WDPN32 PGTW 081500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR SUPER TYPHOON 19W (CHANTHU) WARNING NR 
010//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 15.5N 130.3E
   INITIAL INTENSITY: 135 KTS
   GEOGRAPHIC REFERENCE: 675 NM SOUTH-SOUTHEAST OF KADENA AB
   MOVEMENT PAST 6 HOURS: WESTWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 38 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY THROUGH THE
1200Z HOUR DEPICTS THE FILLING AND ENLARGING OF THE EYE, NOW
ESTIMATED AT 10NM WIDE, LIKELY THE RESULT OF AN EYEWALL REPLACEMENT
CYCLE (EWRC), THOUGH THE LACK OF RECENT MICROWAVE IMAGERY PRESENTS
CHALLENGES TO VERIFYING THAT THE EWRC HAS COMMENCED. ANOTHER
POSSIBILITY IS AN EYEWALL MELD, IN WHICH THE PREVIOUSLY OBSERVED
BAND OF CONVECTION DID NOT REPLACE THE ORIGINAL EYE, ONLY MERGED
WITH IT. ANIMATED 2-MIN HIMAWARI EIR IMAGERY SHOWED THE REAPPEARANCE
OF A VERY SMALL, WARM EYE BY 1400Z, WHICH COULD SUPPORT THE LATER
THEORY. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED
ON THE 10NM EYE IN EIR. THE INITIAL INTENSITY IS LOWERED TO 135
KNOTS AT 1200Z BASED ON THE SLIGHTLY DEGRADED PRESENTATION IN THE
EIR IMAGERY, BUT STILL ABOVE THE AGENCY AND OBJECTIVE ESTIMATES.
ADDITIONAL SUPPORT TO THE INITIAL INTENSITY ASSESSMENT IS PROVIDED
BY A 080917Z SENTINEL-1 SAR IMAGE WHICH INDICATED A FIX INTENSITY
OF 136 KNOTS WITH SOME DATA POINTS NEAR 145 KNOTS. HAVING REACHED
ITS FURTHEST EQUATORWARD POINT, STY 19W IS TRACKING WESTWARD ALONG
THE SOUTHERN PERIPHERY OF A DEEP-LAYER STR. SOME SHORT-TERM WOBBLE
IN THE TRACK IS EXPECTED, BUT OVERALL SHOULD AVERAGE WESTWARD. THE
ENVIRONMENT REMAINS HIGHLY FAVORABLE WITH LOW VWS, GOOD RADIAL
OUTFLOW AND WARM, HIGH OHC WATERS.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA,  SAR DATA. 

CURRENT STEERING MECHANISM: DEEP-LAYER SUBTROPICAL RIDGE (STR) TO
THE NORTH. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T6.5 - 127 KTS
   RJTD: T6.0 - 115 KTS
   KNES: T6.5 - 127 KTS
   CIMSS SATCON: 111 KTS AT 081036Z
   CIMSS ADT: 115 KTS AT 081140Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE RADIAL
   OTHER FACTORS: EYEWALL REPLACEMENT CYCLE. 

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   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: STY 19W LOOKS TO BE ON A FIRM WESTWARD TRACK,
ALONG THE SOUTH SIDE OF A DEEP STR TO THE NORTH, THOUGH SHORT-TERM
WOBBLES ABOUT THE MEAN MOTION VECTOR ARE EXPECTED. OVER THE NEXT 12
HOURS THE STR CENTER IS FORECAST TO MOVE EAST TO NEAR THE 140
LONGITUDE AND BECOME A BIT MORE ELONGATED NORTHWEST-SOUTHEAST,
ALLOWING FOR STY 19W TO TURN NORTHWESTWARD BY TAU 24. THE SYSTEM IS
EXPECTED TO PASS CLOSE TO THE NORTHEASTERN TIP OF LUZON BY 48, BUT
AT THIS POINT IS EXPECTED TO REMAIN JUST OFFSHORE. STARTING AROUND
TAU 48, A WEAK MID-LATITUDE TROUGH IS EXPECTED TO TRANSIT NORTH OF
THE RIDGE. WHILE WEAK, IT HAS JUST ENOUGH DYNAMIC SUPPORT TO ERODE
THE RIDGE ENOUGH FOR A BREAK TO DEVELOP NORTH OF TAIWAN. STY 19W
WILL RESPOND BY TURNING POLEWARD AFTER TAU 72, MAKING LANDFALL
ALONG THE SOUTHEAST CHINA COAST BETWEEN TAU 96 AND 120. DUE TO THE
ONGOING EWRC, STY 19W IS EXPECTED TO CONTINUE WEAKENING OVER THE
NEXT 24 HOURS, DOWN TO 120 KNOTS. AFTER COMPLETION OF THE EWRC, THE
SYSTEM SHOULD REINTENSIFY AS IT MOVES OVER A POOL OF HIGH OHC WATER
OFF THE COAST OF LUZON, REACHING A PEAK OF 130 KNOTS BY TAU 48. THE
UNCERTAIN TIMING OF EWRC COMPLETION, AND THE POTENTIAL FOR A SECOND
EWRC COULD RESULT IN LARGE FLUCTUATIONS IN THE INTENSITY OVER THE
NEXT 24 TO 36 HOURS. AFTER TAU 72 INCREASING VWS AND DECREASING
UPPER-LEVEL OUTFLOW WILL FACILITATE A FAIRLY RAPID WEAKENING TREND,
ACCELERATING AFTER LANDFALL. 

MODEL DISCUSSION: TRACK GUIDANCE IS IN VERY GOOD AGREEMENT THROUGH
TAU 48, WITH THE NOTABLE EXCEPTION OF JGSM WHICH TURNS THE SYSTEM
SHARPLY POLEWARD AFTER TAU 24. THE REMAINDER OF THE GUIDANCE
DISPLAYS A 80NM SPREAD AT TAU 48. THE SPREAD IN THE GUIDANCE
INCREASES SHARPLY AFTER TAU 72, UP TO 760NM BETWEEN THE GFS AND
GALWEM OUTLIERS AT TAU 120. THE MAJORITY OF THE GUIDANCE IS BITING
ON THE SYSTEM MOVING POLEWARD INTO THE BREAK IN THE RIDGE AFTER TAU
72, THOUGH THE NAVGEM, GALWEM AND HWRF INDICATE AN EXCESSIVE AMOUNT
OF TURN, AS IS EXPECTED WITH THESE MODELS WHICH HAVE A STRONG
POLEWARD BIAS. THE GFS MEANWHILE DENIES THE EXISTENCE OF THE BREAK
IN THE RIDGE, AND KEEPS THE SYSTEM ON A WESTWARD TRACK TO THE
VICINITY OF HONG KONG BY TAU 120. ADDITIONAL UNCERTAINTY IS
INTRODUCED WHEN LOOKING AT THE GFS AND ECMWF ENSEMBLES. THE
ENSEMBLE MEMBERS OF BOTH MODELS ARE ROUGHLY EVENLY SPLIT IN A
BIFURCATION BETWEEN THE TWO SCENARIOS DESCRIBED ABOVE. THE JTWC
FORECAST REMAINS CONSISTENT WITH THE PREVIOUS FORECASTS, BUT MOVES
SLIGHTLY CLOSER TO TAIWAN, ON THE SOUTHWESTERN EDGE OF THE TIGHTEST
GROUPING OF TRACKERS. WHILE THERE IS HIGH CONFIDENCE IN THE TRACK
THROUGH TAU 48, THERE IN LOW CONFIDENCE THEREAFTER IN LIGHT OF THE
UNCERTAINTY DESCRIBED ABOVE. INTENSITY GUIDANCE IS IN GENERAL
AGREEMENT ON THE TRENDS AND THE JTWC FORECAST CLOSELY TRACKS THE
HWRF SOLUTION  THROUGH TAU 48 AND THE GFS SHIPS THEREAFTER. THERE
IS LOW CONFIDENCE IN THE FORECAST INTENSITY DUE TO THE UNCERTAINTY
SURROUNDING THE EWRC AND THE SUBSEQUENT IMPACT ON LATER
INTENSITIES.  

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