Advisories

WDPN32 PGTW 150900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 29W (CHAMPI) WARNING 
NR 004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 10.9N 158.4E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 809 NM EAST OF ANDERSEN AFB
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 13 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL DEPRESSION (TD) 29W HAS NOT CHANGED MUCH OVER THE PAST
6-12 HOURS AS IT MOVES WEST-NORTHWESTWARD. ANIMATED MULTISPECTRAL
SATELLITE IMAGERY (MSI) DEPICTS A PARTIALLY EXPOSED LOW-LEVEL
CIRCULATION CENTER (LLCC) WITH PERSISTENT DEEP CONVECTION
ASSOCIATED WITH A MID-LEVEL CIRCULATION DISPLACED TO THE SOUTHWEST
OF THE LLCC. OVERSHOOTING TOPS HAVE REACHED TEMPERATURES AS COLD AS
-94 DEGREES CELSIUS. A 142320Z ASCAT-C PASS REVEALED A PATCH OF
30-KT WINDS CONFINED TO THE SOUTHWEST QUADRANT OF AN ELONGATED,
ASYMMETRIC LLCC. A 150228Z AMSR3 PASS DID NOT REVEAL A WELL-DEFINED
INNER CORE. THE SYSTEM IS TRAVERSING VERY WARM SEA SURFACE
TEMPERATURES (SST) OF 29-30 DEGREES CELSIUS IN A MODERATELY SHEARED
ENVIRONMENT WITH GOOD UPPER-LEVEL DIVERGENCE ALONG THE WESTERN
PERIPHERY. THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE
BASED ON THE PARTLY OBSCURED LLCC IN MSI AND IS LOCATED NEAR THE
TIGHT CLUSTER OF FIXES FROM PGTW, RJTD, AND RCTP. GIVEN THE
RELATIVELY UNCHANGED CONVECTIVE STRUCTURE SINCE THE LAST CYCLE, THE
INITIAL INTENSITY IS HELD AT 30 KTS WITH HIGH CONFIDENCE, WHICH IS
SUPPORTED BY THE EARLIER ASCAT DATA AND THE UNANIMOUS T2.0 DVORAK
CLASSIFICATIONS.

INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).

CURRENT STEERING MECHANISM: SOUTHERN FLANK OF A LOW- TO MID-LEVEL
SUBTROPICAL RIDGE (STR) CENTERED NEAR 30N 160E

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T2.0 - 30 KTS
   RCTP: T2.0 - 30 KTS
   CIMSS SATCON: 36 KTS AT 150239Z
   CIMSS ADT: 32 KTS AT 150540Z
   CIMSS AIDT: 33 KTS AT 150540Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG WESTWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: HIGH
   INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE OVERALL FORECAST TRACK HAS BEEN
SHIFTED SLIGHTLY SOUTHWARD BY 18 NM AT TAU 60.

FORECAST DISCUSSION: OVER THE PAST 12 HOURS, TD 29W HAS MOVED
WEST-NORTHWESTWARD AT AN AVERAGE SPEED OF APPROXIMATELY 11 KTS. THE
SYSTEM WILL GAIN SLIGHTLY MORE SPEED, AVERAGING 12-13 KTS THROUGH
TAU 120, AS IT TRAVELS WEST-NORTHWESTWARD ALONG THE SOUTHERN
PERIPHERY OF THE STR. A TROUGH PASSING WELL TO THE NORTH WILL ERODE
THE RIDGE AROUND TAU 48. HOWEVER, A RIDGE LOCATED OVER THE EAST
CHINA SEA WILL QUICKLY BUILD EASTWARD AND MAINTAIN THE
WEST-NORTHWEST TRAJECTORY. OVER THE NEXT 60 HOURS, MODERATE
VERTICAL WIND SHEAR (VWS) WILL PERSIST, WHILE THE SYSTEM TRAVELS
OVER VERY WARM SST OF 29-30 DEGREES CELSIUS AND HIGH OCEAN HEAT
CONTENT (OHC) OF NEAR 100 KJ PER CM2. INITIAL DEVELOPMENT WILL
LIKELY BE GRADUAL DUE TO THE ASYMMETRIC AND VERTICALLY TILTED
VORTEX. ONCE THE VORTEX BECOMES MORE SYMMETRICAL AND VERTICALLY
ALIGNED, THE RATE OF INTENSIFICATION WILL INCREASE. SHEAR WILL EASE
TO 10-15 KTS BY TAU 72, WHILE THE CIRCULATION MOVES OVER A REGION
OF HIGH SST AND OHC BY TAU 96. THE FORECAST BRINGS TD 29W TO
TYPHOON INTENSITY NEAR TAU 48, FOLLOWED BY A PERIOD OF RAPID
INTENSIFICATION (RI) THROUGH TAU 96. THE FORECAST PEAKS AT 125 KTS
AT TAU 96, CONSERVATIVELY ACCOUNTING FOR LESS PREDICTABLE INTERNAL
DYNAMICS THAT COULD CAUSE INTENSITY FLUCTUATIONS.

MODEL DISCUSSION: TRACK GUIDANCE IS GRADUALLY COMING INTO BETTER
AGREEMENT, WITH THE CROSS-TRACK SPREAD DECREASING FROM 90 NM LAST
CYCLE TO 70 NM THIS CYCLE AT TAU 60. THE SPREAD SPANS FROM ROTA TO
A POINT SLIGHTLY SOUTH OF GUAM. THE BULK OF THE DYNAMICAL MODELS
LIES ON THE POLEWARD SIDE OF THE GUIDANCE ENVELOPE, WHILE THE AI
MODELS PREFER A MORE WESTWARD TRACK. GOOGLE DEEPMIND IS THE MOST
WESTWARD SOLUTION, PASSING THE SYSTEM 14 NM SOUTH OF GUAM. ECMWF IS
CLOSEST TO THE MULTI-MODEL CONSENSUS (CONW), WHILE GFS IS SLIGHTLY
NORTH. THE OVERALL GUIDANCE ENVELOPE SHIFTED SOUTHWARD LAST CYCLE,
AND THE SOLUTIONS THIS CYCLE REMAIN SIMILAR. THIS INCREASES
CONFIDENCE IN A TRACK NEAR THE SOUTHERN MARIANA ISLANDS. THE JTWC
FORECAST HAS THEREFORE BEEN NUDGED SOUTHWARD BY 18 NM AT TAU 60 AND
IS POSITIONED SOUTH OF CONW, BUT 8 NM NORTH OF THE AI-MODEL MEAN
AND 25 NM NORTH OF THE GOOGLE DEEPMIND SOLUTION. MODEST SPREAD
EXISTS AMONG THE INTENSITY AIDS. DEEPMIND AND COAMPS-TC ARE THE
MOST AGGRESSIVE SOLUTIONS, DEPICTING 95-110 KTS AT TAU 60 AND 135
KTS AFTER TAU 72. GFS IS SLOWER AND WEAKER OVERALL. THE JTWC
INTENSITY FORECAST ACCOUNTS FOR THE ASYMMETRIC AND TILTED VORTEX,
CALLING FOR A SLOWER INITIAL RATE OF INTENSIFICATION THROUGH TAU
48, FOLLOWED BY RI THROUGH TAU 96. THIS FORECAST LIES NEAR THE
CENTER OF THE GUIDANCE ENVELOPE THROUGH TAU 48, THEN GRADUALLY
TRENDS TOWARD THE HIGHER END OF THE ENVELOPE. IF TD 29W CAN ACHIEVE
AXISYMMETRIZATION QUICKLY, THE STORM MAY INTENSIFY TO WELL OVER 80
KTS BY THE TIME IT PASSES NEAR THE MARIANA ISLANDS. FURTHERMORE, A
PEAK HIGHER THAN 125 KTS REMAINS WELL WITHIN THE REALM OF
POSSIBILITIES, GIVEN THE INCREASINGLY FAVORABLE ENVIRONMENTAL
CONDITIONS TOWARD THE END OF THE FORECAST PERIOD.

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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