Advisories

WDPN31 PGTW 282100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 30W (IN-FA) WARNING NR 022//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 15.6N 156.3E
   INITIAL INTENSITY: 120 KTS
   GEOGRAPHIC REFERENCE: 427 NM WEST-NORTHWEST OF ENEWETAK
   MOVEMENT PAST 6 HOURS: WESTWARD AT 13 KTS
   SIGNIFICANT WAVE HEIGHT: 55 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY SHOWS A STILL
LARGE AND CLEAR EYE, THOUGH IT HAS BECOME SLIGHTLY MORE RAGGED OVER
THE PAST SIX HOURS. THE SURROUNDING RING OF COLD CLOUD TOPS HAS
PERIODICALLY BECOME BROKEN IN THE NORTHEASTERN QUADRANT, AND THE
DISTRIBUTION OF INNER CORE CONVECTION NOW SKEWS WESTWARD,
INDICATING SOME DISRUPTION BY WHAT MODELS ASSESS TO BE 10-15 KT OF
EASTERLY VERTICAL SHEAR BELOW THE OUTFLOW LAYER. UPPER-LEVEL
OUTFLOW ITSELF CONTINUES TO BE RADIAL IN NATURE AND PRESENT IN ALL
QUADRANTS, THOUGH STRONGEST WESTWARD. OCEANIC CONDITIONS REMAIN
HIGHLY FAVORABLE AS THE STORM TRACKS THROUGH A REGION OF OCEANIC
HEAT CONTENT VALUES IN EXCESS OF 100 KJ PER SQUARE CENTIMETER. THE
INITIAL INTENSITY IS LOWERED TO 120 KT BASED ON A GRADUAL FALLING
TREND IN AUTOMATED INTENSITY ESTIMATES AND AGENCY DVORAK INTENSITY
ESTIMATES.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE TO THE NORTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T6.5 - 127 KTS
   RJTD: T6.5 - 127 KTS
   RCTP: T6.0 - 115 KTS
   CIMSS SATCON: 133 KTS AT 281513Z
   CIMSS ADT: 109 KTS AT 281730Z
   CIMSS AIDT: 122 KTS AT 281730Z
   CIMSS D-MINT: 135 KTS AT 281551Z
   CIMSS D-PRINT: 132 KTS AT 281800Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG RADIAL

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TYPHOON 30W (IN-FA) CONTINUES TO TRACK
SLIGHTLY NORTH OF DUE WEST ALONG THE SOUTHWESTERN PERIPHERY OF THE
SUBTROPICAL RIDGE TO ITS NORTHEAST. THIS GENERAL MOTION IS EXPECTED
TO CARRY IN-FA THROUGH THE NORTHERN MARIANA ISLANDS BETWEEN TAU 36
AND 48. MODERATE EASTERLY SHEAR APPEARS TO BE DISRUPTING THE
STORM'S QUASI-ANNULAR STRUCTURE AND CAUSING SLIGHT WEAKENING, BUT
SHEAR IS EXPECTED TO DECREASE DURING THE NEXT 48 HOURS, AND
NEAR-STEADY INTENSITY IS FORECAST DURING THAT TIME. SOME SLIGHT
FLUCTUATIONS IN INTENSITY, BOTH UP AND DOWN, MAY OCCUR DUE TO
INTERNAL VORTEX DYNAMICS AND SLIGHT CHANGES IN ENVIRONMENTAL SHEAR.
BETWEEN TAU 48 AND 72, A PASSING LONGWAVE TROUGH OVER JAPAN WILL
WEAKEN THE STEERING RIDGE AND IS EXPECTED TO INDUCE 30W TO TURN
NORTHWESTWARD AND SLOW ITS SPEED OF ADVANCE TO THE SOUTHEAST OF IWO
TO. THE PASSING TROUGH WILL MOVE QUICKLY EASTWARD, ALLOWING A
SUBTROPICAL RIDGE TO BUILD BEHIND IT NEAR AND EAST OF TAIWAN,
IMPEDING IN-FA'S NORTHWARD PROGRESS. DYNAMICAL MODELS AGREE THAT
THIS WILL PREVENT 30W FROM RECURVING INITIALLY, BLOCKING THE STORM
AND FORCING IT TO TURN BACK WESTWARD BEYOND TAU 72 TO THE SOUTH OF
IWO TO. MODELS EXHIBIT MODERATE DISAGREEMENT REGARDING THE EXACT
SHAPE AND SPEED OF THE TRACK BEYOND TAU 60. REGARDING THE INTENSITY
FORECAST, THE STORM'S SLOWING BETWEEN TAU 48 AND 72 MAY CAUSE SOME
OCEANIC UPWELLING, THOUGH ITS MAGNITUDE IS UNCERTAIN DUE TO THE
ANTICIPATED TRACK NEAR A TIGHT OCEANIC HEAT CONTENT GRADIENT,
DECREASING QUICKLY FROM SOUTH TO NORTH. MODEST WEAKENING IS THUS
FORECAST AROUND THAT TIME. DYNAMICAL MODELS INDICATE OTHERWISE
GENERALLY FAVORABLE ENVIRONMENTAL CONDITIONS FOR MAINTAINING A
STRONG TYPHOON THROUGH THE FORECAST PERIOD, WITH LOW VERTICAL SHEAR
AND WARM SEA SURFACE TEMPERATURES PERSISTING.

MODEL DISCUSSION: THE JTWC TRACK FORECAST IS PLACED WITH HIGH
CONFIDENCE THROUGH 72 HOURS NEAR THE MULTI-MODEL CONW CONSENSUS,
THEN WITH MEDIUM CONFIDENCE BEYOND 72 HOURS AS MODEL SPREAD
BALLOONS TO OVER 300 NM DUE TO THE COMPLEX STEERING ENVIRONMENT. THE 
JTWC INTENSITY FORECAST IS CLOSE TO A BLEND OF SHIPS, COAMPS-TC, 
HWRF, AND THE GOOGLE DEEPMIND ENSEMBLE MEAN, LEANING A LITTLE HIGHER
BEYOND 72 HOURS DUE TO FAVORABLE OCEANIC AND ATMOSPHERIC
ENVIRONMENTAL CONDITIONS ALONG THAT PORTION OF THE JTWC TRACK
FORECAST.

FORECAST CONFIDENCE:
   TRACK 00-72 HR: HIGH
   TRACK 72-120 HR: MEDIUM
   INTENSITY 00-72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
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