WDPN31 PGTW 081500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (TWENTYONE) 
WARNING NR 010//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 24.0N 147.4E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 337 NM EAST OF IWO TO
   MOVEMENT PAST 6 HOURS: EASTWARD AT 05 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
DEPRESSION 21W WITH A COMPLETELY EXPOSED LOW-LEVEL CIRCULATION
CENTER (LLCC) AND FLARING CONVECTION ALONG THE EASTERN BOUNDARY OF
THE SYSTEM DISPLACED ABOUT 150 NM FROM THE CENTER. 21W WAS
PREVIOUSLY QUASI-STATIONARY, BUT OVER THE PAST 6 HOURS HAS BEGUN
TRACKING MORE QUICKLY IN THE EASTWARD DIRECTION. ENVIRONMENTAL
ANALYSIS INDICATES THAT 21W IS IN A MARGINALLY FAVORABLE
ENVIRONMENT FOR INTENSIFICATION CHARACTERIZED BY MODERATE POLEWARD
OUTFLOW ALOFT, MODERATE TO HIGH (20-25 KTS) VERTICAL WIND SHEAR,
DRY AIR ENTRAINMENT, AND WARM (28-29 C) SEA SURFACE TEMPERATURES.
THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE
EXPOSED LLCC SEEN IN THE ANIMATED EIR SATELLITE IMAGERY. THE
INITIAL INTENSITY OF 30 KTS IS ASSESSED WITH MEDIUM CONFIDENCE
BASED MAINLY ON THE OBJECTIVE INTENSITY ESTIMATES LISTED BELOW. THE
LACK OF CONVECTION OVER THE CENTER HAS CAUSED AGENCY DVORAK
ESTIMATES TO BE CONSIDERED TOO WEAK.

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

CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A LOW TO MID-LEVEL
RIDGE TO THE EAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: NO CURRENT ESTIMATE
   RJTD: T1.5 - 25 KTS
   CIMSS ADT: 25 KTS AT 081200Z
   CIMSS AIDT: 31 KTS AT 081200Z
   CIMSS DPRINT: 27 KTS AT 081200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 20-25 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD
   OTHER FACTORS: EXTENSIVE DRY AIR TO THE WEST.

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   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: 21W IS FORECAST TO BEGIN TRACKING
NORTH-NORTHEASTWARD ALONG THE WESTERN PERIPHERY OF A LOW TO
MID-LEVEL RIDGE TO THE EAST THROUGH TAU 72. THE ENVIRONMENT IS
EXPECTED TO REMAIN SUBPAR FOR 21W AS DRY AIR AND HIGH VERTICAL WIND
SHEAR PERSIST, HINDERING DEVELOPMENT. AS A RESULT, A GRADUAL
INCREASE TO 40 KTS AT TAU 48 IS FORECAST. AFTER TAU 48, 21W WILL
INTENSIFY TO AROUND 55 KTS AS IT APPROACHES THE BAROCLINIC ZONE DUE
TO GREATLY INCREASED POLEWARD OUTFLOW INTO THE JET. HOWEVER, AFTER
TAU 72 21W IS ANTICIPATED TO WEAKEN IN RESPONSE TO SHARPLY
INCREASING VERTICAL WIND SHEAR AND RAPIDLY DROPPING SEA SURFACE
TEMPERATURES AS IT TRANSITIONS INTO AN EXTRATROPICAL LOW WITH AN
EXPANSIVE WIND FIELD. EXTRATROPICAL TRANSITION IS FORECAST TO BE
COMPLETE AROUND TAU 84 TO 96, SOUTH OF THE KAMCHATKA PENINSULA.  

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE CONTINUES TO BE IN GOOD
AGREEMENT ON THE TRACK OF 21W WITH A 140 NM CROSS-TRACK SPREAD AT
TAU 72. AFTER TAU 72, MODELS BEGIN TO DIVERGE WITH DIFFERING TRACK
SPEEDS AS THE SYSTEM INTERACTS WITH THE JET. THE JTWC TRACK
FORECAST IS PLACED CLOSE TO CONSENSUS WITH HIGH CONFIDENCE THROUGH 
TAU 72 WITH MEDIUM CONFIDENCE THEREAFTER. INTENSITY GUIDANCE IS IN 
MODERATE AGREEMENT WITH COAMPS-TC CONTINUING TO BE THE MOST AGGRESSIVE 
IN THE NEAR TERM, SUGGESTING 60 KTS AT TAU 48. GFS AND HAFS-A AGREE 
ON A MORE GRADUAL INTENSITY TREND, WHICH SEEMS MORE REALISTIC 
CONSIDERING THE ENVIRONMENT, WITH PEAKS OF 45 AND 55 KTS RESPECTIVELY 
AT TAU 72. THE JTWC INTENSITY FORECAST IS PALCED CLOSER TO THE GFS 
AND HAFS-A SOLUTIONS WITH OVERALL MEDIUM CONFIDENCE. 

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