Advisories

WDPN32 PGTW 230900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (NARRA) WARNING NR 
011//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 20.6N 108.1E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 126 NM EAST OF HANOI, VIETNAM
   MOVEMENT PAST 6 HOURS: SOUTHEASTWARD AT 01 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
STORM 18W (NARRA) WITH A PARTIALLY EXPOSED LOW-LEVEL CIRCULATION
CENTER (LLCC) AND DEEP CONVECTION CONFINED TO THE WESTERN
PERIPHERY, CLOSE TO THE COAST. A 230610Z AMSR2 89 GHZ MICROWAVE
IMAGE REVEALED DEFINED LOW-LEVEL CLOUDS TIGHTLY WRAPPING INTO THE
CENTER, SUPPORTING THE INITIAL POSITION WITH HIGH CONFIDENCE. THE
INITIAL INTENSITY OF 35 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED
ON THE AGREEMENT OF T2.5 FROM ALL FIXING AGENCIES. ENVIRONMENTAL
ANALYSIS INDICATES THAT 18W IS IN A MARGINALLY FAVORABLE
ENVIROMENT CHARACTERIZED BY MODERATE EQUATORWARD OUTFLOW ALOFT,
LOW TO MODERATE (15-20 KTS) NORTHEASTERLY VERTICAL WIND SHEAR, AND
VERY WARM (30-31 C) SEA SURFACE TEMPERATURES.

INITIAL WIND RADII BASIS: 230213Z METOP-B ASCAT SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: A COMPETING STEERING PATTERN 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: T2.5 - 35 KTS
   RCTP: T2.5 - 35 KTS
   KNES: T2.5 - 35 KTS
   DEMS: T2.5 - 35 KTS
   CIMSS ADT: 39 KTS AT 230610Z
   CIMSS AIDT: 42 KTS AT 230610Z
   CIMSS D-MINT: 36 KTS AT 230601Z
   CIMSS D-PRINT: 39 KTS AT 230610Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   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: TS 18W IS FORECAST TO DRIFT SLOWLY SOUTHWARD
OVER THE NEXT 24 HOURS AS IT REMAINS PLACED WITHIN A COMPETING
STEERING ENVIRONEMENT. THE ACTUAL TRACK OF 18W IS LIKELY TO BE
ERRATIC DURING THIS TIME, WHICH IS NOT CAPTURED BY THE JTWC
FORECAST. AROUND TAU 36, THE NER TO THE SOUTH IS EXPTECTED TO BUILD
TO THE SOUTHEAST OF 18W, CAUSING IT TO MAKE AN EASTWARD AND THEN
NORTHWARD TURN TOWARD THE SOUTHERN COAST OF CHINA. IN TERMS OF
INTENSITY, 18W IS FORECAST TO MAINTAIN ITS CURRENT INTENSITY OF 35
KTS THROUGH TAU 24 AS THE SHEAR REMAINS RELATIVELY LOW. AFTER TAU
24, NORTHEASTERLY SHEAR WILL INCREASE, CAUSING THE SYSTEM TO BEGIN
ON A WEAKENING TREND AS IT APPROACHES THE COAST OF CHINA. 18W IS
THEN FORECAST TO QUICKLY DISSIPATE AFTER MAKING LANDFALL AROUND TAU
72.

MODEL DISCUSSION: MODEL TRACK GUIDANCE CONTINUES TO BE IN POOR
AGREEMENT UNDER THE EFFECTS OF THE COMPETING STEERING ENVIRONMENT.
MODELS GENERALLY SHOW A SOUTHWARD TRACK THROUGH TAU 24 FOLLOWED BY
A POLEWARD TURN, HOWEVER THERE IS SIGNIFICANT SPREAD THROUGHOUT THE
ENTIRE FORECAST DURATION. THE JTWC TRACK FORECAST IS PLACED CLOSER
TO THE ECMWF AND EXPERIMENTAL GOOGLE DEEPMIND ENSEMBLE MEANS WITH
LOW CONFIDENCE. INTENSITY GUIDANCE AGREES ON A STEADY INTENSITY
THROUGH TAU 24 FOLLOWED BY GRADUAL WEAKENING THROUGH TAU 72 WITH NO
SIGNIFICANT OUTLIERS. THE JTWC INTENSITY FORECAST CLOSELY FOLLOWS
THE CONSENSUS WITH MEDIUM CONFIDENCE.

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