WDPN32 PGTW 212100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (EIGHTEEN) WARNING 
NR 005//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 21.0N 108.8E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 162 NM EAST OF HANOI, VIETNAM
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 02 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS DEEP
CONVECTION REDEVELOPING OVER A LOW-LEVEL CIRCULATION DEPICTED IN A
211427Z METOP-C ASCAT PASS. VERTICAL WIND SHEAR REMAINS MODERATE TO
STRONG, AND THE RECENT NEAR-CENTER CONVECTIVE BURST IS LIKELY A
TEMPORARY INCREASE WITHIN A CYCLING PATTERN. THE INITIAL POSITION
IS PLACED WITH HIGH CONFIDENCE BASED ON THE ASCAT DATA. THE INITIAL
INTENSITY OF 35 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON BOTH
THE ASCAT DATA, WHICH INDICATES A COMPACT AREA OF 30-35 KNOT WINDS TO
THE NORTH OF THE LOW-LEVEL CENTER, AND A CONSENSUS OF SUBJECTIVE
AND THE AUTOMATED INTENSITY ESTIMATES LISTED BELOW.

INITIAL WIND RADII BASIS: 211427Z METOP-C ASCAT SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: WEAK STEERING ENVIRONMENT

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T2.0 - 30 KTS
   CIMSS SATCON: 34 KTS AT 211810Z
   CIMSS ADT: 43 KTS AT 211810Z
   CIMSS AIDT: 42 KTS AT 211810Z
   CIMSS D-MINT: 36 KTS AT 211810Z
   CIMSS D-PRINT: 34 KTS AT 211840Z

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

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE FORECAST TRACK HAS BEEN ADJUSTED
TO INDICATE SLOW, LOOPING MOTION IN AN ANTICIPATED WEAK STEERING
ENVIRONMENT.

FORECAST DISCUSSION: TROPICAL STORM (TS) 18W REMAINS WITHIN A WEAK
STEERING ENVIRONMENT, SITUATED BETWEEN A SUBTROPICAL RIDGE OVER
SOUTHEASTERN CHINA AND DEEP-LAYER SOUTHWESTERLY FLOW TO THE SOUTH.
THE SYSTEM HAS MEANDERED NORTHWARD OVER THE PAST SIX HOURS, BUT
FORWARD MOTION IS NOW EXPECTED TO SLOW AND BECOME ERRATIC AS THE
STEERING CONTRIBUTION OF RIDGING TO THE NORTH INCREASES RELATIVE TO
THE COMPETING FLOW TO THE SOUTH. THE TWO STEERING INFLUENCES ARE
EXPECTED TO REMAIN IN OPPOSITION THROUGHOUT THE FORECAST PERIOD. THE 
EXACT TRACK WILL DEPEND ON SUBTLE CHANGES IN THE STEERING FLOW AND 
STRUCTURE OF THE STORM AND FUTURE ADJUSTMENTS TO THE TRACK ARE 
LIKELY. WARM SEA SURFACE TEMPERATURES AND A FAVORABLE RADIAL OUTFLOW 
PATTERN ARE CURRENTLY SUPPORTING INTENSITY, BUT VERTICAL WIND SHEAR 
SHOULD REMAIN MODERATE TO HIGH THROUGHOUT THE FORECAST PERIOD. 
PERSISTENT SHEAR IS EXPECTED TO STIFLE VERTICAL DEVELOPMENT AND 
FACILITATE INTRUSION OF DRY AIR FROM THE SURROUNDING ENVIRONMENT, 
GRADUALLY WEAKENING THE SYSTEM TO DISSIPATION BY TAU 96.

MODEL DISCUSSION: DESPITE CONSIDERABLE SPREAD AMONG CONSENSUS AND
AI TRACK MODELS, THERE IS BROAD AGREEMENT THAT 18W WILL MEANDER
OVER THE GULF OF TONKIN AND UNDERGO ERRATIC, LIKELY LOOPING MOTION
OVER THE NEXT 72 HOURS. NO OBJECTIVE AIDS ARE CLUSTERED AROUND A
PARTICULAR TRACK AT THIS POINT AND EVEN RUN TO RUN SOLUTIONS FROM
ANY GIVEN MODEL OVER THE PAST SEVERAL CYCLES DEPICT HIGH VARIABILITY. 
THE CURRENT TRACK FORECAST ALIGNS WITH PHYSICAL AND AI MODEL 
CONSENSUSES WITH LOW CONFIDENCE. THERE IS HIGHER AGREEMENT AMONG 
INTENSITY FORECAST SOLUTIONS GIVEN THE EXPECTED INFLUENCE OF 
SUSTAINED VERTICAL WIND SHEAR, WHICH MODELS ACROSS THE BOARD 
ANTICIPATE AS A PRIMARY FACTOR LEADING TO GRADUAL WEAKENING AND 
EVENTUAL DISSIPATION. THERE IS MEDIUM CONFIDENCE IN THE INTENSITY 
FORECAST AS THE RATE OF WEAKENING AND POTENTIAL LANDFALL LATER IN THE 
FORECAST PERIOD CONTRIBUTE UNCERTAINTY.

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