WDPN31 PGTW 130300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (TWENTYONE) 
WARNING NR 003//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 14.6N 151.4E
   INITIAL INTENSITY: 25 KTS
   GEOGRAPHIC REFERENCE: 327 NM EAST OF SAIPAN
   MOVEMENT PAST 6 HOURS: EAST-SOUTHEASTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 10 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) SHOWS THE ASSOCIATED
CONVECTION HAS BECOME SEVERELY SHEARED SOUTHEASTWARD, FULLY EXPOSING 
THE LOW LEVEL CIRCULATION CENTER (LLCC) DUE TO STRONG NORTHWESTERLY 
VERTICAL WIND SHEAR, AS INDICATED BY TRANSVERSE BANDING AROUND THE 
EDGES. THE EIR LOOP ALSO SHOWS A TUTT CELL TO THE NORTH CONTINUES TO 
SUPPRESS CONVECTIVE ACTIVITY ALONG THE NORTHERN PERIPHERY. THE 
INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE EXPOSED 
LLCC IN THE MSI LOOP. THE INITIAL INTENSITY OF 25KTS IS BASED ON 
AGENCY DVORAK ESTIMATES AND IS CONSISTENT WITH THE WEAK CONVECTIVE 
SIGNATURE.

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

CURRENT STEERING MECHANISM: WEAK LOW-LAYER NEAR EQUATORIAL RIDGE TO
THE SOUTH.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS
   RJTD: T1.0 - 25 KTS
   CIMSS ADT: 32 KTS AT 122340Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 25-30 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG EASTWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: TRUNCATED THE FORECAST TO TAU 48 IN
ANTICIPATION OF EARLY DISSIPATION VS EXTRA-TROPICAL TRANSITION.  

FORECAST DISCUSSION: TD 21W WILL BEGIN TO TRACK POLEWARD AS A 
SUBTROPICAL RIDGE BUILDS TO THE EAST AND STEERS IT NORTHEASTWARD THEN 
NORTHWARD. THE MARGINALLY FAVORABLE ENVIRONMENT WILL FUEL WEAK 
INTENSIFICATION TO A PEAK OF 30KTS BY TAU 12. AFTERWARD, VWS 
INCREASING FURTHER, AND CONTINUED SUBSIDENCE FROM THE TUTT CELL WILL 
OFFSET THE WARM SST AND STRONG OUTFLOW, LEADING TO DISSIPATION BY TAU 
48, LIKELY SOONER. COINCIDENTALLY, AROUND TAU 24, A SECONDARY 
CIRCULATION WILL FORM APPROXIMATELY 6 DEGREES TO THE NORTHEAST. 

MODEL DISCUSSION: NUMERICAL MODELS ARE IN GOOD AGREEMENT THROUGHOUT
THE FORECAST DURATION. HOWEVER DUE TO THE UNCERTAINTY IN THE SPEED
OF DETERIORATION OF THE SYSTEM DUE TO HIGH VWS, THERE IS LOW
CONFIDENCE IN THE JTWC TRACK FORECAST.  

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