WDPS32 PGTW 050300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 31P (THIRTYONE) WARNING 
NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 13.8S 171.8E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 460 NM WEST-NORTHWEST OF SUVA, FIJI
   MOVEMENT PAST 6 HOURS: SOUTHEASTWARD AT 05 KTS
   SIGNIFICANT WAVE HEIGHT: 18 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS AN
ASYMMETRIC VORTEX WITH CONVECTIVE BANDING WRAPPING INTO THE CENTER
PRIMARILY ON THE EASTERN SIDE. LOW-LEVEL CLOUD BANDS ARE VISIBLE IN
THE NORTHWESTERN QUADRANT, HOWEVER, THE LOW-LEVEL CIRCULATION
CENTER (LLCC) IS ALMOST COMPLETELY OBSCURED. ANIMATED WATER VAPOR
IMAGERY REVEALS MODERATE POLEWARD AND EQUATORWARD OUTFLOW, AS WELL
AS A POCKET OF RELATIVELY DRY AIR SITUATED SOUTHWEST OF THE
ASSESSED CENTER LOCATION. OWING TO A LACK OF RECENT MICROWAVE
IMAGERY AND WIND SPEED DATA, THE INITIAL POSITION IS ASSESSED WITH
MEDIUM CONFIDENCE BASED ON THE ANIMATED MSI. THE INITIAL INTENSITY
OF 40 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON AGENCY DVORAK
FIXES AND A 042141 METOP-B ASCAT PASS THAT SHOWED A SMALL SWATH OF
30 KT WINDS NORTH OF THE CENTER POSITION. THAT SAID, THE ASYMMETRIC
WIND FIELD EXPOSED IN MUCH EARLIER DATA SUGGESTS THAT THE MAXIMUM
WIND SPEEDS ARE CONFINED TO THE EASTERN SEMICIRCLE, SO THE INITIAL
INTENSITY COULD BE A BIT HIGHER. THE CURRENT ENVIRONMENT IS
ANALYZED TO BE FAVORABLE FOR INTENSIFICATION WITH MODERATE WESTERLY
VERTICAL WIND SHEAR (VWS), VERY WARM SEA SURFACE TEMPERATURES
(29-30 C), AND GOOD OUTFLOW ALOFT.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE CENTERED TO THE EAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   KNES: T2.5 - 35 KTS
   PHFO: T2.5 - 35 KTS
   CIMSS D-PRINT: 45 KTS AT 050000Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD AND EQUATORWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: TROPICAL CYCLONE (TC) 31P HAS TRACKED SLOWLY
SOUTHEASTWARD OVER THE PAST 12 HOURS UNDER THE RELATIVELY WEAK
STEERING INFLUENCE OF THE STR TO ITS EAST. THE SYSTEM WILL CONTINUE
TO ADVANCE SOUTHEAST AT A LEISURELY PACE UNDER THE WEAK STEERING
FLOW THROUGH TAU 36, AT WHICH TIME A LONGWAVE TROUGH APPROACHING
FROM THE WEST TIGHTENS THE GRADIENT AND CAUSES IT TO ACCELERATE
SOUTHEASTWARD THROUGH THE REMAINDER OF THE FORECAST PERIOD. A
STEADY INTENSITY INCREASE IN A FAVORABLE ENVIRONMENT, FURTHER
ENHANCED BY INCREASED POLEWARD OUTFLOW AFTER TAU 36 FROM A JET
STREAK ASSOCIATED WITH THE AFOREMENTIONED TROUGH, IS FORECAST
THROUGH ABOUT TAU 60. TC 31P IS EXPECTED TO PEAK AT 85-90 KTS NEAR
TAU 60, AFTER WHICH AN INCREASINGLY HOSTILE SHEAR ENVIRONMENT AND
DRY AIR ENTRAINMENT ARE FORECAST TO INITIATE A STEADY WEAKENING
PHASE THROUGH THE END OF THE FORECAST PERIOD. THE SYSTEM IS
EXPECTED TO BEGIN EXTRATROPICAL TRANSITION AROUND TAU 120 UNDER THE
INFLUENCE OF THE JET STREAK PREVIOUSLY MENTIONED.

MODEL DISCUSSION: MODEL TRACK GUIDANCE IS IN GENERALLY GOOD
AGREEMENT ABOUT THE SOUTHEASTWARD TRAJECTORY, HOWEVER, THE
CROSS-TRACK SPREAD OF 45 NM AT TAU 12 STEADILY EXPANDS TO AROUND
400 NM BY TAU 120. NAVGEM AND GFS DETERMINISTIC MODELS CONSTITUTE
THE EASTERNMOST OUTLIERS AND HAVE BEEN EXCLUDED FROM THE
CROSS-TRACK MEASUREMENTS. ALONG-TRACK AGREEMENT IS SIMILARLY
VARIABLE THROUGHOUT THE FORECAST PERIOD. THE SLOW SPEED OF ADVANCE
AND WEAK STEERING FLOW IN THE FIRST 36-48 HOURS IS REFLECTED IN THE
GUIDANCE TRACKS AS A SLIGHT MEANDERING CENTERED ABOUT A MEAN
SOUTHWESTWARD DIRECTION, WHICH IS THE LIKELY SOURCE OF EXPANDING
CROSS- AND ALONG-TRACK SPREAD IN LATER TAUS. THE JTWC TRACK
FORECAST IS PLACED CLOSE TO THE TES1 CONSENSUS, WITH MEDIUM
CONFIDENCE THROUGH TAU 72 AND LOW CONFIDENCE THEREAFTER. THE
INTENSITY GUIDANCE IS IN GOOD AGREEMENT ABOUT THE TIMING AND RATE
OF INTENSIFICATION AND WEAKENING WITH THE EXCEPTION OF RAPID
INTENSIFICATION AIDS (COAMPS-TC AND RIDE) AND HAFS-A, WHICH SUGGEST
PEAK INTENSITIES OF 105-110 KTS. THE REMAINDER OF GUIDANCE REMAINS
TIGHTLY GROUPED, WITH PEAK INTENSITIES RANGING FROM 80-90 KTS. THE
JTWC INTENSITY FORECAST IS PLACED CLOSE TO THE CONSENSUS WITH
MEDIUM CONFIDENCE.

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