Advisories

WDPN32 PGTW 240300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (NARRA) WARNING NR 
014//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 20.3N 107.6E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 104 NM EAST-SOUTHEAST OF HANOI, VIETNAM
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 02 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
STORM (TS) 18W (NARRA) WITH A PERSISTENT AREA OF CONVECTION
CENTERED TO THE WEST OF THE LOW-LEVEL CIRCULATION (LLC) WITH A FEW
BANDS OF CONVECTION WRAPPING IN ALONG THE SOUTHERN FLANK. A LARGER
AREA OF CIRRUS OBSCURES THE AREA IN A FILAMENT PATTERN, INDICATING
CONTINUED STRONG OUTFLOW ALOFT. A 232234Z SSMIS 91 GHZ MICROWAVE
IMAGE DEPICTS SOME LOW-LEVEL CLOUD BANDS WRAPPING INTO AN AREA TO
THE EAST OF THE CENTER OF CONVECTION. ADDITIONALLY, ANIMATED RADAR
IMAGERY DEPICTS THE BROAD CIRCULATION MOVING GENERALLY
SOUTHWESTWARD OVER THE PAST SIX HOURS. ENVIRONMENTAL ANALYSIS
REVEALS MARGINALLY FAVORABLE CONDITIONS OF VERY WARM (30-31 C) SEA
SURFACE TEMPERATURES AND THE AFOREMENTIONED FAVORABLE OUTFLOW ALOFT
WHICH ARE BEING OFFSET BY MODERATE (15-20 KTS) VERTICAL WIND SHEAR
(VWS). THE INITIAL POSITION IS ASSESSED WITH MEDIUM CONFIDENCE
BASED ON A COMBINATION OF RADAR AND MICROWAVE IMAGERY. THE INITIAL
INTENSITY IS ASSESSED WITH HIGH CONFIDENCE BASED ON A 232242Z 
SENTINEL-1D SAR PASS DEPICTING 40 KTS WRAPPED AROUND ALL BUT THE 
SOUTHWESTERN QUADRANT OF THE SYSTEM. 

INITIAL WIND RADII BASIS: PERSISTENCE 

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

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
   CIMSS ADT: 32 KTS AT 232350Z
   CIMSS AIDT: 40 KTS AT 232350Z
   CIMSS D-MINT: 36 KTS AT 232235Z
   CIMSS D-PRINT: 34 KTS AT 240010Z

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

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: HIGH
   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 FORECAST TO CONTINUE AN OVERALL
COUNTERCLOCKWISE LOOP WITHIN THE GULF OF TONKIN, ALTHOUGH ERRATIC
MOTION IN THE EXACT TRACK CAN STILL BE EXPECTED AS THE SYSTEM
REMAINS WITHIN A COMPETING STEERING ENVIRONMENT. TS 18W IS
CURRENTLY UNDER THE STEERING INFLUENCE OF A SOUTHWARD EXTENSION OF
A SUBTROPICAL RIDGE (STR) CENTERED TO THE NORTHWEST AND A NORTHWARD
EXTENSION OF A NEAR-EQUATORIAL RIDGE (NER) CENTERED TO THE
SOUTHWEST, CAUSING THE WOBBLING BUT GENERAL SOUTHWARD MOTION OUT TO
AROUND TAU 24. AFTER THIS POINT, BOTH RIDGES ARE EXPECTED TO
WEAKEN AS A SEPARATE NER CENTERED TO THE SOUTHEAST BUILDS, CAUSING
A POLEWARD TURN BY TAU 36. NEAR THE END OF THE FORECAST PERIOD, THE
STR CENTERED TO THE NORTH WILL BUILD ONCE AGAIN, CAUSING A
NORTHWESTWARD TURN BETWEEN TAUS 60 AND 72 ONCE TS 18W MAKES
LANDFALL. REGARDING INTENSITY, VWS IS EXPECTED TO REMAIN MODERATE
FOR THE NEXT 12 TO 24 HOURS, ALLOWING NARRA TO MAINTAIN TROPICAL
STORM FORCE. AFTER THIS POINT, VWS IS EXPECTED TO INCREASE TO OVER
25 KTS AND OUTFLOW ALOFT WILL BECOME LESS FAVORABLE, CAUSING A
STEADY WEAKENING TREND OVER WATER UNTIL NARRA MAKES LANDFALL OVER
SOUTHERN CHINA BETWEEN TAUS 48 AND 60. AT THIS POINT, INTERACTION
WITH THE TERRAIN WILL HASTEN THE WEAKENING OF THE SYSTEM, WITH
DISSIPATION FORECAST NO LATER THAN TAU 72.

MODEL DISCUSSION: TRACK MODEL GUIDANCE CONTINUES TO COME INTO
BETTER AGREEMENT ON THE COUNTERCLOCKWISE MOTION OF TS 18W, WITH THE
PREVIOUS OUTLIERS OF GFS AND GFS ENSEMBLE MEAN NOW FALLING WITHIN
THE BULK OF THE GUIDANCE, ALTHOUGH BOTH STILL DEPICT A SLOWER TRACK
SPEED AND DISSIPATION OVER WATER. THE OFFICIAL JTWC FORECAST TRACK
IS PLACED WITH MEDIUM CONFIDENCE USING AN ADJUSTED MULTI-MODEL
CONSENSUS TO EXCLUDE GFS, GFS ENSEMBLE MEAN, AND THE ADDITIONAL
OUTLIER OF NAVGEM. REGARDING INTENSITY, GUIDANCE REMAINS IN GOOD
AGREEMENT WITH THE EXCEPTION OF GFS-BASED COAMPS-TC AND HAFS WHICH
BOTH DEPICT TEMPORARY SPIKES BETWEEN TAUS 24 TO 48. THERE REMAINS
SOME UNCERTAINTY IN THE SHORT-TERM DUE TO THE DIFFERING
ENVIRONMENTAL CONDITIONS PREDICTED BY GFS AND ECMWF, WITH GFS
DEPICTING A MUCH MORE HOSTILE ENVIRONMENT OF HIGH VWS BEYOND TAU
36, WHILE ECMWF REMAINS BELOW 30 KTS OF VWS FOR THE REMAINDER OF
THE FORECAST PERIOD. THUS, THE OFFICIAL JTWC INTENSITY FORECAST IS
ASSESSED WITH MEDIUM CONFIDENCE, CLOSELY IN LINE WITH THE
MULTI-MODEL CONSENSUS.

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