WDPN32 PGTW 120900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 19W (CHANTHU) WARNING NR 025//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 25.2N 122.3E
   INITIAL INTENSITY: 110 KTS
   GEOGRAPHIC REFERENCE: 44 NM EAST OF TAIPEI, TAIWAN
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 14 KTS
   SIGNIFICANT WAVE HEIGHT: 42 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TY 19W APPEARS TO HAVE COMPLETED ANOTHER EYEWALL REPLACEMENT CYCLE
(ERC) BASED ON THE SINGLE EYEWALL EVIDENT IN A PARTIAL 120650Z GMI
89GHZ MICROWAVE IMAGE. THE VISIBLE AND INFRARED EYE BY 0600Z HAD
SHRUNK DOWN TO APPROXIMATELY 10-NM ONCE AGAIN. ANIMATED RADAR
IMAGERY DEPICTED A SMALL CORE OF INTENSE CONVECTION AND A WELL
DEFINED EYE. HOWEVER, BY THE 0600Z HOUR, INFRARED IMAGERY REVEALED
WEAKENING CONVECTION AS EVIDENCED BY WARMING CLOUD TOP TEMPERATURES
AND THE RADAR EYE WAS SHOWING SIGNS OF BECOMING INCREASINGLY
RAGGED. THE CENTER OF TY 19W HAD TRACKED IN A SHALLOW ARC TO THE
EAST BETWEEN 0000Z AND 0600Z, HOWEVER THE TRACK MADE GOOD OVER THAT
TIME WAS DUE NORTH FROM THE PREVIOUS POSITION. THE INITIAL POSITION
WAS ASSESSED WITH HIGH CONFIDENCE BASED ON THE MSI AND RADAR DATA.
THE INITIAL INTENSITY WAS ASSESSED WITH MEDIUM CONFIDENCE BASED ON
THE PGTW AND KNES CURRENT INTENSITY ESTIMATES BETWEEN 5.5-6.0 AND
THE ADT ESTIMATE. RJTD DATA-T NUMBERS ARE 5.5 (102 KTS), WHILE THE
CURRENT INTENSITY REMAINS HELD CONSERVATIVELY HIGH AT 6.5 (127
KTS). THE SYSTEM IS MOVING NORTHWARD THROUGH A GENERALLY FAVORABLE
ENVIRONMENT WITH LOW VWS AND ROBUST POLEWARD AND EQUATORWARD
OUTFLOW CHANNELS. WHILE SSTS REMAIN WARM (28-29C), THE SYSTEM IS
CURRENTLY MOVING INTO AN EDDY OF LOW OHC WATERS NORTHEAST OF
TAIWAN.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA. 

CURRENT STEERING MECHANISM: DEEP-LAYERED, NORTH-SOUTH ORIENTED
SUBTROPICAL RIDGE (STR) ENTRENCHED TO THE EAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T6.0 - 115 KTS
   RJTD: T6.5 - 127 KTS
   KNES: T5.5 - 102 KTS
   CIMSS ADT: 107 KTS AT 120540Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: STRONG POLEWARD AND EQUATORWARD
   OTHER FACTORS: SYSTEM IS MOVING OVER AN EDDY OF LOW OHC WATERS. 

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   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: TY 19W IS FORECAST TO CONTINUE TRACKING
NORTHWARD ALONG THE WESTERN PERIPHERY OF THE DEEP-LAYER STR TO THE
EAST THROUGH TAU 24, BEFORE ENTERING A WEAK STEERING ENVIRONMENT
BETWEEN A TRIO OF RIDGES TO THE NORTHWEST, EAST AND SOUTHWEST. ONCE
THE SYSTEM ENTERS THE WEAK STEERING PATTERN, IT WILL SLOW
SIGNIFICANTLY AND STALL IN THE VICINITY OF SHAHGHAI, REMAINING
QUASI-STATIONARY OR DRIFTING SLIGHTLY POLEWARD BETWEEN TAUS 36 AND
72. AN APPROACHING DEEP MID-LATITUDE TROUGH WILL BEGIN TO ERODE THE
RIDGE TO THE WEST BY TAU 72 AND BY TAU 96, TY 19W WILL BEGIN TO
ACCELERATE TO THE EAST ALONG THE NORTHWEST BOUNDARY OF THE DEEP STR
TO THE SOUTHEAST AND AHEAD OF THE TROUGH. AS THE SYSTEM MOVES OVER
THE SOUTHERN COAST OF KOREA BY TAU 120 IT IS EXPECTED TO BEGIN
EXTRATROPICAL TRANSITION (ETT) AS IT INTERACTS WITH THE BAROCLINIC
ZONE, EMBEDS WITHIN THE UPPER-LEVEL WESTERLIES AND DEVELOPS
MODERATE THERMAL ADVECTION. TY 19W IS EXPECTED TO STEADILY WEAKEN
OVER THE NEXT 72 HOURS AS IT ENCOUNTERS MID-LEVEL NORTHWESTERLY
SHEAR, DECREASING OUTFLOW AND DECREASING MID-LEVEL RELATIVE
HUMIDITY. ONCE IT BECOMES QUASI-STATIONARY NEAR SHANGHAI. ONCE IT
BEGINS MOVING NORTHEAST, SYSTEM WILL MAINTAIN MINIMAL TROPICAL
STORM STRENGTH AS THE COMPETING EFFECTS OF STRONG POLEWARD OUTFLOW
AND MODERATE TO STRONG VWS OFFSET ONE ANOTHER PRIOR TO ETT. 

MODEL DISCUSSION: TRACK GUIDANCE IS IN TIGHT AGREEMENT, WITH A 60NM
SPREAD AT TAU 24. THEREAFTER GUIDANCE IS HIGHLY UNCERTAIN AS THE
SYSTEM ENTERS THE WEAK STEERING PATTERN. ALL GUIDANCE SUGGESTS A
QUASI-STATIONARY MOTION OVERALL, THOUGH MODELS ARE DIVIDED INTO TWO
CAMPS; ONE CONSISTING OF THE HWRF, ECMWF, JGSM AND TO A LESSER
DEGREE THE GFS, INDICATE A DRIFT OF THE CIRCULATION FAIRLY FAR
INLAND TO THE WEST OF SHANGHAI BEFORE LOOPING BACK TO THE EAST
AFTER TAU 72. THE REMAINDER OF THE CONSENSUS MEMBERS KEEP THE
CENTER OFFSHORE OF SHANGHAI AND THEN RESUME THE EASTWARD TRACK
AFTER TAU 72. THE JTWC FORECAST TRACK LIES CLOSE TO THE GFS
SOLUTION WITH LOW CONFIDENCE DUE TO THE LARGE UNCERTAINTIES
ASSOCIATED WITH THE MODEL SOLUTIONS. IN THE EXTENDED FORECAST, ALL
GUIDANCE AGREES ON AN EJECTION OF THE SYSTEM TO THE EAST BY TAU 96
WITH A 225NM SPREAD BY TAU 120. ALONG-TRACK SPREAD INCREASES AS THE
SYSTEM UNDERGOES ETT BUT THE BULK OF THE GUIDANCE IS CLUSTERED
AROUND THE JTWC FORECAST, WITH LOW CONFIDENCE. INTENSITY GUIDANCE
IS IN GOOD AGREEMENT ON THE OVERALL SCENARIO, AND THE JTWC FORECAST
LIES JUST ABOVE THE CONSENSUS MEAN THROUGH THE FORECAST PERIOD.
CONFIDENCE IS LOW DUE THE UNCERTAINTIES SURROUNDING THE EXACT TRACK
AND INTERACTIONS WITH TERRAIN OVER EASTERN CHINA AND HOW STRONG THE
SYSTEM MAY BE ONCE IT RESUMES THE EASTWARD TRACK. 

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: LOW
   TRACK 72-120 HR: LOW
   INTENSITY 0 - 72 HR: LOW
   INTENSITY 72-120 HR: LOW//
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