Advisories

WDPN32 PGTW 250300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (NARRA) WARNING NR 
018//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.3N 107.5E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 136 NM SOUTHEAST OF HANOI, VIETNAM
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHWESTWARD AT 02 KTS
   SIGNIFICANT WAVE HEIGHT: 11 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS THE NOW
FULLY EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC) OF TROPICAL STORM
(TS) 18W (NARRA) WITH FLARING CONVECTION ON THE WESTERN FLANK
CONTINUING TO BE QUICKLY SHEARED TO THE WEST. ADDITIONALLY, MSI AND
ANIMATED RADAR IMAGERY BOTH DEPICT THE LOW-LEVEL CIRCULATION OF 18W
BECOMING INCREASINGLY ELONGATED. ENVIRONMENTAL CONDITIONS ARE
ASSESSED AS MARGINALLY UNFAVORABLE AS WARM (29-30 C) SEA SURFACE
TEMPERATURES ARE BEING OFFSET BY HIGH (25-30 KTS) OF VERTICAL WIND
SHEAR (VWS) AND WEAK OUTFLOW ALOFT. THE INITIAL POSITION IS PLACED
WITH HIGH CONFIDENCE BASED ON THE EXPOSED LLCC IN MSI. THE INITIAL
INTENSITY IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE AGENCY
DVORAK FIXES AND CIMSS OBJECTIVE ESTIMATES LISTED BELOW.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: NORTHWARD EXTENSION OF A
NEAR-EQUATORIAL RIDGE CENTERED TO THE SOUTH.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T2.0 - 30 KTS
   RCTP: T2.5 - 35 KTS
   KNES: T2.5 - 35 KTS
   CIMSS SATCON: 37 KTS AT 242110Z
   CIMSS ADT: 34 KTS AT 242310Z
   CIMSS AIDT: 34 KTS AT 242310Z
   CIMSS D-MINT: 35 KTS AT 241855Z
   CIMSS D-PRINT: 35 KTS AT 242340Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 25-30 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: WEAK EQUATORWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   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: DESPITE ITS TEMPORARY SOUTHWESTWARD JOG OVER
THE PAST SIX HOURS, TS 18W IS FORECAST TO CONTINUE GENERALLY
EASTWARD AND THEN BEGIN A POLEWARD TURN WITHIN THE NEXT 12 HOURS AS
THE NORTHWARD EXTENSION OF A NEAR-EQUATORIAL RIDGE (NER) CENTERED
TO THE SOUTH CONTINUES TO BUILD, DRIVING 18W ON A NORTHEASTWARD
TRACK TOWARDS THE LEIZHOU PENINSULA FOR THE REMAINDER OF THE
FORECAST PERIOD. LANDFALL OVER THE WESTERN COAST OF THE PENINSULA
IS FORECAST TO OCCUR JUST AFTER TAU 24. REGARDING INTENSITY, TS 18W
IS FORECAST TO BEGIN WEAKENING IMMINENTLY AS VWS INCREASES TO OVER
30 KTS, FURTHER SHEARING THE UPPER-LEVEL CONVECTION AWAY FROM THE
LLCC AND INTRODUCING MID-LEVEL DRIER AIR. THE COMBINATION OF THESE
UNFAVORABLE CONDITIONS AND TERRAIN INTERACTION UPON LANDFALL AROUND
TAU 24 IS EXPECTED TO CAUSE DISSIPATION OF TS 18W OVER SOUTHERN
CHINA NO LATER THAN TAU 36.

MODEL DISCUSSION: TRACK MODEL GUIDANCE IS IN GOOD AGREEMENT FOR THE
REMAINDER OF THE FORECAST PERIOD WITH THE EXCEPTION OF JGSM FAR TO
THE WEST AND NAVGEM, WHICH DEPICTS A MUCH SLOWER TRACK SPEED.
EXCLUDING THESE OUTLIERS, CROSS TRACK SPREAD IS 55 NM AT TAU 36 AND
ALONG TRACK SPREAD IS 50 NM, LENDING CONFIDENCE TO THE FORECAST.
THE OFFICIAL JTWC FORECAST IS PLACED WITH MEDIUM CONFIDENCE,
CLOSELY IN LINE WITH THE AI CONSENSUS AND THE MULTI-MODEL CONSENSUS
EXCLUDING NAVGEM. INTENSITY MODEL GUIDANCE IS ALSO IN GOOD
AGREEMENT, WITH GFS CONTINUING TO DEPICT A TEMPORARY SPIKE AT TAU
12 WHICH WAS DISREGARDED AS AN OUTLIER. IT IS POSSIBLE THAT TS 18W
COULD DISSIPATE SOONER THAN TAU 36 DEPENDING ON HOW QUICKLY
INCREASED VWS AND THE EFFECTS OF LAND INTERACTION TAKE HOLD,
HOWEVER THIS IS NOT REFLECTED IN THE OFFICIAL FORECAST. 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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