WDPN31 PGTW 061500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (TWENTYONE) 
WARNING NR 002//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 18.5N 145.5E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 21 NM SOUTH-SOUTHWEST OF AGRIHAN
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS
CONVECTIVE BANDING WRAPPING AROUND THE NORTHERN PERIPHERY OF
TROPICAL DEPRESSION 21W. DEEP CONVECTION CONTINUES TO BUILD AND
OBSCURE THE LOW-LEVEL CIRCULATION CENTER (LLCC). THE INITIAL
POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON A 071104Z METOP-B
ASCAT IMAGE DEPICTING A DISTINCT LLCC WITH A BAND OF WINDS REACHING
35KTS WRAPPING AROUND THE NORTHEAST PERIPHERY. THE INITIAL
INTENSITY OF 30 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE
AFOREMENTIONED SCATTEROMETRY DATA WITH SUPPORT FROM THE AGENCY AND
OBJECTIVE FIXES LISTED BELOW.

INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS
   RJTD: T1.0 - 25 KTS
   KNES: T1.5 - 25 KTS
   CIMSS ADT: 28 KTS AT 061140Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 10-15 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE EASTWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TROPICAL DEPRESSION (TD) 21W WILL CONTINUE TO
TRACK NORTHEASTWARD UNDER THE STEERING INFLUENCE OF A SUBTROPICAL
RIDGE (STR) TO THE SOUTHEAST. BETWEEN TAU 72-120, THE SYSTEM WILL
BECOME INFLUENCED BY A DEEP-LAYER TROUGH FROM THE NORTHWEST,
ACCELERATING THE TRACK SPEED AS THE STEERING ENVIRONMENT TRANSFERS
TO A DEEP-LAYER RIDGE TO THE NORTHEAST. FOR THE DURATION OF THE
FORECAST, VERTICAL WIND SHEAR WILL BE MODERATE WHILE LOCATED UNDER
A SHORTWAVE RIDGE. WESTERLY DRY AIR IS EXPECTED CONTRIBUTE TO THE
LIMITED INTENSIFICATION RATE THROUGHOUT THE FORECAST PERIOD. THE
UPPER-LEVEL ENVIRONMENT WILL BE FAVORABLE UNTIL TAU 72, WHEN TD 21W
APPROACHES AN UPPER-LEVEL TROUGH, CAUSING THE UPPER-LEVEL
ENVIRONMENT TO BECOME UNFAVORABLE AND CONFUSED. BAROCLINIC FORCING
WILL ALLOW THE SYSTEM TO CONTINUE TO INTENSIFY AFTER TAU 96 WHEN THE
SYSTEM ENTERS THE BAROCLINIC ZONE AND BECOMES EMBEDDED IN THE
UPPER-LEVEL JET TO THE NORTH. TD 21W WILL BE FULLY EXTRATROPICAL BY
TAU 120.

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE IS IN GOOD AGREEMENT
THAT THE SYSTEM WILL TRACK NORTHEASTWARD THOUGH THE END OF THE
FORECAST PERIOD. THE CROSS-TRACK SPREAD BY TAU 120 IS 220NM,
CONTRIBUTING TO THE HIGH CONFIDENCE IN THE JTWC FORECAST. THE
INTENSITY GUIDANCE IS IN GOOD AGREEMENT THAT INTENSIFICATION WILL
STRUGGLE IN THE EARLY-TERM FORECAST BUT BEGIN TO SPREAD AROUND TAU
72 WHEN THE MODEL SOLUTIONS INTERPRET DIFFERENT INTERACTIONS WITH
THE UPPER-LEVEL TROUGH. COAMPS-TC AND HAFS-A BOTH DEPICT HIGHER
POTENTIAL INTENSITIES WHILE THE JTWC FORECAST LIES CLOSER TO THE
MULTI-MODEL CONSENSUS MEAN.

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