WDPN31 PGTW 060900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (TWENTYONE) 
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.8N 145.5E
   INITIAL INTENSITY: 25 KTS
   GEOGRAPHIC REFERENCE: 25 NM SOUTHWEST OF PAGAN
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 11 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS CONVECTION
BUILDING OVER A PREVIOUSLY EXPOSED LOW-LEVEL CIRCULATION CENTER.
LOW-LEVEL FORMATIVE BANDING ENCIRCLES THE LLCC. THE CIRCULATION
APPEARS TO BE DEVELOPING ON THE EASTERN PERIPHERY OF A DEEP-LAYER
CYCLONE. THE SYSTEM SLOWLY DRIFTS NORTHWARD IN A REGION OF HIGH
VERTICAL WIND SHEAR DUE TO CLOSE PROXIMITY TO AN UPPER-LEVEL RIDGE.
THE UPPER-LEVEL ENVIRONMENT IS MODERATELY FAVORABLE FOR
INTENSIFICATION WITH MODERATE EASTWARD OUTFLOW ASSOCIATED WITH THE
FLOW OF THE UPPER-LEVEL RIDGE. EARLIER OCEANSAT-3 OSCAT DATA
REVEALS WIND SPEEDS BETWEEN 25-30KTS IN THE NORTHERN PERIPHERY OF
THE CIRCULATION. THE INITIAL POSITION IS PLACED WITH HIGH
CONFIDENCE BASED ON THE LOW-LEVEL BANDING WRAPPING IN TO THE
OBSCURED LLCC ON THE AFOREMENTIONED ANIMATED MSI. THE INITIAL
INTENSITY OF 25 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 EAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS
   RJTD: T1.0 - 25 KTS
   KNES: T2.0 - 30 KTS

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

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE WARNING GRAPHIC DOES NOT DISPLAY
A SHIP AVOIDANCE AREA FROM TAU 0 TO TAU 24.

FORECAST DISCUSSION: TROPICAL DEPRESSION (TD) 21W WILL TRACK
NORTH-NORTHEASTWARD OVER THE NEXT 24 HOURS ON THE NORTHWESTERN
PERIPHERY OF A STR. THE STEERING ENVIRONMENT WILL TRANSFER TO A
DIFFERENT STR AT TAU 36, AT WHICH TD 21W WILL TRACK SLIGHTLY MORE
NORTHEASTWARD. TD 21W WILL BE IN A MARGINALLY FAVORABLE ENVIRONMENT
DUE TO MODERATE VERTICAL WIND SHEAR THAT IS INDUCED BY AN
UPPER-LEVEL RIDGE THAT WILL PERSIST FOR THE FIRST 72 HOURS OF THE
FORECAST. OTHER ENVIRONMENTAL FACTORS, SUCH AS SEA SURFACE
TEMPERATURE AND UPPER-LEVEL DIVERGENCE WILL BE FAVORABLE. AS THE
RIDGE MOVES EASTWARD, THE ENVIRONMENT FOR INTENSIFICATION WILL BE
CAPPED BY WESTERLY DRY AIR INTRUDING IN TO THE LOW-LEVEL
CIRCULATION CENTER (LLCC) AS WELL BETWEEN TAU 36-72. AN UPPER-LEVEL
TROUGH APPROACHING FROM THE WEST WILL INCREASE VERTICAL WIND SHEAR
AND CAUSE THE UPPER-LEVEL ENVIRONMENT TO BECOME UNFAVORABLE AS THE
SYSTEM REACHES A PEAK OF 50KTS BY TAU 120. 

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE REPRESENTS A
CONSISTENT TREND THAT THE SYSTEM WILL TRACK NORTH-NORTHEASTWARD TO
NORTHEASTWARD THROUGH THE END OF THE FORECAST PERIOD. THE JTWC
MULTI-MODEL CONSENSUS MEMBERS SUPPORT HIGH CONFIDENCE IN THE
FORECAST WITH A LOW 174NM CROSS-TRACK SPREAD BY TAU 120. THE JTWC
INTENSITY GUIDANCE HAS A LARGER SPREAD THAT ONLY OPENS BETWEEN TAU
96-120. THE GFS-BASED COAMPS-TC GENERALLY SUPPORTS SLIGHTLY HIGHER
INTENSIFICATION AT THE BEGINNING OF THE FORECAST WHILE HAFS-A AND
GFS REMAIN LOW, BUT HAFS-A SPIKES TO A PEAK OF 90KTS BETWEEN TAU
96-120. THE JTWC INTENSITY FORECAST IS MADE WITH HIGH CONFIDENCE IN
THE NEAR-TERM, AND MEDIUM CONFIDENCE IN THE LATE-TERM DUE TO THE
SPIKE IN HAFS-A.

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