WDPN32 PGTW 130900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 19W (CHANTHU) WARNING NR 029//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 30.7N 123.4E
   INITIAL INTENSITY: 75 KTS
   GEOGRAPHIC REFERENCE: 102 NM EAST-SOUTHEAST OF SHANGHAI, CHINA
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 17 KTS
   SIGNIFICANT WAVE HEIGHT: 30 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TY 19W CONTINUES TO RAPIDLY WEAKEN AS IT TRACKS NORTHWARD TO THE
EAST OF SHANGHAI. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI)
DEPICTS A COMPACT CORE OF MODERATE CONVECTIVE ACTIVITY SURROUNDED
BY A MOAT OF WEAK CONVECTION AND SPIRAL BANDS OF WEAK CONVECTION
AROUND THE PERIPHERY OF THE SYSTEM. A 130455Z AMSR2 89GHZ MICROWAVE
IMAGE SHOWED MODERATE CONVECTION ON THE NORTHERN HEMISPHERE OF THE
LOW LEVEL CIRCULATION CENTER, WITH MINIMAL CONVECTION TO THE SOUTH.
THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON AN
EXTRAPOLATION OF THE WEAK LLCC EVIDENT IN THE AMSR2 IMAGERY
SUPPORTED BY ANALYSIS OF ANIMATED RADAR DATA. THE INITIAL INTENSITY
IS ASSESSED WITH HIGH CONFIDENCE AT 75 KNOTS, HEDGED BELOW THE
AGENCY CURRENT INTENSITY ESTIMATES OF 90 KNOTS IN LIGHT OF THE MUCH
LOWER DATA-T NUMBERS RANGING BETWEEN T4.0-4.5 (65-75 KTS).
ADDITIONALLY, THE AMSR2 WINDSPEED PRODUCT INDICATED MAXIMUM WINDS
OF 75 KNOTS. TY 19W IS MOVING THROUGH A MARGINALLY FAVORABLE
ENVIRONMENT WITH MODERATE TO STRONG (20-25 KTS) VWS OFFSET BY
ROBUST POLEWARD OUTFLOW. SSTS HAVE COOLED TO 27C BUT STILL REMAIN
WARM ENOUGH TO SUSTAIN THE TROPICAL CYCLONE.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF DEEP-LAYERED
SUBTROPICAL RIDGE (STR) TO THE EAST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.0 - 90 KTS
   RJTD: T5.0 - 90 KTS
   KNES: T5.0 - 90 KTS
   CIMSS SATCON: 85 KTS AT 130523Z
   CIMSS ADT: 63 KTS AT 130540Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 20-25 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: STRONG POLEWARD

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: TY 19W HAS CONTINUED TO TRACK RATHER QUICKLY
POLEWARD ALONG THE WESTERN PERIPHERY OF THE STR OVER THE PAST SIX
HOURS, THOUGH IS SHOWING SOME SIGNS OF SLOWING DOWN AFTER THE 0600Z
HOUR. A BUILDING RIDGE TO THE WEST OVER EAST-CENTRAL CHINA IS
EXPECTED TO EXTEND EAST ACROSS THE SHANDONG PENINSULA INTO THE
YELLOW SEA AND EFFECTIVELY BLOCK FURTHER POLEWARD MOVEMENT AFTER
TAU 12. ONCE THIS RIDGE IS IN PLACE, TY 19W IS FORECAST TO REMAIN
QUASI-STATIONARY OR DRIFT SLOWLY EASTWARD IN A COMPETING STEERING
FLOW BETWEEN THE RIDGE TO THE WEST AND THE LARGE STR EAST-SOUTHEAST
OF JAPAN. WHILE VWS IS FORECAST TO DECREASE AFTER TAU 12, THE
QUASI-STATIONARY MOTION OVER RELATIVELY COOL AND LOW OHC WATERS,
COMBINED WITH A REDUCTION IN OUTFLOW ALOFT, WILL RESULT IN STEADY
WEAKENING TO 40 KNOTS BY TAU 72. AFTER TAU 48, AN APPROACHING
MID-LATITUDE TROUGH WILL ERODE THE WESTERN RIDGE AND BY TAU 72 WILL
EJECT TY 19W OUT OF THE WEAK STEERING FLOW AND ACCELERATE
NORTHEASTWARD. THE SYSTEM WILL BEGIN EXTRA-TROPICAL TRANSITION
(ETT) BY TAU 96 AS IT BEGINS TO INTERACT WITH THE BAROCLINIC ZONE.
AS THE SYSTEM ACCELERATES NORTHEAST ALONG THE SOUTHERN COAST OF
KOREA, IT WILL INCREASE SLIGHTLY IN INTENSITY AS ROBUST POLEWARD
OUTFLOW OFFSETS INCREASING VWS. BY TAU 120 THE SYSTEM WILL COMPLETE
ETT AS IT MOVES UNDER THE 200MB JET AND DEVELOPS FRONTAL
CHARACTERISTICS. 

MODEL DISCUSSION: THOUGH ALL MODEL TRACKERS AGREE THAT THE SYSTEM
WILL ENTER THE WEAK STEERING PATTERN AND BECOME QUASI-STATIONARY,
THEY SHOW CONSIDERABLE UNCERTAINTY IN REGARDS TO THE DIRECTION OF
THE DRIFT, WITH THE ECMWF AND JGSM DRIFTING TOWARDS SHANGHAI AND
THE REMAINDER SHOWING A DRIFT EAST FOLLOWED BY A CLOCKWISE LOOP
THROUGH TAU 72. THE JTWC FORECAST LIES ON THE NORTHERN SIDE OF THE
BULK OF THE EASTERN GROUPING OF TRACKERS WITH LOW CONFIDENCE
THROUGH TAU 72. ALL GUIDANCE MEMBERS AGREE ON THE EJECTION OF THE
SYSTEM TO THE NORTHEAST BY TAU 72, AND ARE IN REMARKABLY GOOD
CROSS-TRACK AGREEMENT THROUGH TAU 120. THE JTWC FORECAST TRACK LIES
JUST NORTH AND SLIGHTLY SLOWER THAN THE CONSENSUS MEAN WITH MEDIUM
CONFIDENCE. THE MAJORITY OF THE INTENSITY GUIDANCE IS IN GOOD
AGREEMENT, THOUGH THERE REMAINS A FAIR AMOUNT OF UNCERTAINTY IN
REGARDS TO THE PACE OF THE WEAKENING AND THUS THE STARTING
INTENSITY IN THE EXTENDED RANGE FORECAST, LENDING MEDIUM TO LOW
CONFIDENCE IN THE INTENSITY FORECAST. 

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