WDPN32 PGTW 090900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 13W (PEILOU) WARNING NR 
010//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 21.2N 142.7E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 229 NM SOUTH-SOUTHEAST OF IWO TO
   MOVEMENT PAST 6 HOURS: EAST-SOUTHEASTWARD AT 13 KTS
   SIGNIFICANT WAVE HEIGHT: 18 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
STORM (TS) 13W (PEILOU) AS A WELL-DEFINED, ALBEIT SHEARED,
PARTIALLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC), WITH THE
DEEP CONVECTION CONFINED TO THE SOUTHEASTERN SEMICIRCLE. THE
090602Z SSMIS 91GHZ IMAGE REVEALS THE SYSTEM IS VERY COMPACT, WITH
DEFINED LOW-LEVEL CLOUD BANDING AND A DISCONTINUOUS AND DISORGANIZED 
BAND OF DEEP CONVECTION EXTENDING FROM THE EASTERN QUADRANT AND 
WRAPPING TO THE SOUTH. ENVIRONMENTAL ANALYSIS REVEALS A MARGINALLY 
FAVORABLE ENVIRONMENT, CHARACTERIZED BY LOW TO MODERATE (15-20 KTS) 
VERTICAL WIND SHEAR (VWS). WARM (29-30 C) SEA SURFACE TEMPERATURES 
(SST) AND STRONG EQUATORWARD OUTFLOW, OFFSET BY A RELATIVELY DRY 
ENVIRONMENT ALONG THE NORTHERN PERIPHERY OF THE SYSTEM. THE INITIAL 
POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE PARTIALLY EXPOSED 
LLCC IN ANIMATED MSI, SUPPLEMENTED BY THE 090602Z SSMIS MICROWAVE 
DATA. THE INITIAL INTENSITY OF 35 KTS IS ASSESSED WITH HIGH CONFIDENCE 
BASED ON THE 090044Z ASCAT DATA, CIMSS AUTOMATED INTENSITY ESTIMATES, 
AND THE AGENCY DVORAK FIXES LISTED BELOW.

INITIAL WIND RADII BASIS: ESTABLISHED WIND RADII USING THE 090044Z
SCATTEROMETER DATA.

CURRENT STEERING MECHANISM: THE NORTHERN PERIPHERY OF AN ELONGATED
NEAR-EQUATORIAL RIDGE (NER) AND A BELT OF STRONG LOW-LEVEL
WESTERLIES TO THE SOUTH OF THE SYSTEM.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T1.5 - 25 KTS
   CIMSS SATCON: 42 KTS AT 090640Z
   CIMSS ADT: 31 KTS AT 090710Z
   CIMSS AIDT: 30 KTS AT 090640Z
   CIMSS D-MINT: 33 KTS AT 090602Z
   CIMSS D-PRINT: 35 KTS AT 090640Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG EASTWARD

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 13W IS FORECAST TO CONTINUE TRACKING
EAST-SOUTHEASTWARD FOR THE NEXT 12 HOURS, WHILE IT REMAINS ON THE
NORTHERN PERIPHERY OF THE STEERING NER. BY AROUND TAU 12, 13W WILL
CURVE NORTHEASTWARD AS IT GETS TRAPPED BY THE ENHANCED
WEST-NORTHWESTERLY GRADIENT FLOW ASSOCIATED WITH TS 14W (CHAN-HOM),
WHICH WILL ALSO CAUSE AN ACCELERATION IN FORWARD MOTION.
SIMULTANEOUSLY, 13W WILL BE PROPELLED INTO THE STEERING INFLUENCE
OF A BUILDING SUBTROPICAL RIDGE CURRENTLY CENTERED TO ITS
NORTHEAST, WHICH WILL REINFORCE THE NORTHEASTWARD TRACK THROUGH TAU
72. REGARDING INTENSITY, PEILOU IS FORECAST TO STEADILY INTENSIFY
THROUGH TAU 36, AS THE SYSTEM IS ABLE TO FEND OFF DRY AIR
ENTRAINMENT AND MAINTAIN A GOOD EASTWARD OUTFLOW CHANNEL DUE TO AN
UPPER-LEVEL TROUGH POSITIONED TO THE NORTH. AFTER ABOUT TAU 36, THE
UPPER-LEVEL TROUGH IS EXPECTED TO PASS OVER THE SYSTEM, INDUCING
HIGHER SHEAR AND DRY AIR. PEILOU IS FORECAST TO QUICKLY WEAKEN IN
THE INCREASINGLY UNFAVORABLE ENVIRONMENT AND DISSIPATE BY TAU 72.
HOWEVER, SINCE 13W IS A COMPACT AND SHALLOW SYSTEM, IT IS MORE
SUSCEPTIBLE TO ENVIRONMENTAL CHANGES. THEREFORE, IT MAY DISSIPATE
SOONER THAN ANTICIPATED IF THE UPPER-LEVEL TROUGH EXTENDS FURTHER
EQUATORWARD THAN CURRENTLY ANTICIPATED. ALTERNATIVELY, PEILOU MAY
SUSTAIN TROPICAL DEPRESSION INTENSITY BEYOND 72 HOURS IF THE
ENVIRONMENT REMAINS MARGINALLY FAVORABLE PAST TAU 36.

MODEL DISCUSSION: DETERMINISTIC MODEL TRACK GUIDANCE IS IN MODERATE
AGREEMENT THROUGH TAU 72, WITH MINIMAL ALONG-TRACK SPREAD BUT A
CROSS-TRACK SPREAD OF 22NM AT TAU 72. NAVGEM IS THE NOTABLE
OUTLIER, AND DEPICTS A SIGNIFICANT DEVIATION FROM THE REST OF MODEL
GUIDANCE. DISCOUNTING NAVGEM, THE JTWC TRACK FORECAST IS PLACED
WITH MEDIUM CONFIDENCE. RELIABLE INTENSITY GUIDANCE IS IN GOOD
AGREEMENT FOR THE DURATION OF THE FORECAST PERIOD, WITH ALL
RELIABLE MODELS DEPICTING A PEAK BETWEEN 40-50 KTS AT AROUND TAU
24-36. THE JTWC INTENSITY FORECAST IS IN LINE WITH THE HAFS-A, HWRF,
GFS, AND COAMPS-TC SOLUTIONS, AND THE MULTI-MODEL CONSENSUS, AND
IS ASSESSED WITH MEDIUM CONFIDENCE. UNCERTAINTY IN THE INTENSITY
FORECAST STEMS FROM THE PLACEMENT OF THE UPPER-LEVEL TROUGH NEAR
THE END OF THE FORECAST.

FORECAST CONFIDENCE:
   TRACK 00-72 HR: MEDIUM
   INTENSITY 00-72 HR: MEDIUM//
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