Advisories

WDPN32 PGTW 220300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (EIGHTEEN) WARNING 
NR 
006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 21.1N 108.7E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 157 NM EAST OF HANOI, VIETNAM
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 01 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
STORM (TS) 18W AS A QUASI-STATIONARY SYSTEM WITH THE LOW LEVEL
CIRCULATION (LCC) COMPLETELY OBSCURED BY CIRRUS OVERCAST. BURSTS OF 
CONVECTION JUST OFF THE COAST CONTINUE TO PUFF AND BE QUICKLY SHEARED 
TO THE NORTHWEST. ENVIRONMENTAL CONDITIONS ARE ASSESSED AS MARGINAL 
DUE TO THIS MODERATE (15-20 KTS) VERTICAL WIND SHEAR (VWS) AND 
INTRUSION OF DRIER AIR FROM THE SOUTHEAST, OFFSET BY MODERATE RADIAL 
OUTFLOW ALOFT AND WARM (29-30 C) SEA SURFACE TEMPERATURES. THE 
INITIAL 
POSITION IS PLACED WITH LOW CONFIDENCE BASED ON RADAR IMAGERY 
DEPICTING A BROAD AREA OF CIRCULATION. THE INITIAL INTENSITY IS 
ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE SPREAD OF AGENCY DVORAK 
FIXES AND CIMSS OBJECTIVE ESTIMATES LISTED BELOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: WEAK STEERING ENVIRONMENT

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.0 - 30 KTS
   RCTP: T1.5 - 25 KTS
   KNES: T3.0 - 45 KTS
   CIMSS ADT: 49 KTS AT 212350Z
   CIMSS AIDT: 41 KTS AT 212350Z
   CIMSS D-PRINT: 36 KTS AT 212350Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE RADIAL

ANALYSIS CONFIDENCE:
   INITIAL POSITION: LOW
   INITIAL INTENSITY: MEDIUM
   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 MAINTAIN A SLOW,
MEANDERING TRACK WITHIN THE GULF OF TONKIN AS IT REMAINS IN A WEAK
STEERING ENVIRONMENT COMPRISED OF STRONG LOW-LEVEL SOUTHWESTERLIES
AND A SUBTROPICAL RIDGE (STR) CENTERED TO THE NORTH. THIS LENDS
LOWER CONFIDENCE TO THE EXACT TRACK OF 18W AS THESE TWO STEERING
INFLUENCES CONTINUE TO COMPETE, FURTHERED BY THE CURRENT LOW
CONFIDENCE OF THE INITIAL POSITION OF TS 18W. ULTIMATELY, TS 18W IS
FORECAST TO MOVE SLOWLY SOUTHWARD BUT REMAIN NORTH OF HAINAN BEFORE
MAKING LANDFALL OVER SOUTHERN CHINA. INTENSITY IS EXPECTED TO
REMAIN STEADY OUT TO TAU 48 AND THEN DECREASE AS INCREASED
VWS AND MID-LEVEL DRY AIR ENTRAINMENT WEAKEN THE SHALLOW SYSTEM. 

MODEL DISCUSSION: TRACK MODEL GUIDANCE IS IN POOR AGREEMENT
THROUGHOUT THE FORECAST PERIOD, BUT SLIGHTLY IMPROVED FROM THE
PREVIOUS CYCLE, AS A MAJORITY NOW SHOW OVERALL SOUTHWARD MOVEMENT
THROUGH TAU 60. AFTER THIS POINT, THE LOCATION OF LANDFALL VARIES,
WITH THE MULTI-MODEL CONSENSUS AND AI CONSENSUS TRACKING EAST OF
THE LEIZHOU PENINSULA WHILE THE EXPERIMENTAL GOOGLE DEEPMIND TRACKS
OVER THE SOUTHERN TIP OF THE PENINSULA. THE OFFICIAL JTWC FORECAST
TRACK IS PLACED WITH LOW CONFIDENCE, FOLLOWING THE SAME GENERAL
ROUTE AS AI GUIDANCE BUT HEDGING TO THE NORTH, IN GOOD AGREEMENT WITH 
THE MULTI-MODEL CONSENSUS EXCLUDING NAVGEM AFTER TAU 60. MODEL 
INTENSITY GUIDANCE IS IN GOOD AGREEMENT REGARDING A PLATEAU IN 
INTENSITY OUT TO TAU 36 AND A GENERAL WEAKENING TREND THEREAFTER.
THE OFFICIAL JTWC INTENSITY FORECAST IS ASSESSED WITH MEDIUM
CONFIDENCE FOR THE REMAINDER OF THE FORECAST PERIOD, IN LINE WITH
THE MULTI-MODEL CONSENSUS IN THE SHORT-TERM AND SLIGHTLY LOWER FROM 
TAU 72 ONWARD.

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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