Advisories

WDPN32 PGTW 230300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (NARRA) WARNING NR 
010//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 20.8N 107.9E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 113 NM EAST OF HANOI, VIETNAM
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
STORM (TS) 18W (NARRA) GAINING IMPROVED CONVECTION AND STRUCTURE
OVER THE PAST SIX HOURS AND HAS COMPLETED A SMALL COUNTERCLOCKWISE
LOOP. RADAR IMAGERY INDICATES A SECOND WELL-DEFINED RAIN BAND
DEVELOPING ALONG THE WESTERN FLANK OF THE SYSTEM, LYING OVER THE
COAST OF NORTHEAST VIETNAM. THIS IMPROVED DEFINITION IN CONVECTIVE
BANDING IS FURTHER SUPPORTED BY A 222240Z WSF-M 89 GHZ MICROWAVE
IMAGE DEPICTING THE TWO BANDS. WHILE THE CONVECTION REMAINS
PRIMARILY TO THE NORTHWEST OF THE LOW LEVEL CIRCULATION CENTER
(LLCC), THE LLCC ITSELF IS STILL OBSCURED BY THE EXPANDING SWATH OF
CIRRUS CLOUDS OVER THE GULF OF TONKIN. REGARDING ENVIRONMENTAL
CONDITIONS, VERTICAL WIND SHEAR HAS SLIGHTLY ABATED AND OUTFLOW
ALOFT HAS VISUALLY IMPROVED AS EVIDENCED BY THE FILAMENT APPEARANCE
AND RADIAL OUTFLOW OF THE CIRRUS SHIELD IN MSI. SEA SURFACE
TEMPERATURES REMAIN VERY WARM AT 30 TO 31 C, LEADING TO A
MARGINALLY FAVORABLE ASSESSMENT OF CONDITIONS. THE INITIAL POSITION
IS PLACED WITH MEDIUM CONFIDENCE BASED ON THE RADAR IMAGERY. THE
INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON A
SURFACE OBSERVATION FROM BACH LONG VY TO THE SOUTH OF THE SYSTEM OF
27 KTS, VISUAL IMPROVEMENT OF TS 18W, AND AGENCY DVORAK FIXES AND
CIMSS OBJECTIVE ESTIMATES LISTED BELOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: COMPETITIVE STEERING ENVIRONMENT OF A
SOUTHWARD EXTENSION OF A SUBTROPICAL RIDGE CENTERED TO THE
NORTHWEST AND A NEAR-EQUATORIAL RIDGE CENTERED TO THE SOUTHWEST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.5 - 35 KTS
   KNES: T2.0 - 30 KTS
   CIMSS SATCON: 40 KTS AT 222140Z
   CIMSS ADT: 34 KTS AT 222340Z
   CIMSS AIDT: 40 KTS AT 222340Z
   CIMSS D-PRINT: 43 KTS AT 222340Z

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

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 18W IS EXPECTED TO TRACK GENERALLY
SOUTHWARD OVER THE NEXT 36 HOURS AS IT IS STEERED BY THE SOUTHWARD
EXTENSION OF A SUBTROPICAL RIDGE (STR) CENTERED TO THE NORTHWEST.
ERRATIC MOTION IS STILL EXPECTED AS THE STR COMPETES FOR STEERING
INFLUENCE WITH A NEAR-EQUATORIAL RIDGE CENTERED FARTHER TO THE
SOUTHWEST OF TS 18W. AFTER TAU 36, BOTH RIDGES ARE EXPECTED TO 
WEAKEN, CAUSING TS 18W TO BEGIN A POLEWARD TURN TOWARDS THE COAST OF 
SOUTHERN CHINA UNDER THE STRONG LOW-LEVEL SOUTHWESTERLY FLOW THAT 
DOMINATES THE AREA, ULTIMATELY MAKING LANDFALL BY TAU 96 TO THE WEST 
OF THE LEIZHOU PENINSULA. REGARDING INTENSITY, TS 18W IS CURRENTLY IN 
A SLIGHTLY MORE FAVORABLE ENVIRONMENT OF LOWER VERTICAL WIND SHEAR 
AND IMPROVED OUTFLOW ALOFT, WHICH IS EXPECTED TO ALLOW 18W TO 
MAINTAIN TROPICAL STORM FORCE OUT TO TAU 36. AFTER THIS POINT, VWS 
WILL SHARPLY INCREASE TO OVER 30 KTS BY TAU 48, DETERIORATING THE 
STRUCTURE OF THE SYSTEM
AND CAUSING DISSIPATION BY NO LATER THAN TAU 96.

MODEL DISCUSSION: TRACK MODEL GUIDANCE HAS REMAINED SPREAD OUT IN
THE INITIAL DIRECTION OF 18W, BUT AGREES ON A GENERALLY SOUTHWARD
TRACK FOR THE FIRST 36 HOURS FOLLOWED BY A POLEWARD TURN
THEREAFTER, TO VARYING DEGREES OF SHARPNESS. GFS AND THE GFS
ENSEMBLE MEAN REMAIN OUTLIERS TO THE WEST AND SHOW DISSIPATION OVER
WATER, WHILE THE PREVIOUSLY EASTERNMOST OUTLIERS OF GALWEM AND THE
EXPERIMENTAL GOOGLE DEEPMIND HAVE SHIFTED WESTWARD TO SKIRT ALONG
THE WEST COAST OF THE LEIZHOU PENINSULA VICE TRANSITING INTO THE
SOUTH CHINA SEA AS PREVIOUSLY DEPICTED. THE OFFICIAL JTWC FORECAST
TRACK IS PLACED WITH LOW CONFIDENCE BASED ON THE CONTINUED
COMPETITIVE STEERING ENVIRONMENT AND SPREAD OF MODEL GUIDANCE, BUT
REMAINS CLOSELY IN LINE WITH THE MULTI-MODEL CONSENSUS EXCLUDING
NAVGEM AS WELL AS THE ECMWF DETERMINISTIC SOLUTION. MODEL INTENSITY
GUIDANCE IS IN GOOD AGREEMENT ON A SMALL INTENSIFICATION PERIOD IN
THE SHORT-TERM DUE TO THE TEMPORARILY IMPROVED ENVIRONMENTAL
CONDITIONS FOLLOWED BY A WEAKENING PHASE THEREAFTER. THE OFFICIAL
JTWC INTENSITY FORECAST IS ASSESSED WITH MEDIUM CONFIDENCE, CLOSELY
IN LINE WITH THE BULK OF GUIDANCE, EXCLUDING COAMPS-TC AS AN
OUTLIER.

FORECAST CONFIDENCE:
   TRACK 00-72 HR: MEDIUM
   TRACK 72-120 HR: MEDIUM
   INTENSITY 00-72 HR: HIGH
   INTENSITY 72-120 HR: HIGH//
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