WDPS31 PGTW 041500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 15P (FIFTEEN) WARNING 
NR 005//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 21.5S 172.4E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 395 NM WEST-SOUTHWEST OF SUVA, FIJI
   MOVEMENT PAST 6 HOURS: EASTWARD AT 18 KTS
   SIGNIFICANT WAVE HEIGHT: 17 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
PARTIALLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC), LOCATED ON
THE NORTHWESTERN EDGE OF A BALL OF FLARING DEEP CONVECTION. A
041010Z AMSU-B 89GHZ MICROWAVE IMAGE REVEALED AN IRREGULAR AND
ELONGATED LLCC, WITH DEEP CONVECTIVE BANDS ALONG THE SOUTHERN
PERIPHERY AND SHALLOW BANDING FEATURES WRAPPING IN FROM THE NORTH.
THE INITIAL SYSTEM IS ASSESSED WITH MEDIUM CONFIDENCE USING THE
LOW-RESOLUTION MICROWAVE IMAGERY AND PARTIALLY EXPOSED LLCC IN THE
EIR. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE,
HEDGED A NOTCH HIGHER THAN THE SOLE AGENCY FIX FROM PGTW, IN LIGHT
OF THE OBJECTIVE ESTIMATES NOTED ABOVE AND PERSISTENCE WITH A SAR
PASS FROM 040708Z THAT INDICATED WINDS WERE STILL ELEVATED ABOVE 40
KNOTS AT THAT TIME. THE ENVIRONMENT IS TRENDING RAPIDLY TOWARDS THE
UNFAVORABLE SIDE OF THE SCALE, WITH WESTERLY SHEAR NOW EXCEEDING 35
KNOTS, AND MID-LEVEL DRY AIR INTRUDING FROM THE NORTH AND WEST,
THOUGH POLEWARD OUTFLOW REMAINS ROBUST AND SSTS ARE WARM. THE
VORTEX IS SHARPLY TILTED AND BEING OVERRUN BY DRY AIR AT A RAPID
CLIP. THE LATEST PHASE CLASSIFICATION WORKSHEET INDICATES THE
SYSTEM IS A BORDERLINE SUBTROPICAL OR HYBRID SYSTEM ALREADY.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: SOUTHWESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) CENTERED TO THE NORTHEAST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS
   CIMSS ADT: 33 KTS AT 041130Z
   CIMSS AIDT: 37 KTS AT 041240Z
   CIMSS D-PRINT: 36 KTS AT 041210Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 30+ KTS
   SST: 27-28 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: DRY MID-LEVEL AIR MOVING IN FROM THE NORTHWEST. 

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TC 15P WILL CONTINUE TO TRACK EASTWARD FOR THE
NEXT 12 HOURS OR SO ALONG THE SOUTHWESTERN SIDE OF THE DEEP STR TO
THE NORTHEAST. HOWEVER, AS THE SYSTEM WEAKENS AND THE VORTEX
SHALLOWS OUT, THE STEERING LEVEL WILL LOWER TO THE MID-LEVELS,
WHERE THE RIDGE ORIENTATION IS MORE NORTHWEST-SOUTHEAST. TC 15P
WILL TURN SOUTHEASTWARD AFTER TAU 12 AS IT COMES UNDER THE STEERING
INFLUENCE OF THE LOW- TO MID-LEVEL FLOW. IN TERMS OF INTENSITY, THE
SYSTEM WILL MAINTAIN INTENSITY FOR THE NEXT 12 TO 18 HOURS BEFORE
IT WEAKENS TO 30 KNOTS BY TAU 24, AFTER THE LLCC IS COMPLETELY
ENGULFED IN DRY AIR AND THE VERY STRONG SHEAR PROHIBITS SIGNIFICANT
CONVECTIVE DEVELOPMENT NEAR THE CENTER. THE SYSTEM WILL BE
UNDERGOING SUBTROPICAL TRANSITION (STT), OR MORE PROPERLY,
MAINTAINING ITSELF AS A HYBRID SYSTEM DURING THROUGH TAU 36, WHEN
IT WILL BEGIN EXTRATROPICAL TRANSITION. BY THE END OF THE FORECAST,
TC 15P WILL BECOME EMBEDDED UNDER A 500MB SHORTWAVE TROUGH AND MOVE
UNDER THE JET STREAM, WITH REINTENSIFICATION EXPECTED DUE TO
BAROCLINIC FORCING AS IT COMPLETES EXTRATROPICAL TRANSITION (ETT). 


MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS MIXED, WITH THE
MODELS SPREADING OUT IN A FAN-SHAPED FASHION, TO A 350NM SPREAD BY
TAU 48. GFS AND GEFS TURN THE SYSTEM SHARPLY SOUTHWARD AFTER ABOUT
TAU 24 AND MARK THE WESTERN SIDE OF THE GUIDANCE ENVELOPE. THE
ECMWF AND ECENS TAKE A SHALLOW, FLATTER APPROACH TO THE TRACK, AND
MARK THE EASTERN SIDE OF THE ENVELOPE. THE JTWC FORECAST HEDGES
SLIGHTLY INSIDE (WEST) OF THE CONSENSUS MEAN THOUGH CONFIDENCE IS
MEDIUM DUE TO THE UNCERTAINTY SURROUNDING HOW MUCH AND WHEN THE
STEERING LEVEL WILL LOWER AND THE RESULT ON THE TRACK. INTENSITY
GUIDANCE IS IN GOOD AGREEMENT THAT THE SYSTEM WILL WEAKEN TO 30
KNOTS WITHIN THE NEXT 24 HOURS FOLLOWED BY A REINTENSIFICATION AS
IT UNDERGOES ETT. THE JTWC FORECAST GENERALLY FOLLOWS THE MESOSCALE
MODELS (HWRF, HAFS-A AND COAMPS-TC) FOR THE BULK OF THE FORECAST
PERIOD, WITH MEDIUM CONFIDENCE. 

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