
   CIMSS AIDT: 38 KTS AT 300540Z
   CIMSS D-MINT: 32 KTS AT 300638Z
   CIMSS D-PRINT: 30 KTS AT 300540Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 15-20 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD
   OTHER FACTORS: TERRAIN INTERACTION

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TS 20W IS FORECAST TO IMMINENTLY MAKE LANDFALL
AND TRACK WESTWARD THROUGHOUT THE REMAINDER OF THE FORECAST PERIOD.
20W WILL DISSIPATE QUICKLY ONCE OVER LAND DUE TO RUGGED TERRAIN
INTERACTION WITH THE ANNAMITE MOUNTAIN RANGE. DISSIPATION IS
FORECAST TO OCCUR NO LATER THAN TAU 24 OVER NORTHERN THAILAND. 

MODEL DISCUSSION: MODELS ARE IN GOOD AGREEMENT, WITH A 60NM
CROSS-TRACK SPREAD AT TAU 24. THE JTWC TRACK FORECAST IS PLACED
CLOSE TO THE MULTI-MODEL CONSENSUS WITH HIGH CONFIDENCE. THE
INTENSITY GUIDANCE IS IN GOOD AGREEMENT ON A RAPID WEAKENING TREND
OVER THE NEXT 24 HOURS. AS A RESULT, THE JTWC INTENSITY FORECAST IS
ALSO PLACED WITH HIGH CONFIDENCE.

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