WDPN31 PGTW 072100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (TWENTYONE) WARNING 
NR 007//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 24.0N 147.4E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 337 NM EAST OF IWO TO
   MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 18 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS
DEVELOPING, FLARING CONVECTION OVER A LOW-LEVEL CIRCULATION CENTER
(LLCC), THAT WAS INITIALLY FULLY EXPOSED, ASSOCIATED WITH THE TROPICAL 
STORM (TS) 21W. THE SYSTEM IS TRACKING NORTH-NORTHEASTWARD, UNDER AN 
ELONGATED AREA OF HIGH VERTICAL WIND SHEAR (VWS), WITH 25-30 KTS WINDS 
FROM THE SOUTHWEST, LEADING TO THE CONVECTIVE ACTIVITY BEING DECOUPLED 
FROM THE LLCC AND PUSHED NORTHEASTWARD. THE INITIAL POSITION IS PLACED 
WITH HIGH CONFIDENCE BASED ON THE LLCC BEING FULLY EXPOSED, JUST PRIOR 
TO THE CONVECTION DEVELOPING OVER THE LAST HOUR. THE INITIAL INTENSITY 
OF 35 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON PERSISTENCE, AS
WELL AS THE AGENCY AND OBJECTIVE ESTIMATES LISTED BELOW.

INITIAL WIND RADII BASIS: SAR DATA 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T2.5 - 35 KTS
   CIMSS SATCON: 40 KTS AT 071541Z
   CIMSS ADT: 34 KTS AT 071730Z
   CIMSS DPRNT: 28 KTS AT 07800Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 20-25 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE EASTWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   INITIAL WIND RADII: HIGH

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TS 21W IS FORECAST TO CONTINUE TRACKING
NORTH-NORTHEASTWARD, UNDER THE INFLUENCE OF A SUBTROPICAL RIDGE TO
THE EAST OF THE SYSTEM. GIVEN THE MARGINALLY FAVORABLE
ENVIRONMENTAL CONDITIONS - HIGH SEA SURFACE TEMPERATURES AND
MODERATE UPPER-LEVEL OUTFLOW, THE SYSTEM IS EXPECTED TO STEADILY
INTENSIFY AND REACH THE PEAK WIND SPEEDS OF 65 KTS BETWEEN TAUS 72
AND 96. BY TAU 72, TS 21W WILL START INTERACTING WITH THE
BAROCLINIC ZONE, AND A SUBTROPICAL JET FURTHER ENHANCING THE
OUTFLOW, AS THE SYSTEM WILL BEGIN EXTRATROPICAL TRANSITION (ETT).
BY TAU 96, ETT IS EXPECTED TO BE COMPLETED.

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE REMAINS IN GOOD
AGREEMENT IN REGARD TO THE TRACK THROUGHOUT THE FORECAST, AVERAGING
60 NM CROSS-TRACK SPREAD. JTWC FORECAST IS LAID IN CLOSE PROXIMITY
TO THE MULTI-MODEL CONSENSUS. ALONG-TRACK SPREAD AGREEMENT IS GOOD,
UNTIL TAU 72, WHEN TS 21W IS EXPECTED TO BEGIN INTERACTING WITH THE
BAROCLINIC ZONE, BY WHICH THE DISTANCE BETWEEN MODEL GUIDANCE IS
OVER 300 NM. INTENSITY GUIDANCE IS ALSO IN GOOD AGREEMENT, WITH ALL
THE CONSENSUS MEMBERS SUGGESTING STEADY INTENSIFICATION OUT TO TAU
72-84, FOLLOWED BY A RAPID WEAKENING BEGINNING AT THE END OF THE
FORECAST PERIOD.

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