WDPN34 PGTW 152100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 27W (USAGI) WARNING NR 
020//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 22.3N 120.0E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 187 NM SOUTH-SOUTHWEST OF TAIPEI, TAIWAN
   MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 15 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
STORM 27W (USAGI) JUST SOUTHWEST OF TAIWAN WITH A VERY RAGGED
APPEARANCE. INCREASING VERTICAL WIND SHEAR HAS WEAKENED THE VORTEX
CONSIDERABLY. ENVIRONMENTAL ANALYSIS INDICATES THAT 27W IS IN AN
UNFAVORABLE ENVIRONMENT CHARACTERIZED BY WEAK POLEWARD OUTFLOW
ALOFT, MODERATE TO HIGH (20-25 KTS) VERTICAL WIND SHEAR, AND
COOLING (25-26 C) SEA SURFACE TEMPERATURES. THE INITIAL POSITION IS
PLACED WITH LOW CONFIDENCE BASED ON THE RAGGED APPEARANCE IN
ANIMATED EIR IMAGERY AND LACK OF RECENT MICROWAVE IMAGERY TO AID IN
POSITIONING. THE INITIAL INTENSITY OF 35 KTS IS ASSESSED WITH
MEDIUM CONFIDENCE BASED ON THE AGENCY DVORAK FINAL-T NUMBERS QUICKLY 
FALLING.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: LOW TO MID-LEVEL SUBTROPICAL RIDGE
(STR) POSITIONED TO THE EAST, WITH A WEAK EXTENSION TO THE NORTH.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   RJTD: T3.0 - 45 KTS
   RCTP: T2.0 - 30 KTS
   KNES: T3.0 - 45 KTS
   DEMS: T3.0 - 45 KTS
   CIMSS DPRINT: 32 KTS AT 151800Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 20-25 KTS
   SST: 25-26 CELSIUS
   OUTFLOW: WEAK POLEWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT FROM THE WEST.

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: 27W IS FORECAST TO TRACK NORTHEASTWARD AND
MAKE LANDFALL ALONG THE SOUTHWESTERN COAST OF TAIWAN AS IT
CONTINUES TO WEAKEN. INCREASING TERRAIN INTERACTION, HIGH VERTICAL
WIND SHEAR, AND DRY AIR WILL ALL CONTRIBUTE TO DEGRADING THE
VORTEX. DISSIPATION OVER TAIWAN NEAR TAU 36 IS CURRENTLY
FORECASTED, BUT THERE IS LARGE UNCERTAINTY IN THE ACTUAL TRACK 27W
TAKES. 

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN EXTREMELY POOR
AGREEMENT AFTER TAU 12 AS MODELS FAN OUT NEARLY IN EVERY DIRECTION
DUE TO THE WEAKENED VORTEX. THE JTWC TRACK FORECAST IS PLACED WITH
VERY LOW CONFIDENCE AS A RESULT. INTENSITY GUIDANCE IS IN BETTER
AGREEMENT, SUGGESTING CONTINUED WEAKENING AS MODELS QUICKLY LOSE
THE VORTEX. THE JTWC INTENSITY FORECAST IS PLACED WITH MEDIUM
CONFIDENCE DUE TO THIS.

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