WDPN31 PGTW 070900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (TWENTYONE) WARNING 
NR 005//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 21.7N 146.5E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 342 NM EAST-SOUTHEAST OF IWO TO
   MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 19 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A PARTIALLY
EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC) WITH DEEP CONVECTION
DISPLACED EAST OF CENTER DUE TO WESTERLY 20-25KT VERTICAL WIND
SHEAR (VWS). WESTERLY DRY AIR ASSOCIATED WITH A DEEP-LAYER TROUGH
COMPETES AGAINST EASTERLY MOIST AIR TO ERODE THE VORTEX.
UPPER-LEVEL DIVERGENCE REMAINS FAVORABLE DUE TO UPPER-LEVEL RIDGING
ENHANCING EASTWARD OUTFLOW. A 070618Z F18 SSMIS COLOR-ENHANCED
89GHZ MICROWAVE IMAGE HIGHLIGHTS THE SHEARED AND EXPOSED NATURE OF
THE SYSTEM WITH CONVECTIVE BANDING WRAPPING AROUND THE EASTERN SIDE
OF TROPICAL STORM (TS) 21W. THE INITIAL POSITION IS PLACED WITH
HIGH CONFIDENCE BASED ON THE LLCC IDENTIFIED IN THE AFOREMENTIONED
MSI AND MICROWAVE IMAGERY. THE INITIAL INTENSITY OF 40 KTS IS
ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE AGENCY AND OBJECTIVE
FIXES LISTED BELOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: A SUBTROPICAL RIDGE (STR) TO THE EAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T2.0 - 30 KTS
   CIMSS SATCON: 37 KTS AT 070405Z
   CIMSS ADT: 37 KTS AT 070530Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 20-25 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: STRONG EASTWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE SLOW TRACK SPEED OVER THE LAST
SIX HOURS HAS DELAYED THE ANTICIPATED EXTRATROPICAL TRANSITION
(ETT) IN THE LATE-TERM FORECAST. OTHERWISE, THERE ARE NO
SIGNIFICANT CHANGES FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TS 21W WILL CONTINUE TO BE DRIVEN
NORTHEASTWARD UNDER THE STEERING INFLUENCE OF A SUBTROPICAL RIDGE
TO THE SOUTHEAST THROUGH THE REMAINDER OF THE FORECAST PERIOD. THE
ALONG-TRACK SPEED WILL INCREASE SIGNIFICANTLY AFTER TAU 48 DUE TO
CONSTRUCTIVE INTERFERENCE ENHANCING THE STEERING FLOW AS A
DEEP-LAYER TROUGH APPROACHES FROM THE WEST. MODERATE TO HIGH
VERTICAL WIND SHEAR WILL CONTINUE TO CURB THE INTENSIFICATION RATE
OF TS 21W THROUGH TAU 48. THE HEIGHTENED UPPER-LEVEL DIVERGENCE AND
WARM SEA SURFACE TEMPERATURES (28-29C) WILL OVERTAKE THE
UNFAVORABLE VERTICAL WIND SHEAR ALLOWING SLOW DEVELOPMENT OVER
TIME. AFTER TAU 72, BAROCLINIC FORCING ASSOCIATED WITH AN
UPPER-LEVEL TROUGH WILL BEGIN TO INFLUENCE THE SYSTEM, AND IT WILL
START ETT BETWEEN TAU 72-96. WHILE TRANSITIONING, THE SYSTEM WILL
INTENSIFY TO A PEAK OF 65KTS AT TAU 96, AND WEAKEN SLIGHTLY BY TAU
120 WHEN THE SYSTEM BECOMES FULLY EXTRATROPICAL.

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE IS IN GOOD AGREEMENT
IN THE NEAR-TERM BUT OPENS FROM A CROSS-TRACK SPREAD OF 175NM AT
TAU 96 TO 439NM AT TAU 120. THE SPREAD AFTER TAU 96 CAN BE
ASSOCIATED WITH DIFFERING INTERPRETATIONS OF HOW STRONGLY THE
UPPER-LEVEL TROUGH WILL INFLUENCE THE TRACK OF TS 21W, AND HOW FAR
EAST OR WEST THE RIDGE TO THE EAST WILL INFLUENCE THE TRACK. THE
INTENSITY GUIDANCE IS IN GOOD AGREEMENT WITH A SPREAD OF 15KTS
BETWEEN THE PEAK INTENSITIES AT TAU 96. THE GFS SOLUTION REMAINS
THE LOWEST THROUGHOUT THE FORECAST PERIOD WITH A MAXIMUM INTENSITY
OF 60KTS AT TAU 96, WHILE HAFS-A REFLECTS A SIMILAR LOW INTENSITY
IN THE EARLY-TERM FORECAST AND HAS A HIGHER PEAK OF 80KTS AT TAU
96. THE JTWC FORECAST IS CLOSEST TO GFS DUE TO THE EXPECTED
PERSISTENT SHEAR AND DRY AIR IMPACTING INTENSIFICATION FOR THE NEXT
120 HOURS.

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