WDPN33 PGTW 130900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 08W (AMPIL) WARNING NR 
004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 25.1N 138.5E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 615 NM SOUTH OF YOKOSUKA, JAPAN
   MOVEMENT PAST 6 HOURS: NORTHEASTWARD AT 05 KTS
   SIGNIFICANT WAVE HEIGHT: 17 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A LLCC FULLY
OBSCURED BY A RECENTLY DEVELOPED CDO. THE LACK OF RECENT MICROWAVE
IMAGERY PRECLUDES AN ACCURATE ASSESSMENT OF THE LOW LEVEL
STRUCTURE, BUT IT ASSUMED THAT THE PREVIOUSLY DOCUMENTED PRECESSION
OF THE VORTEX IS CONTINUING WITH VERTICAL ALIGNMENT OF THE VORTEX
FROM ITS PREVIOUSLY SOUTHWARDLY TILTED STRUCTURE. THE INITIAL
POSITION IS PLACED WITH LOW CONFIDENCE BASED ON THE ANIMATED
INFRARED AND A 130428Z ATMS 88.2GHZ MICROWAVE IMAGERY. THE SYSTEM
PRESENTLY IS ASSOCIATED WITH SPIRAL BANDS OF CONVECTION PRIMARILY
TO THE EAST AND SOUTH, WITH AN INTENSE AND QUICK DEVELOPMENT OF A
CDO FEATURE. THE INITIAL INTENSITY OF 40 KTS IS ASSESSED WITH LOW
CONFIDENCE BASED ON EARLIER SCATTEROMETRY DATA AS WELL AS
PERSISTENCE, AND SUBJECTIVE AND OBJECTIVE INTENSITY ESTIMATES
GENERALLY IN LINE WITH THE OVERALL ASSESSMENT. ANALYSIS REVEALS A
FAVORABLE ENVIRONMENT WITH VERY WARM SSTS, GOOD EQUATORWARD OUTFLOW
AND LOW VERTICAL WIND SHEAR.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: NEAR-EQUATORIAL RIDGE TO THE SOUTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T3.0 - 45 KTS
   KNES: T3.0 - 45 KTS
   CIMSS SATCON: 37 KTS AT 130430Z
   CIMSS ADT: 39 KTS AT 130600Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 31-32 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: LOW
   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: TS 08W (AMPIL) IS CURRENTLY TRACKING
NORTHEASTWARD DRIVEN PRIMARILY BY THE NEAR-EQUATORIAL RIDGE (NER)
CENTERED OVER CNMI. THE NER OPENS UP INTO RIDGING AFTER TAU 24,
WHICH ALONG WITH THE PASSAGE OF TS 09W TO THE NORTHEAST, WILL
WEAKEN THE STEERING GRADIENT AND ALLOWING TS 09W TO SLOW DOWN
SLIGHTLY. A STRONG STR LOCATED NEAR 40N WILL THEN BEGIN MOVING
SOUTHWARD, AND BECOME THE DOMINANT STEERING INFLUENCE AROUND
TAU 36. TS 08W WILL TURN NORTHWARD AND ACCELERATE AFTER TAU 36,
HEADING TOWARDS THE TOKYO AREA. THE SYSTEM REACHES THE AXIS OF THE
STR AROUND TAU 72, THEN TURNS SHARPLY NORTHEASTWARD AS IT PASSES
THE RIDGE AXIS AND CONTINUES EAST-NORTHEASTWARD ALONG THE NORTHWEST
SIDE OF THE STR THROUGH THE END OF THE FORECAST PERIOD.  THE TIMING
OF THE PASSAGE OF THE RIDGE AXIS WILL DETERMINE HOW QUICKLY AMPIL
WILL MAKE A NORTH-EASTWARDLY TURN AND HOW CLOSE THE CPA TO MAINLAND
JAPAN WILL BE. IN THE NEAR-TERM, THE ASYMMETRIC VORTEX STRUCTURE
AND AN EXPECTED INFLUX OF DRY AIR FROM THE WEST AROUND TAU 24, WILL
KEEP A LID ON THE PACE OF INTENSIFICATION UP TO TAU 36. THERAFTER,
TS 08W WILL BE UNDER INCREASINGLY FAVORABLE CONDITIONS, INCLUDING
LOW VERTICAL SHEAR, POTENTIALLY DEVELOPING AN UPPER-LEVEL
ANTICYCLONE, ENABLING FOR MORE INTENSE OUTFLOW IN THE POLAR
DIRECTION, AS WELL AS VERY HIGH SEA SURFACE TEMPERATURES, NEARLY
ALL THE WAY TOWARDS MAINLAND JAPAN - 30-31 DEGREES CELSIUS. UNDER
THOSE CIRCUMSTANCES, THE SYSTEM IS EXPECTED TO RAPIDLY INTENSIFY
TAU 48 AND TAU 72, REACHING INTENSITY MAXIMUM OF 100 KTS SHORTLY
WHILE APPROACHING CPA TO JAPAN. OF NOTE, AS THE CURRENT CPA TO
JAPAN FALLS BETWEEN TWO TAUS, THE ACTUAL CPA MAY FALL CLOSER TO
MAINLAND JAPAN THAN DEPICTED BY THE TRACK INTERPOLATION.

MODEL DISCUSSION: CURRENT MODEL GUIDANCE IS IN GOOD AGREEMENT WITH
CROSS-TRACK SPREAD OF 140 NM AND ALONG TRACK SPREAD OF 145 AT TAU
72. PAST TAU 72 THE AGREEMENT CONTINUES TO BE GOOD, HOWEVER THERE
IS A DISCREPANCY IN REGARD TO HOW THE TIMING OF SYSTEM INTERACTION
WITH THE RIDGING TO THE EAST WILL IMPACT THE POTENTIAL FOR LANDFALL
OVER JAPAN. JTWC TRACK FORECAST IS STILL LEANING CLOSE TOWARDS THE
MULTI-MODEL CONSENSUS. ENSEMBLE GUIDANCE (ECENS AND GEFS) CONTINUE
TO SHOW A ROUGHLY 50-50 SPLIT WITH HALF OF THE MEMBERS TRACKING THE
SYSTEM WEST OF TOKYO WHILE THE OTHER HALF REMAIN OVER WATER. THE
ENSEMBLE MEANS ARE ROUGHLY CONSISTENT HOWEVER WITH THE MULTI-MODEL
CONSENSUS AND THE JTWC FORECAST TRAKC. INTENSITY GUIDANCE IS COMING
INTO BETTER AGREEMENT, BUT STILL SHOWS A SPREAD IN PEAK INTENSTIES
OF ABOUT 40 KNOTS (70-110 KTS). THE JTWC FORECAST IS ON THE HIGH
END OF THE GUIDANCE, THOUGH BOTH THE HWRF AND CTCX MODELS DEPICT A
SLIGHTLY HIGHER PEAK. THE JTWC INTENSITY FORECAST PEAKS
APPROXIMATELY 20 KTS HIGHER, DUE TO THE INCREASINGLY FAVORABLE
ENVIRONMENTAL CONDITIONS AND THE HIGH PROBABILITY OF RI. 

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