WDPN31 PGTW 130900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 15W (MERBOK) 
WARNING NR 011//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 24.7N 163.1E
   INITIAL INTENSITY: 55 KTS
   GEOGRAPHIC REFERENCE: 378 NM NORTH-NORTHWEST OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 22 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS FLARING DEEP
CONVECTION IN THE CORE OF THE SYSTEM AND IMPROVING CLOUD BANDS
WRAPPING INTO THE LLCC. A 130420Z SSMIS 91GHZ MICROWAVE IMAGE SHOWS
A PARTIAL PASS OF THE SYSTEM BUT DOES INDICATE THAT 15W HAS A
SINGLE SOLID BAND OF DEEP CONVECTION WRAPPING AROUND THE SOUTHERN
SEMICIRCLE, BUT FRAGMENTED BANDING TO ITS NORTH AND EAST. THE
INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON MSI AND
SSMIS IMAGERY. THE INITIAL INTENSITY OF 55 KTS IS ASSESSED WITH
MEDIUM CONFIDENCE BASED ON A SPREAD OF AGENCY DVORAK INTENSITY
ESTIMATES AND CIMSS INTENSITY ESTIMATES SHOWING AN ENVELOPE OF
47-65 KTS.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A SUB-TROPICAL
RIDGE (STR) TO THE EAST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.0 - 65 KTS
   RJTD: T3.5 - 55 KTS
   KNES: T4.0 - 65 KTS
   CIMSS SATCON: 61 KTS AT 130206Z
   CIMSS ADT: 47 KTS AT 130530Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE RADIAL

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   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.

FORECAST DISCUSSION: TS 15W IS FORECAST TO CONTINUE TRACKING ON THE
WESTERN PERIPHERY OF THE STR TO THE EAST. AS THE SYSTEM ROUNDS THE
STR AXIS, A GENERALLY NORTH-NORTHWESTWARD TRACK WILL BECOME
NORTH-NORTHEASTWARD. AS THIS TAKES PLACE, 15W IS FORECAST TO
CONTINUE INTENSIFYING IN A FAVORABLE ENVIRONMENT CHARACTERIZED BY
WARM SEA SURFACE TEMPERATURES (SST) (29-30C), LOW VERTICAL WIND
SHEAR (VWS) (10-15KTS) AND IMPROVING OUTFLOW ALOFT. AFTER TAU 36,
AFTER REACHING A POTENTIAL PEAK INTENSITY OF 85 KTS, SST VALUES
WILL BEGIN TO COOL AND VWS WILL INCREASE AS A MID LATITUDE TROUGH
APPROACHES FROM THE WEST. AFTER TAU 48, 15W WILL INCREASE ITS
INTERACTION WITH THIS TROUGH AND PROGRESS THROUGH EXTRA-TROPICAL
TRANSITION (ETT). BY TAU 72, NOW FULLY DEVOID OF TROPICAL
CHARACTERISTICS AND STILL TYPHOON STRENGTH 15W WILL COMPLETE ETT.

MODEL DISCUSSION: NUMERICAL MODELS ARE IN TIGHT AGREEMENT THROUGH
THE FORECAST PERIOD WITH A 120NM SPREAD AT TAU 72. FOR THIS REASON
THE JTWC FORECAST TRACK IS PLACED WITH HIGH CONFIDENCE. RELIABLE
MODEL INTENSITY GUIDANCE IS IN FAIR AGREEMENT THROUGH TAU 36 WITH
ALL MEMBERS SHOWING INTENSIFICATION. AFTER TAU 36, COAMPS-TC (GFS
VERSION) PLUMMETS FROM 85KTS TO 25KTS IN 24 HRS, WHILE THE
REMAINING GUIDANCE FOLLOWS THE JTWC FORECAST INTENSITY AS THE
SYSTEM PROGRESSES THOUGH ETT. FOR THESE REASONS THE JTWC FORECAST
INTENSITY IS PLACED WITH MEDIUM CONFIDENCE.

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