Advisories

WDPN32 PGTW 231500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (NARRA) WARNING NR 
012//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 20.5N 108.1E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 127 NM EAST-SOUTHEAST OF HANOI, VIETNAM
   MOVEMENT PAST 6 HOURS: SOUTHWARD AT 01 KTS
   SIGNIFICANT WAVE HEIGHT: 13 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
STORM 18W (NARRA) NEARLY STATIONARY OVER THE PAST 6 HOURS WITH A
PARTIALLY EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC) AND DEEP
CONVECTION CONFINED TO THE WESTERN PERIPHERY. NORTHEASTERLY SHEAR
IS EVIDENT, INDICATED BY A SHARP EDGE IN THE NORTHEASTERN PORTION
OF THE CONVECTION. THE INITIAL POSITION IS PLACED WITH HIGH
CONFIDENCE BASED PRIMARILY ON ANIMATED RADAR IMAGERY SHOWING A
DEFINED CIRCULATION. THE INITIAL INTENSITY OF 40 KTS IS ALSO
ASSESSED WITH HIGH CONFIDENCE BASED ON THE CLOSE AGREEMENT BETWEEN
THE AGENCY DVORAK FIXES AND CIMSS INTENSITY ESTIMATES LISTED BELOW.
ADDITIONALLY, SURFACE OBSERVATIONS FROM BACH LONG VI REPORTED 37
KTS FROM THE WEST-NORTHWEST AT 231200Z, FURTHER SUPPORTING THE
INITIAL INTENSITY ESTIMATE. ENVIRONMENTAL ANALYSIS INDICATES THAT
18W IS IN A MARGINALLY FAVORABLE ENVIRONMENT 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: PERSISTENCE 

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: 37 KTS AT 231210Z
   CIMSS AIDT: 38 KTS AT 231210Z
   CIMSS D-MINT: 40 KTS AT 231326Z
   CIMSS D-PRINT: 40 KTS AT 231210Z

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 ENVIRONMENT. 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 40
KTS THROUGH TAU 36 AS THE SHEAR REMAINS RELATIVELY LOW. AFTER TAU
36, EASTERLY SHEAR AND DRY AIR WILL INCREASE, CAUSING THE SYSTEM TO
START 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. HAFS-A AND COAMPS-TC SHOW A BRIEF PERIOD OF FURTHER
INTENSIFICATION THROUGH TAU 24 TO AROUND 50 KTS, WHICH IS NOT
OUTSIDE THE REALM OF POSSIBILITY IF THE SHEAR DOES NOT HINDER
THE VORTEX TOO MUCH. MODELS THEN AGREE ON A STEADY WEAKENING TREND
AFTER TAU 24. 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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