WDPN32 PGTW 230900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (NORU) 
WARNING NR 006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 18.0N 131.8E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 556 NM SOUTH-SOUTHEAST OF KADENA AB
   MOVEMENT PAST 6 HOURS: WESTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 16 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A DEEP
CONVECTIVE STRUCTURE WITH A PARTIALLY EXPOSED LOW-LEVEL CIRCULATION
(LLCC), WHICH IS BECOMING MORE VERTICALLY STACKED AS SEEN ON THE
MSI LOOP. A SLIGHT TILT STILL EXISTS WITH THE LLCC, HOWEVER, THE
EASTERLY SHEAR IS BEGINNING TO RELAX, WHICH IS HELPING THE SYSTEM
REGAIN ITS VERTICAL STRUCTURE. THE INITIAL POSITION IS PLACED WITH
HIGH CONFIDENCE USING THE MSI LOOP AND A 230458Z AMSR2 89GHZ
MICROWAVE IMAGE. A LATER 230713Z SSMIS 91GHZ FURTHER CORROBORATES
THE LATEST POSITION ALONG WITH THE CONVECTIVE STRUCTURE AND SLIGHT
TILT. THE INITIAL INTENSITY IS PLACED WITH MEDIUM CONFIDENCE BASED
ON A COMBINATION OF SUBJECTIVE AND OBJECTIVE AGENCY DVORAK
INTENSITY ESTIMATES LISTED BELOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE (STR) TO THE
NORTHEAST 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.5 - 35 KTS
   KNES: T3.0 - 45 KTS
   CIMSS SATCON: 37 KTS AT 230447Z
   CIMSS ADT: 35 KTS AT 230550Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: WEAK WESTWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   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 18W IS BEGINNING TO SLOW ONCE AGAIN ON ITS
WESTWARD TRACK TOWARDS LUZON. OVER THE LAST 6 HOURS THE SYSTEM HAS
BATTLED THROUGH A MARGINALLY FAVORABLE ENVIRONMENT, CHARACTERIZED
BY WARM SEA SURFACE TEMPERATURES (SSTS) (29-30C), WEAK RADIAL 
OUTFLOW, OFFSET BY MODERATE VERTICAL WIND SHEAR (15-20 KNOTS), WITH 
MID LEVEL DRY AIR SURROUNDING THE SYSTEM. DURING THE NEXT 12 TO 24 
HOURS, THE SYSTEM WILL CONTINUE TRACKING WESTWARD UNDER THE OUTER 
INFLUENCE OF A DEEP-LAYER SUBTROPICAL RIDGE TO THE NORTHEAST. THIS 
DOMINANT STEERING FEATURE WILL STEER 18W TOWARD THE COAST OF LUZON, 
BEFORE MAKING LANDFALL BETWEEN TAU 36 AND 48. RAPID INTENSIFICATION
GUIDANCE CONTINUES TO BE TRIGGERED, HOWEVER, DUE TO THE STRONGER
EASTERLY CONVERGENT FLOW ALOFT, AND WEAK UPPER-LEVEL OUTFLOW, ALONG
WITH THE MID-LEVEL DRY AIR, THE FORECAST WILL MAINTAIN A GRADUAL
INTENSIFICATION UP TO 60 KNOTS BEFORE LANDFALL AND OVER LUZON. THE
LAND INTERACTION WILL HALT ANY FURTHER INTENSIFICATION DURING ITS
SHORT DURATION OVER LAND. ONCE TS NORU MOVES OVER THE SOUTH CHINA
SEA IT WILL ENJOY THE RIPENED ENVIRONMENT OF THE VERY WARM WATERS,
LOW VERTICAL WIND SHEAR, AND ENHANCED UPPER-LEVEL ENVIRONMENT. THE
FAVORABLE ENVIRONMENT WILL HELP INTENSIFY THIS SYSTEM JUST SOUTH OF
HAINAN, CHINA. POTENTIAL FOR RAPID INTENSIFICATION DURING TAUS 72
AND 96. HOWEVER, THE OFFICIAL JTWC INTENSITY PEAK AT THIS TIME WILL
BE 80 KNOTS. BY TAU 120, TS NORU WILL MAKE ITS SECONDARY LANDFALL
OVER VIETNAM.

MODEL DISCUSSION: NUMERICAL MODELS ARE IN OVERALL TIGHT AGREEMENT
WITH NVGM THE NORTHERN OUTLIER AND GFS THE SOUTHERNMOST OUTLIER.
HOWEVER, MODEL TRACK GUIDANCE ONLY SPREADS TO A MAX OF 110NM OVER
THE NEXT 96 HOURS, AND ONLY SPREADS SLIGHTLY UPON THE SECONDARY
LANDFALL OVER VIETNAM TO 210NM. INTENSITY GUIDANCE INDICATES
UNCERTAINTY DUE TO RAPID INTENSIFICATION GUIDANCE BEING TRIGGERED
THROUGH TAU 48, HOWEVER, INTENSITY GUIDANCE BECOMES MORE IN LINE
FROM TAU 72 THROUGH TAU 96. THUS, THE JTWC INTENSITY FORECAST IS
PLACED WITH MEDIUM CONFIDENCE OVERALL.

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: HIGH
   TRACK 72-120 HR: MEDIUM
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
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