WDPN31 PGTW 080900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 24W (YINXING) WARNING NR 021//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 18.4N 118.1E
   INITIAL INTENSITY: 90 KTS
   GEOGRAPHIC REFERENCE: 321 NM SOUTHEAST OF HONG KONG
   MOVEMENT PAST 6 HOURS: WESTWARD AT 12 KTS
   SIGNIFICANT WAVE HEIGHT: 40 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A
CLOUD-FILLED EYE, WITH A DIAMETER OF ROUGHLY 25NM. OVER THE PAST
COUPLE OF HOURS, DEEP CONVECTION HAS STARTED TO ORGANIZE INTO
DEFINED BANDS AND BEGUN TO ROTATE UPSHEAR ON THE EASTERN SIDE OF
THE CIRCULATION. WHILE THE EYE REMAINS CLOUD-FILLED, OR MORE
APPROPRIATELY CLOUD-COVERED (BY UPPER-LEVEL CIRRUS), THE CIRRUS IS
THIN ENOUGH TO SEE FURTHER DOWN INTO THE EYE AND SEE THAT IT IS
BECOMING INCREASINGLY SYMMETRICAL. THE INITIAL POSITION IS ASSESSED
WITH HIGH CONFIDENCE AND PLACED IN THE CENTER OF THE 25NM WIDE EYE
IN THE MSI. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM
CONFIDENCE, WITH A WIDE SPREAD BETWEEN THE AGENCY DVORAK CURRENT
INTENSITY ESTIMATES AND THE AGGRESSIVE CIMSS ADT ESTIMATE. THE
ENVIRONMENT REMAINS FAVORABLE, WITH LOW VWS, WARM SSTS AND MODERATE
POLEWARD OUTFLOW. HOWEVER, THE POLEWARD OUTFLOW IS STEADILY
WEAKENING AND TY 24W IS BECOMING MORE ISOLATED FROM THE LARGER
ENVIRONMENT, WITH DRY AIR ALMOST COMPLETELY SURROUNDING THE SYSTEM.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: SOUTHERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) WHICH EXTENDS FROM WEST OF HONG KONG, OVER
TAIWAN AND THENCE INTO THE WESTERN PACIFIC TO THE SOUTH OF JAPAN. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.5 - 102 KTS
   RJTD: T5.0 - 90 KTS
   RCTP: T5.5 - 102 KTS
   KNES: T5.5 - 102 KTS
   DEMS: T5.5 - 102 KTS
   CIMSS ADT: 104 KTS AT 080600Z
   CIMSS AIDT: 90 KTS AT 080600Z
   CIMSS D-PRINT: 87 KTS AT 080630Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE RADIAL
   OTHER FACTORS: WEAKENING POLEWARD OUTFLOW CHANNEL

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 24W WILL CONTINUE TO TRACK ON A
WEST-NORTHWESTWARD TRACK, STEADILY GAINING LATITUDE, AS THE
STEERING RIDGE ORIENTATION SHIFTS FROM EAST-WEST TO A MORE
SOUTHEAST-NORTHWEST AXIS. BY TAU 36, A DEEP MID-LATITUDE TROUGH
CURRENTLY PASSING OVER MYANMAR WITH MOVE INTO EASTERN CHINA, AND
BREAK THE RIDGE IN THE VICINITY OF HONG KONG. TY 24W WILL SLOW DOWN
WHILE MAKING A HEAD FAKE TO THE NORTH AFTER TAU 24, REACHING IT
FURTHEST NORTHWARD POINT BY TAU 36. OVER THE NEXT 24 TO 36 HOURS,
ENVIRONMENTAL CONDITIONS WILL FAVOR INTENSIFICATION, THOUGH THE
ISOLATION OF THE SYSTEM FROM ANY OUTFLOW CHANNELS ALOFT AND SOURCES
OF ADDITIONAL MOISTURE INFLUX MEANS THAT THE RATE OF INTENSIFICATION 
WILL BE SLOW. THE PEAK OF 100 KNOTS IS FORECAST BY TAU 24. BY TAU 36 
THE SYSTEM WILL BEGIN ITS WEAKENING TREND AS SOUTHEASTERLY FLOW ALOFT, 
ALONG THE WEST SIDE OF A STRONG 200MG RIDGE EAST OF LUZON BEGINS TO 
PUNCH IN OVER THE SYSTEM. BY TAU 48, SOUTHWESTERLY SHEAR WILL INCREASE 
DRAMATICALLY, AND USHER IN A WEDGE OF MID-LEVEL DRY AIR INTO THE CORE 
OF THE ALREADY WEAKENING SYSTEM. AT THIS POINT, THE SYSTEM WILL BE 
RAPIDLY OVERWHELMED BY DRY MID-LEVEL AIR AND SHEAR EXCEEDING 30 KNOTS, 
LEADING TO A DECAPITATION OF THE VORTEX AND THE ONSET OF RAPID 
WEAKENING. AS THE SYSTEM RAPIDLY WEAKENS AND SHALLOWS OUT, IT WILL TURN 
TOWARDS THE SOUTHWEST, ALONG THE LOW-LEVEL NORTHERLY FLOW WHICH EXTENDS 
FROM HAINAN TO THE COAST OF VIETNAM. TY 24W WILL DISSIPATE BELOW 25
KNOTS NO LATER THAN TAU 120 AS IT SKIRTS THE COAST OF VIETNAM, BUT
MAY IN FACT DISSIPATE AS EARLY AS TAU 96. 

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE REMAINS CONSISTENT
AND IN GOOD AGREEMENT, SHOWING LITTLE IN THE WAY OF UNCERTAINTY
THROUGH TAU 48. AFTER THE VORTEX IS DECAPITATED, THE MODEL GUIDANCE
BEGINS TO SPREAD OUT QUICKLY, LEADING TO A SPREAD OF 130NM BY TAU
72. THEREAFTER, BOTH ALONG- AND CROSS-TRACK UNCERTAINTY INCREASES
DUE TO DIFFERENCES IN THE LOW-LEVEL FLOW DIRECTION AND SPEED ACROSS
THE VARIOUS MODELS. THE GFS KEEPS THE SYSTEM OFFSHORE OF VIETNAM
WHILE MOVING SOUTH AT NEARLY 30 KNOTS, WHILE THE ECMWF DISSIPATES
IT EAST OF DA NANG, AND THE NAVGEM, UKMET, GALWEM AND ECENS
ENSEMBLE TAKE THE VORTEX INTO SOUTHERN LAOS BY TAU 120. THE JTWC
FORECAST REMAINS CONSISTENT WITH THE PREVIOUS FORECAST AND NEAR THE
CONSENSUS MEAN THROUGH THE ENTIRE FORECAST. INTENSITY GUIDANCE IS
IN GOOD AGREEMENT, WITH THE JTWC FORECAST TRACING THE HAFS-A, HWRF
AND CONSENSUS MEAN FOR THE DURATION OF THE FORECAST PERIOD. 

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