WDPN32 PGTW 171500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 19W (CHANTHU) WARNING 
NR 046//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 33.8N 131.5E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 42 NM SOUTHWEST OF IWAKUNI, JAPAN
   MOVEMENT PAST 6 HOURS: EASTWARD AT 20 KTS
   SIGNIFICANT WAVE HEIGHT: 16 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TS 19W HAS CONTINUED TO ACCELERATE EASTWARD ALONG THE NORTHERN
PERIPHERY OF THE DEEP STR TO THE SOUTH AND AHEAD OF A 500MB
SHORTWAVE TROUGH. ANIMATED ENHANCED INFRARED (EIR) SATELLITE
IMAGERY DEPICTED AN AREA OF PERSISTENT CONVECTION WHICH OBSCURED
THE LOW LEVEL CIRCULATION CENTER (LLCC). DUE TO THE LOSS OF HIGH
RESOLUTION VISIBLE IMAGERY AND THE OBSCURED LLCC, AGENCY FIXES WERE
WIDELY SCATTERED WITH PGTW AND KNES OVER LAND SOUTHEAST OF FUKUOKA,
WHILE RCTP WAS WELL TO THE EAST. HOWEVER, THE DENSE JAPANESE SURFACE
OBSERVATION NETWORK AND RADAR DATA PROVIDED HIGH CONFIDENCE TO THE
INITIAL POSITION, SITUATED AT 1200Z JUST WEST OF HIMEJIMA ISLAND IN
THE SUO NADA. THE INITIAL INTENSITY IS ASSESSED AT A GENEROUS 40
KNOTS WITH MEDIUM CONFIDENCE. OVER WATER AGENCY FIXES WERE 2.5, AS
WAS THE ADT. AND WHILE SURFACE WIND REPORTS WERE RELATIVELY LOW,
PRESSURE READINGS AROUND 994MB FROM STATIONS SURROUNDING THE SUO
NADA SUPPORT A 38-40 KNOT INTENSITY USING THE KNAFF-ZEHR WIND
PRESSURE RELATIONSHIP. ADDITIONALLY, A PARTIAL 171106Z ASCAT-A PASS
SHOWED 30-35KT SOUTHERLY WINDS IN THE BUNGO CHANNEL EXTENDING
NORTHWARD ALONG THE EASTERN EDGE OF THE CIRCULATION. THE SYSTEM IS
MOVING IN PHASE WITH A 500MB SHORTWAVE TROUGH, THUS PROVIDING A
SOURCE OF ROBUST, DIVERGENT POLEWARD OUTFLOW. SHEAR IS STEADILY
INCREASING AND NOW THAT THE SYSTEM IS TRACKING THROUGH AN INLAND
SEA SURROUNDED BY RUGGED TERRAIN IT WILL STEADILY WEAKEN FROM THIS
POINT FORWARD IN AN INCREASINGLY UNFAVORABLE ENVIRONMENT.

INITIAL WIND RADII BASIS: PERSISTENCE AND SURFACE OBSERVATIONS. 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: NO CURRENT ESTIMATE
   RJTD: T2.5 - 35 KTS
   RCTP: T2.5 - 35 KTS
   CIMSS ADT: 35 KTS AT 171140Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 25-30 KTS
   SST: 24-25 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: TERRAIN INTERACTION AND DRY AIR ENTRAINMENT. 

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: LOW

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING. THE FORECAST HAS BEEN
TRUNCATED TO 48 HOURS. 

FORECAST DISCUSSION: TS 19W IS FORECAST TO CONTINUE TRACKING
EASTWARD THROUGH THE DURATION OF THE FORECAST PERIOD ALONG THE
NORTH SIDE OF THE SUBTROPICAL RIDGE. OVER THE NEXT 24 HOURS, TS 19W
WILL WEAKLY INTERACT WITH THE BAROCLINIC ZONE, AND WILL BEGIN
EXTRA-TROPICAL TRANSITION (ETT) AS IT MOVES IN PHASE WITH THE 500MB
TROUGH. THE PREVIOUSLY MENTIONED ASCAT PASS INDICATED THE PRESENCE
OF A SHARP SURFACE TROUGH EXTENDING TO THE NORTHEAST INTO THE
EASTERN SEA OF JAPAN. GLOBAL MODELS CONTINUE TO INDICATE A SHIFT IN
THE PRIMARY VORTICITY SIGNATURE TO THE NORTH ALONG THIS TROUGH,
LEADING TO DEVELOPMENT OF A SECONDARY OR TRIPLE POINT LOW PRESSURE
SYSTEM WEST OF CENTRAL HONSHU. AS 19W CONTINUES TO WEAKEN IT WILL
BECOME INCREASINGLY SHALLOW AND THE MID-LEVEL TROUGH WILL PULL OUT
AHEAD OF THE SYSTEM BY TAU 36, WITH THE SECONDARY LOW BECOMING THE
DOMINANT SYSTEM TO THE NORTH WITH A WEAK FRONTAL BAND EXTENDING
SOUTHWARD TO TS 19W. ONCE SOUTH OF TOKYO, THE 500MB TROUGH HAVING
OUTRUN IT, TS 19W WILL RESIDE UNDER UPPER-LEVEL CONVERGENT FLOW
BEHIND THE TROUGH, LEADING TO STEADY WEAKENING AS AN EXTRA-TROPICAL
LOW SOUTHEAST OF TOKYO. THE SYSTEM COMES UNDER THE LOW LEVEL
STEERING INFLUENCE OF A STRONG HIGH PRESSURE AREA TO THE NORTH AND
THE REMNANTS OF TS 19W WILL TURN SOUTHWARD BY TAU 48. 

MODEL DISCUSSION: TRACK GUIDANCE IS IN GOOD AGREEMENT THROUGH THE
NEXT 12 HOURS, BUT DIVERGES SIGNIFICANTLY THEREAFTER AS THE
DETERMINISTIC MODEL GUIDANCE STRUGGLES WITH THE WEAKENING VORTEX
AND THE EXTRA-TROPICAL TRANSITION. THE GALWEM AND HWRF REMAIN THE
POLEWARD OUTLIERS, TAKING THE SYSTEM ON A STRAIGHT NORTHEAST TRACK
ON THE WEST SIDE OF THE JAPANESE ALPS. THE REMAINDER OF THE
GUIDANCE GENERALLY AGREES ON A SLOWING TRACK TO THE EAST FOLLOWED
BY A TURN TO THE SOUTH AROUND TAU 36. THE JGSM AND NAVGEM SHOW THE
FASTEST AND MOST AGGRESSIVE TURN SOUTHWARD BY TAU 24, WHILE THE
REMAINDER OF THE CONSENSUS MEMBERS INDICATE A SHALLOWER TURN BY TAU
36. THE JTWC FORECAST LIES SOUTH OF THE CONSENSUS MEAN THROUGH TAU
48 WITH MEDIUM CONFIDENCE. INTENSITY GUIDANCE IS IN GENERAL
AGREEMENT THOUGH MOST TRACKERS INDICATE A FASTER RATE OF WEAKENING
THAN THE JTWC FORECAST, WHICH TRACKS THE SHAPE OF THE INTENSITY
TREND WHILE REMAINING ABOVE IT THROUGH THE DURATION OF THE
FORECAST. DUE TO THE UNCERTAINTIES OF THE TERRAIN INTERACTION AND
THE TIMING OF ONSET OF THE CONVERGENT FLOW ALOFT, THERE IS MEDIUM
CONFIDENCE IN THE JTWC INTENSITY FORECAST.  

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