WDPN32 PGTW 240900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (NARRA) WARNING NR 
015//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.9N 107.3E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 103 NM SOUTHEAST OF HANOI, VIETNAM
   MOVEMENT PAST 6 HOURS: SOUTHWESTWARD AT 05 KTS
   SIGNIFICANT WAVE HEIGHT: 13 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
STORM 18W (NARRA) WITH VIGOROUS BURSTS OF DEEP CONVECTION OFFSET TO
THE WEST OF A PARTIALLY EXPOSED LOW-LEVEL CIRCULATION CENTER
(LLCC). STRONG NORTHEASTERLY SHEAR CONTINUES TO IMPACT THE VORTEX,
CAUSING A SOUTHWESTWARD TILT WITH HEIGHT AND PREVENTING CONVECTION
FROM BUILDING OVER THE CENTER. THE INITIAL POSITION IS PLACED WITH
HIGH CONFIDENCE BASED ON A COMBINATION OF THE ANIMATED MSI AND
RADAR IMAGERY SHOWING A DEFINED CIRCULATION. THE INITIAL INTENSITY
OF 40 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE AGENCY
DVORAK FIXES AND CIMSS INTENSITY ESTIMATES LISTED BELOW.
ENVIRONMENTAL ANALYSIS INDICATES THAT 18W IS IN A MARGINALLY
FAVORABLE ENVIRONMENT CHARACTERIZED BY STRONG EQUATORWARD OUTFLOW
ALOFT AND VERY WARM (30-31 C) SEA SURFACE TEMPERATURES OFFSET BY
MODERATE TO HIGH (20-25 KTS) NORTHEASTERLY SHEAR AND DRY AIR
ENTRAINMENT.

INITIAL WIND RADII BASIS: 240215Z METOP-C ASCAT SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: A COMPETING STEERING ENVIRONMENT WITH A
NEAR EQUATORIAL RIDGE (NER) TO THE SOUTH AND A SUBTROPICAL RIDGE
(STR) TO THE NORTH

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.0 - 30 KTS
   RCTP: T2.5 - 35 KTS
   KNES: T2.5 - 35 KTS
   DEMS: T2.5 - 35 KTS
   CIMSS ADT: 35 KTS AT 240610Z
   CIMSS AIDT: 40 KTS AT 240610Z
   CIMSS D-MINT: 43 KTS AT 240625Z
   CIMSS D-PRINT: 36 KTS AT 240710Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 20-25 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: STRONG EQUATORWARD
   OTHER FACTORS: SLIGHT DRY AIR ENTRAINMENT FROM THE SOUTH AND
EAST

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   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: TS 18W IS FORECAST TO BEGIN TRACKING SLOWLY
EASTWARD OVER THE NEXT 24 HOURS AS THE STR RIDGE TO THE NORTH
WEAKENS AND THE NER TO THE SOUTH BECOMES THE PRIMARY STEERING
MECHANISM. AFTER TAU 24, 18W IS EXPECTED TO TURN POLEWARD, USHERED
BY THE NER BUILDING TO THE SOUTHEAST OF THE SYSTEM. LANDFALL WITHIN
SOUTHERN CHINA IS FORECAST TO OCCUR AROUND TAU 48 NEAR THE LEIZHOU
PENINSULA. REGARDING INTENSITY, 18W IS FORECAST TO MAINTAIN ITS
CURRENT INTENSITY THROUGH TAU 12 AS THE SHEAR REMAINS MODERATE.
AFTER TAU 12, EASTERLY SHEAR AND DRY AIR ENTRAINMENT IS EXPECTED TO
INCREASE, CAUSING THE SYSTEM TO WEAKEN AS IT APPROACHES THE COAST.
AFTER LANDFALL, TERRAIN INTERACTION WILL QUICKLY WEAKEN THE VORTEX
WITH DISSIPATION ANTICIPATED TO OCCUR BY TAU 60. 

MODEL DISCUSSION: MODEL TRACK GUIDANCE CONTINUES TO BE IN BETTER
AGREEMENT THIS MODEL RUN WITH A CROSS-TRACK SPREAD OF ABOUT 70 NM
AT TAU 48. THE JTWC TRACK FORECAST IS PLACED CLOSE TO THE
MULTI-MODEL CONSENSUS WITH MEDIUM CONFIDENCE. INTENSITY GUIDANCE
AGREES ON STEADY WEAKENING THROUGHOUT THE FORECAST PERIOD WITH NO
NOTABLE OUTLIERS PRESENT. AS A RESULT, THE JTWC INTENSITY FORECAST
IS ALSO PLACED NEAR THE CONSENSUS WITH MEDIUM CONFIDENCE.

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