WDPN31 PGTW 211500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 01W (SANVU) WARNING NR 
009//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 10.4N 156.1E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 244 NM NORTH-NORTHWEST OF POHNPEI
   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 03 KTS
   SIGNIFICANT WAVE HEIGHT: 16 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A SMALL
SLIGHTLY NORTH-SOUTH ELONGATED CENTRAL DENSE OVERCAST (CDO) WHOSE
WESTERN PERIPHERY IS DEVOID OF DEEP CONVECTION. WITH VERY LIMITED
SATELLITE DATA TO INTERROGATE, A PARTIAL 210823Z SSMIS 89GHZ COLOR
COMPOSITE MICROWAVE IMAGE REVEALS FAINT LOW LEVEL CLOUD LINES IN
THE EXPOSED WESTERN PERIPHERY WRAPPING UNDER THE DEEP CONVECTION
OBSCURING THE LLCC. THE INITIAL POSITION IS PLACED WITH MEDIUM
CONFIDENCE BASED ON EIR AND SSMIS IMAGERY. THE INITIAL INTENSITY OF
45 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON A CONSENSUS
AMONGST REPORTING DVORAK AGENCIES OF T3.0. AUTOMATED INTENSITY
ESTIMATES APPEAR DISCORDANT WITH CIMSS ADT SHOWING T3.4 THOUGH MUCH
LIKE PGTW THEIR RAW T NUMBER HAS FALLEN TO T2.5 AND CIMSS OPEN-AIR
HAS DROPPED TO 30KTS.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: WEAK BUT INCREASING STEERING INFLUENCE
OF A SUB-TROPICAL RIDGE (STR) BUILDING TO THE NORTH.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   RJTD: T3.0 - 45 KTS
   KNES: T3.0 - 45 KTS
   CIMSS ADT: 53 KTS AT 211230Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 10-15 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE RADIAL

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TS 01W (SANVU) IS FORECAST TO EXIT A WEAK
STEERING ENVIRONMENT OVER THE NEXT SEVERAL HOURS AS A STR TO THE
NORTH CONTINUES TO BUILD ITS INFLUENCE. DURING THIS TIME, THE
ENVIRONMENT SURROUNDING 01W IS ANTICIPATED TO GRADUALLY DETERIORATE
AS THE POSITIVE FACTORS OF WARM SSTS AND GOOD OUTFLOW ALOFT ARE
COUNTERACTED BY STEADILY INCREASING VERTICAL WIND SHEAR (VWS) AND
DRY AIR ENTRAINMENT. IT IS IMPORTANT TO KEEP IN MIND THAT A FEW
DEGREES NORTH OF THE SYSTEM, LIES A SHARP GRADIENT OF BOTH MOISTURE
AND VWS AND AS THE SYSTEM TRACKS INTO THAT GRADIENT THE INTENSITY
IS FORECAST TO FALL. IF HOWEVER, THE STR BUILDS FASTER THAN
PREVIOUSLY FORECAST, 01W MAY STEADY UP ON A COURSE CLOSER TO THE
12TH PARALLEL AND THEREFOR STAVE OFF SOME WEAKENING, THEREBY
STRETCHING THE EXTENDED TRACK FORECAST CLOSER TO GUAM. THAT
NOTWITHSTANDING, THE PREPONDERANCE OF EVIDENCE STILL INDICATES A
GRADUAL WEAKENING TREND AS VWS AND DRY AIR ENTRAINMENT CONTINUE TO
INCREASE EVENTUALLY DISSIPATING THE SYSTEM BY TAU 96, WELL EAST OF
GUAM.

MODEL DISCUSSION: NUMERICAL MODELS ARE IN GOOD AGREEMENT WITH ALL
MEMBERS INDICATING A GRADUAL WESTWARD TURN AS THE STR TAKES
STEERING CONTROL. THERE IS STILL A MILD AMOUNT OF CROSS TRACK
SPREADING WITH NVGM, HWRF AND COAMPS-TC POLEWARD OF THE CONSENSUS
WHILE JGSM, ECMWF AND ITS ENSEMBLE ARE EQUATORWARD. FOR THIS REASON
THE JTWC TRACK FORECAST IS PLACED CLOSE TO THE MULTI-MODEL
CONSENSUS WITH MEDIUM CONFIDENCE. WITH THE EXCEPTION OF
DECAY-SHIPS, RELIABLE MODEL INTENSITY GUIDANCE IS IN FAIR AGREEMENT
THAT 01W WILL GRADUALLY WEAKEN THROUGH THE FORECAST PERIOD. FOR
THIS REASON THE JTWC INTENSITY FORECAST IS PLACED INITIALLY WITH
MEDIUM CONFIDENCE AND WITH HIGH CONFIDENCE IN THE EXTENDED
FORECAST.

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