Advisories

WDPN32 PGTW 232100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (NARRA) WARNING NR 
013//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 20.4N 107.8E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 113 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 ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
STORM (TS) 18W (NARRA) MAINTAINING A STRONG AREA OF CONVECTION
OFFSET TO THE WEST OF THE LOW LEVEL CIRCULATION (LLC). SOME LOW LEVEL
TURNING ON THE EASTERN FLANK OF THE SYSTEM IS VISIBLE IN EIR, BUT
REMAINS PARTIALLY OBSCURED BY THE CONVECTION AND CIRRUS. ANIMATED
RADAR IMAGERY DEPICTS THE OVERALL WEST-SOUTHWESTWARD TRACK OF TS
18W OVER THE PAST SIX HOURS. ADDITIONALLY, SURFACE OBSERVATIONS
RECORDED FROM BACH LONG VY IN THE CENTER OF THE GULF OF TONKIN HAVE
RECORDED BACKING WINDS OVER THE PAST THREE HOURS GOING FROM 29 KTS
OF WESTERLIES AT 231500Z TO 29 KTS OF WEST-SOUTHWESTERLIES AT
231800Z, LENDING FURTHER CONFIDENCE TO THE OVERALL MOTION OF THE
SYSTEM. ENVIRONMENTAL ANALYSIS REVEALS MARGINALLY FAVORABLE
CONDITIONS WITH VERY WARM (30-31 C) SEA SURFACE TEMPERATURES (SSTS)
AND STRONG EQUATORWARD OUTFLOW ALOFT BEING OFFSET BY MODERATE TO
HIGH (20-25 KTS) OF NORTHEASTERLY VERTICAL WIND SHEAR (VWS). THE
INITIAL POSITION IS PLACED WITH LOW CONFIDENCE BASED ON THE BROAD
CIRCULATION DEPICTED IN RADAR IMAGERY. THE INITIAL INTENSITY IS
ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE SURFACE OBSERVATIONS
FROM BACH LONG VY AS WELL AS THE AGENCY DVORAK FIXES AND CIMSS
OBJECTIVE ESTIMATES LISTED BELOW.

INITIAL WIND RADII BASIS: WIND RADII ADJUSTED BASED ON 231343Z
METOP-C ASCAT DATA

CURRENT STEERING MECHANISM: COMBINATION 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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.5 - 35 KTS
   KNES: T2.5 - 35 KTS
   DEMS: T2.5 - 35 KTS
   CIMSS ADT: 32 KTS AT 231740Z
   CIMSS AIDT: 39 KTS AT 231740Z
   CIMSS D-MINT: 42 KTS AT 231454Z
   CIMSS D-PRINT: 38 KTS AT 231710Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 20-25 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: STRONG EQUATORWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: LOW
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: HIGH

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 CONTINUE ITS SLOW, ERRATIC 
MOTION IN THE GULF OF TONKIN, TRACKING IN AN OVERALL SOUTHWARD MOTION 
OUT TO TAU 24 UNDER THE COMBINED STEERING INFLUENCES 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, RESTRICTING WESTWARD MOVEMENT OF TS 18W. AFTER TAU 24, A 
SEPARATE NER IS EXPECTED TO BUILD TO THE SOUTHEAST OF 18W, CAUSING A 
POLEWARD TURN AND ULTIMATELY DRIVING TS 18W OVER LAND AND INTO 
SOUTHERN CHINA BETWEEN TAUS 48 AND 60. REGARDING INTENSITY, A SHORT 
WINDOW OF MORE FAVORABLE ENVIRONMENTAL CONDITIONS IS EXPECTED FROM TAU 
12 TO 24 AS VWS IS PREDICTED TO ABATE, ALLOWING TS 18W TO MAINTAIN 40 
KT INTENSITY TO THIS POINT. VWS IS EXPECTED TO SHARPLY INCREASE AFTER
TAU 36, LEADING TO A WEAKENING TREND, WHICH WILL BE FURTHER ENHANCED 
BY LANDFALL OVER SOUTHERN CHINA BY TAU 60, ULTIMATELY DISSIPATING AS A 
TROPICAL CYCLONE BY TAU 72.

MODEL DISCUSSION: TRACK MODEL GUIDANCE IS IN FAIR AGREEMENT ON AN
INITIAL SOUTHWARD TRACK FOLLOWED BY A POLEWARD TURN BY TAU 36. GFS AND 
GFS ENSEMBLE MEAN REMAIN OUTLIERS TO THE WEST AND ARE THE ONLY MODELS 
TO SHOW DISSIPATION OVER WATER. LARGER SPREAD REMAINS WITH THE TIMING 
AND LOCATION OF LANDFALL, RANGING FROM GALWEM DEPICTING THE 
EASTERNMOST TRACK OVER THE EASTERN COAST OF THE LEIZHOU PENINSULA, 
WHILE A MAJORITY OF MODELS PREDICT A SHARPER NORTHWESTWARD TURN 
BETWEEN TAUS 60 AND 72. THE OFFICIAL JTWC FORECAST IS PLACED WITH LOW 
CONFIDENCE, USING AN ADJUSTED CONSENSUS TRACK EXCLUDING THE OUTLIERS 
OF GFS, GFS ENSEMBLE MEAN, AND GALWEM. MODEL INTENSITY GUIDANCE IS IN 
GOOD AGREEMENT ON A SHORT-TERM PLATEAU IN INTENSITY FOLLOWED BY A 
WEAKENING TREND BEGINNING BY TAU 36, AND THE MAJORITY OF GUIDANCE 
EXPECTING DISSIPATION BY TAU 72. THE ECMWF AND COAMPS-TC MODELS 
PREDICT A SLIGHT INTENSIFICATION AT TAU 36, AND WHILE THIS SPIKE IS 
NOT REFLECTED IN THE JTWC FORECAST, IT REMAINS A POSSIBILITY DUE TO 
THE UNCERTAINTY OF THE ENVIRONMENTAL CONDITIONS AT THAT TIME. THE 
OFFICIAL JTWC INTENSITY FORECAST IS ASSESSED WITH MEDIUM CONFIDENCE, 
CLOSELY IN LINE WITH THE MULTI-MODEL CONSENSUS.

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