Advisories

WDPN32 PGTW 242100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (NARRA) WARNING NR 
017//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.5N 107.7E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 136 NM SOUTHEAST OF HANOI, VIETNAM
   MOVEMENT PAST 6 HOURS: EAST-SOUTHEASTWARD AT 03 KTS
   SIGNIFICANT WAVE HEIGHT: 11 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
STORM (TS) 18W (NARRA) WITH A WEAKENING CONVECTIVE STRUCTURE THAT
IS CURRENTLY BEING SHEARED TO THE SOUTHWEST, PARTIALLY EXPOSING THE
LOW-LEVEL CIRCULATION CENTER (LLCC). ANIMATED RADAR IMAGERY FURTHER
SUPPORTS TS 18W MAKING AN EASTWARD TURN OVER THE PAST SIX HOURS AND
THE WEAKENING STRUCTURE OF THE SYSTEMS RAIN BANDS. ENVIRONMENTAL
CONDITIONS ARE ASSESSED TO BE MARGINAL WITH HIGH (25-30 KTS) VERTICAL 
WIND SHEAR (VWS) FROM THE NORTHEAST BEING OFFSET ONLY MARGINALLY BY 
WARM (29-30 C) SEA SURFACE TEMPERATURES (SSTS) AND MODERATE OUTFLOW 
ALOFT. THE INITIAL POSITION IS ASSESSED WITH MEDIUM CONFIDENCE BASED 
ON ANIMATED EIR AND RADAR IMAGERY. THE INITIAL INTENSITY IS ASSESSED 
WITH HIGH CONFIDENCE BASED ON THE VISIBLE DETERIORATION OF THE SYSTEM 
AND OVERALL AGREEMENT BETWEEN ALL AGENCY DVORAK FIXES AND CIMSS 
OBJECTIVE ESTIMATES LISTED BELOW. 

INITIAL WIND RADII BASIS: WIND RADII ADJUSTED BASED ON 241302
METOP-B ASCAT DATA

CURRENT STEERING MECHANISM: NEAR-EQUATORIAL RIDGE CENTERED TO THE
SOUTH AND TO A LESSER DEGREE A SUBTROPICAL RIDGE CENTERED TO THE
NORTHWEST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.0 - 30 KTS
   KNES: T2.5 - 35 KTS
   DEMS: T2.5 - 35 KTS
   CIMSS ADT: 35 KTS AT 241740Z
   CIMSS AIDT: 35 KTS AT 241740Z
   CIMSS D-MINT: 38 KTS AT 241305Z
   CIMSS D-PRINT: 34 KTS AT 241710Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 25-30 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: HIGH
   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 FORECAST TO CONTINUE ITS EASTWARD
MOTION OVER THE NEXT 12 TO 24 HOURS AS A SUBTROPICAL RIDGE (STR)
CENTERED TO THE NORTHWEST CONTINUES TO WEAKEN, ALLOWING FOR THE
NEAR-EQUATORIAL RIDGE (NER) CENTERED TO THE SOUTH TO BECOME THE
PRIMARY STEERING INFLUENCE. THE NER IS EXPECTED TO BUILD A
NORTHWARD EXTENSION IMMINENTLY, CAUSING 18W TO TURN POLEWARD BY TAU
12 AND SKIRT ALONG THE WESTERN COAST OF THE LEIZHOU PENINSULA,
MAKING LANDFALL BETWEEN TAUS 36 AND 48 OVER THE PENINSULA.
REGARDING INTENSITY, TS 18W IS EXPECTED TO BEGIN A WEAKENING TREND
BY TAU 12 AS ENVIRONMENTAL CONDITIONS ARE FORECAST TO BECOME
INCREASINGLY HOSTILE, WITH VWS EXPECTED TO EXCEED 30 KTS BY TAU 36
AND INTRODUCE DRIER MID-LEVEL AIR TO THE SYSTEM. LANDFALL AND
SUBSEQUENT TERRAIN INTERACTION WILL FURTHER HASTEN THE
DETERIORATION OF THE VORTEX WITH DISSIPATION FORECAST TO OCCUR NO
LATER THAN TAU 48.

MODEL DISCUSSION: TRACK MODEL GUIDANCE IS GOOD AGREEMENT THROUGHOUT
THE REMAINDER OF THE FORECAST PERIOD WITH A CROSS-TRACK SPREAD OF
100 NM AT TAU 48. THE OFFICIAL JTWC FORECAST TRACK IS PLACED WITH
MEDIUM CONFIDENCE, CLOSELY IN LINE WITH THE MULTI-MODEL CONSENSUS
EXCLUDING NAVGEM AND JGSM AS OUTLIERS TO THE WEST. INTENSITY MODEL
GUIDANCE IS ALSO IN GOOD AGREEMENT WITH THE ONLY OUTLIER OF GFS
DEPICTING A TEMPORARY SPIKE IN INTENSITY AT TAU 12, BUT AGREEING ON
A SHARP WEAKENING TREND THEREAFTER. THE OFFICIAL JTWC INTENSITY
FORECAST IS ASSESSED WITH MEDIUM CONFIDENCE, PLACED SLIGHTLY ABOVE
THE MULTI-MODEL CONSENSUS. 

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