WDPN31 PGTW 071500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (TWENTYONE) WARNING 
NR 006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 23.2N 147.2E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 337 NM EAST-SOUTHEAST OF IWO TO
   MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 16 KTS
   SIGNIFICANT WAVE HEIGHT: 19 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A FULLY
EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC) WITH CONVECTIVE BANDING
DISLOCATED TO THE EAST DUE TO HIGH 20-25KT VERTICAL WIND SHEAR
(VWS). THE VERTICAL WIND SHEAR IS ASSOCIATED WITH DRY DEEP-LAYER
TROUGHING EVIDENT TO THE WEST OF THE SYSTEM ON WATER VAPOR IMAGERY.
WATER VAPOR IMAGERY ALSO HIGHLIGHTS ENHANCED EASTWARD OUTFLOW ALOFT
WHICH IS INDUCED BY AN UPPER-LEVEL RIDGING OVER TROPICAL STORM (TS)
21W. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON
THE AFOREMENTIONED EXPOSED LLCC AND A 071138Z METOP-C AMSU-B 89GHZ
MICROWAVE IMAGE EXHIBITING THE SHEARED ENVIRONMENT WITH A FULLY
EXPOSED LOW-LEVEL BANDING AND CONVECTIVE BANDS FAR TO THE EAST. THE
INITIAL INTENSITY OF 35 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED
ON AN EARLIER 070825Z RCM-2 SYNTHETIC APERTURE RADAR (SAR) IMAGE
EXHIBITING A BAND OF ENHANCED WINDS WRAPPING AROUND THE EAST AND
NORTHEAST PERIPHERY OF TS 21W.

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: 38 KTS AT 070912Z
   CIMSS ADT: 32 KTS AT 071130Z

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 WILL TRACK NORTHEASTWARD BY A STR TO
THE SOUTHEAST FOR THE NEXT 96 HOURS. THE TRACK SPEED WILL INCREASE
PROGRESSIVELY THROUGHOUT THE FORECAST, BUT ESPECIALLY AFTER TAU 48
WHEN A DEEP-LAYER TROUGH WILL ENHANCE THE TRACK SPEED. THE
INTENSITY WILL INCREASE STEADILY THROUGHOUT THE FORECAST PERIOD
WHILE IN A MARGINALLY FAVORABLE ENVIRONMENT. A BALANCE STRUCK
BETWEEN UNFAVORABLE FEATURES SUCH AS HIGH VERTICAL WIND SHEAR ABOVE
20KTS AND DRY AIR ENTRAINMENT WILL COMBAT THE FAVORABLE FEATURES OF
HIGH SURFACE TEMPERATURES AND STRONG OUTFLOW. THE OUTFLOW ALOFT IS
INITIALLY ENHANCED BY THE UPPER-LEVEL RIDGE THAT REMAINS OVER TS
21W UNTIL TAU 48 UNTIL AN UPPER-LEVEL TROUGH ENHANCES OUTFLOW
POLEWARD. THE INFLUENCE OF THE JET AT TAU 72 WILL ALSO INITIATE
EXTRATROPICAL TRANSITION. THE SYSTEM IS EXPECTED TO BECOME FULLY
EXTRATROPICAL AT TAU 96 AS IT PEAKS AT AN INTENSITY OF 65KTS. 

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE IS IN GOOD AGREEMENT
THAT TS 21W WILL TRACK NORTHEASTWARD FOR THE REMAINDER OF THE
FORECAST PERIOD BUT BEGINS TO DISAGREE AFTER EXTRATROPICAL
TRANSITION BEGINS. THE TRACK AT TAU 96 OPENS TO 500NM BASED ON
DIFFERING INTERPRETATIONS OF HOW BAROCLINIC FORCING WILL SHALLOW
THE SYSTEM AND CAUSE IT TO BE STEERED ON THE MID- TO LOW-LEVELS.
THE INTENSITY GUIDANCE IS IN GOOD AGREEMENT THAT THE PEAK INTENSITY
WILL BE 60-65KTS BETWEEN TAU 72-96. THE ONLY OUTLYING MEMBER IS
COAMPS-TC, WHICH PREDICTS UNREALISTIC RAPID WEAKENING FOLLOWING TAU
72. 

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