WDPN32 PGTW 161500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 19W (CHANTHU) WARNING 
NR 042//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 31.7N 125.9E
   INITIAL INTENSITY: 50 KTS
   GEOGRAPHIC REFERENCE: 210 NM WEST-SOUTHWEST OF SASEBO, JAPAN
   MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 22 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TS 19W CONTINUES TO SLOWLY ACCELERATE TO THE NORTHEAST WHILE SLOWLY
WEAKENING. ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY
DEPICTS BROAD SPIRAL BANDS OF LOW-LEVEL CLOUDS WRAPPING INTO A
LOW-LEVEL CIRCULATION CENTER (LLCC) OBSCURED BY A RECENT BURST OF
MODERATE CONVECTION. ANIMATED KOREAN RADAR DATA IS SHOWING THE
FIRST GLIMPSE OF THE LLCC TO THE SOUTH-SOUTHEAST OF CHEJU DO, AND
IN CONJUNCTION WITH THE ANIMATED EIR, PROVIDED HIGH CONFIDENCE TO
THE INITIAL POSITION. THE INITIAL INTENSITY IS ASSESSED AT 50 KNOTS
WITH HIGH CONFIDENCE, ON THE HIGHER END OF MULTI-AGENCY CURRENT
INTENSITY ESTIMATES AND OBJECTIVE ADT AND SATCON ESTIMATES IN LIGHT
OF RECENT SCATTEROMETER DATA FROM 160930Z INDICATING MAXIMUM WINDS
OF 45-50 KNOTS. A PARTIAL 161126Z ASCAT-A PASS AND BUOY DATA SOUTH
OF CHEJU DO PROVIDED GOOD CONFIDENCE IN THE ADJUSTED WIND RADII, AT
LEAST ON THE EASTERN SIDE OF THE SYSTEM. UPPER-AIR SOUNDINGS FROM
THE REGION STILL SHOW THE PRESENCE OF A WEDGE OF DRY AIR IN THE
MID-LEVELS TO THE NORTH OF THE SYSTEM, WHICH IS IN AGREEMENT WITH
THE SATELLITE DEPICTION OF A LACK OF DEEP CONVECTION ON THE NORTH
SIDE OF THE SYSTEM. AS WAS THE CASE AT 0600Z, THE CIMSS SHEAR
ANALYSIS CONTINUES TO INDICATE SOUTHERLY SHEAR AROUND 15-20 KNOTS,
HOWEVER THE ANIMATED EIR SHOWS THE RECENT FLARE OF CONVECTION BEING
SHEARED TO THE EAST OF THE LLCC, INDICATIVE OF WESTERLY SHEAR. THE
HWRF AND CIMSS ATMOSPHERIC MOTION VECTORS (AMV) CONTINUE TO
INDICATE A SMALL ANTICYCLONE JUST EAST OF SHANGHAI, WITH WESTERLY
WINDS OVER TS 19W. THE ENVIRONMENT, WHILE CURRENTLY MARGINALLY
FAVORABLE, IS BECOMING INCREASINGLY MARGINAL DUE TO THE CONTINUED
PRESENCE OF DRY AIR IMPINGING CONVECTIVE DEVELOPMENT AND INCREASING
SHEAR OFFSETTING WARM SSTS AND ROBUST POLEWARD OUTFLOW.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: NORTHERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) TO THE SOUTH. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   RJTD: T3.0 - 45 KTS
   RCTP: T3.5 - 55 KTS
   KNES: T3.5 - 55 KTS
   CIMSS SATCON: 49 KTS AT 161032Z
   CIMSS ADT: 47 KTS AT 161140Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: MID-LEVEL DRY AIR TO THE NORTH. 

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: TS 19W IS NOW FIRMLY ESTABLISHED ON A
NORTHEAST TRACK, AND IS FORECAST TO CONTINUE TO ACCELERATE
NORTHEASTWARD, TURNING MORE EASTWARD THROUGH THE DURATION OF THE
FORECAST, ALONG THE NORTHERN PERIPHERY OF A DEEP-LAYER STR TO THE
SOUTHEAST. THE FORECAST CALLS FOR A LANDFALL ALONG THE NORTHWEST TIP
OF KYUSHU NEAR 170600Z, FOLLOWED BY A TRACK THROUGH THE INLAND SEA
AND CROSSING JUST SOUTH OF TOKYO BY 181200Z BEFORE CONTINUING INTO
THE WESTERN NORTH PACIFIC. THE WINDOW OF OPPORTUNITY FOR THE SYSTEM 
TO TAKE ADVANTAGE OF THE MARGINALLY FAVORABLE CONDITIONS IS GETTING 
SHORTER EVERY HOUR, AND THE FORECAST NOW CALLS FOR ASTEADY 50 KNOTS 
INTENSITY THROUGH LANDFALL. HOWEVER, THE SYSTEM STILL HAS A FEW 
HOURS WHERE THE RELATIVELY LOW VWS COULD BE OFFSET BY THE ROBUST 
POLEWARD OUTFLOW AND AS THIS COINCIDES WITH THE LOCAL DIURNAL 
CONVECTIVE MAXIMUM, COULD ALLOW FOR A BRIEF EXCURSION TO 55 KNOTS 
BETWEEN WARNING TIMES. BY TAU 12 THOUGH, THE SYSTEM WILL START TO 
FEEL THE FULL EFFECTS OF INCREASING SHEAR AND BEGIN TO WEAKEN PRIOR 
TO LANDFALL. ONCE ASHORE THE SYSTEM WILL STEADILY WEAKEN UNDER THE 
COMBINED EFFECTS OF DRAMATICALLY INCREASED SHEAR AND TERRAIN 
INTERACTION. BY TAU 24, TS 19W WILL START TO INTERACT WITH A WEAK 
BAROCLINIC ZONE AND BEGIN EXTRA-TROPICAL TRANSITION (ETT). THE 
SYSTEM IS FORECAST TO COMPLETE ETT BY TAU 48 AS IT APPROACHES THE 
TOKYO METRO AREA, WITH AN EXPANDING AND ASYMMETRIC WIND FIELD AND 
DECREASED CONVECTION. AS WITH THE PREVIOUS FORECAST, THERE REMAINS 
AN ALTERNATE SCENARIO, WITH SEVERAL GLOBAL MODELS CONTINUING TO 
INDICATE THE DEVELOPMENT OF A SECONDARY LOW PRESSURE AREA WHICH 
SPLITS OFF OF TS 19W AFTER LANDFALL AND TRACKS TO THE WEST OF 
HONSHU, ULTIMATELY TRANSITIONING INTO AN EXTRA-TROPICAL LOW
EAST OF MISAWA. MEANWHILE TS 19W CONTINUES TRACKING EASTWARD THROUGH
THE INLAND BUT DOES NOT MOVE RAPIDLY NORTHEASTWARD WHILE UNDERGOING
ETT, BUT RATHER TURNS SOUTH AND STALLS SOUTH OF TOKYO AS A WEAK
EXTRA-TROPICAL OR SUBTROPICAL LOW, WHICH DISSIPATES BY TAU 72. AT
THIS TIME THE FORECAST PHILOSOPHY HAS NOT CHANGED AND REMAINS
CONSISTENT WITH AN ETT TO THE EAST OF TOKYO.  

MODEL DISCUSSION: TRACK GUIDANCE IS IN GOOD AGREEMENT THROUGH TAU
24, WITH ALL GUIDANCE CLOSELY GROUPED IN A 30NM ENVELOPE AT
LANDFALL, INCREASING TO 50NM BY TAU 24. AFTER TAU 24, THE GUIDANCE
DIVERGES RAPIDLY IN BOTH SPEED AND DIRECTION, WITH THE HWRF AND
GALWEM TRACKING ALONG WESTERN HONSHU BEFORE EMERGING INTO THE
WESTERN PACIFIC SOUTH OF HONSHU. THE NAVGEM, GFS, ECMWF AND EGRR
TRACK THE SYSTEM ON A FLATTER AND SLOWER TRAJECTORY KEEPING THE
CENTER SOUTH OF HONSHU AFTER TAU 36 AND TURNING IT SOUTHWARD BY TAU
72. THE REMAINDER OF THE GUIDANCE INCLUDING THE GFS, ECMWF AND
UKMET ENSEMBLE MEANS ARE CLOSELY CLUSTERED AROUND THE JTWC FORECAST
TRACK. THE ENSEMBLE MEMBERS CONTINUE TO SHOW AN EXTREME AMOUNT OF
UNCERTAINTY WITH INDIVIDUAL MEMBERS EXTENDING FROM KYUSHU TO EAST
OF KAMCHATKA. NEAR-TERM FORECAST CONFIDENCE THROUGH TAU 24 IS HIGH.
HOWEVER, IN LIGHT OF THE VERY LARGE SPREAD IN ENSEMBLE GUIDANCE AS
WELL AS THE SIGNIFICANT DIFFERENCES IN DETERMINISTIC GUIDANCE,
THERE IS OVERALL LOW CONFIDENCE IN THE FORECAST TRACK. INTENSITY
GUIDANCE REMAINS IN GOOD AGREEMENT AND CLOSELY TRACKS THE CONSENSUS
MEAN. THE JTWC FORECAST REMAINS SLIGHTLY HIGHER THAN THE MEAN, BUT
WITH THE UNCERTAINTY SURROUNDING THE TRACK FORECAST, THERE IS ONLY
LOW CONFIDENCE IN THE OVERALL INTENSITY FORECAST. 

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