WDXS31 PGTW 042100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 12S (TWELVE) WARNING 
NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 9.7S 96.9E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 150 NM NORTH OF THE COCOS ISLANDS
   MOVEMENT PAST 6 HOURS: SOUTHEASTWARD AT 13 KTS
   SIGNIFICANT WAVE HEIGHT: 19 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TC 12S HAS DEVELOPED INTO A TROPICAL CYCLONE DURING THE PAST SIX
HOURS. ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS
DEEP CONVECTION PERSISTING OVER THE NORTHERN AND WESTERN PORTIONS
OF A PARTIALLY EXPOSED LOW-LEVEL CIRCULATION DEPICTED IN A 041420Z
METOP-B ASCAT PASS. THE INITIAL POSITION IS PLACED WITH MEDIUM
CONFIDENCE BASED ON RECENT SATELLITE POSITION FIXES FROM PGTW AND
APRF AND THE 041420Z ASCAT DATA. THE INITIAL INTENSITY OF 40 KTS IS
ASSESSED WITH MEDIUM CONFIDENCE BASED ON WIND SPEEDS EXCEEDING 35
KTS EVIDENT WITHIN THE NORTHERN PORTION OF THE CIRCULATION DEPICTED
IN THE ASCAT DATA.

INITIAL WIND RADII BASIS: 041420Z METOP-B ASCAT SCATTEROMETER DATA
AND OBJECTIVE BEST TRACK

CURRENT STEERING MECHANISM: EXTENSION OF NEAR-EQUATORIAL RIDGE TO
THE NORTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   DEMS: T2.0 - 30 KTS
   APRF: T2.0 - 30 KTS
   CIMSS D-PRINT: 23 KTS AT 041800Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE POLEWARD

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TC 12S WILL TRACK SOUTH-SOUTHEASTWARD FOR THE
NEXT 12 HOURS UNDER THE CONTINUED STEERING INFLUENCE OF A
NEAR-EQUATORIAL RIDGE EXTENSION TO THE NORTHEAST. THEREAFTER, A
BUILDING SUBTROPICAL RIDGE TO THE SOUTHEAST IS EXPECTED TO BECOME
THE DOMINANT STEERING MECHANISM, INITIALLY GUIDING THE SYSTEM
SOUTH-SOUTHWESTWARD BEFORE TURNING THE SYSTEM TOWARD A GENERALLY 
WESTWARD TRAJECTORY AFTER TAU 48. SLOW INTENSIFICATION IS
ANTICIPATED OVER THE NEXT 24 HOURS DUE TO IMPROVING UPPER-LEVEL
CONDITIONS INCLUDING A TEMPORARY REDUCTION IN VERTICAL WIND SHEAR
AND INCREASING POLEWARD OUTFLOW. VERTICAL WIND SHEAR WILL AGAIN 
INCREASE AS THE SYSTEM TURNS WESTWARD AND APPROACHES THE EDGE OF
AN UPPER-LEVEL RIDGE, TRACKING BELOW THE ASSOCIATED PERIPHERAL FLOW.
PERSISTENT SHEAR AND ENCROACHMENT OF DRIER AIR FROM THE SURROUNDING
ENVIRONMENT WILL LEAD TO DISSIPATION BY TAU 96.

MODEL DISCUSSION: NUMERICAL MODEL TRACK FORECAST GUIDANCE IS IN
GOOD AGREEMENT REGARDING THE OVERALL SCENARIO AND ASSOCIATED
VORTEX TRACKERS ARE TIGHTLY PACKED THROUGH TAU 48. AFTER TAU 48,
SPREAD INCREASES IN BOTH THE ALONG AND CROSS-TRACK DIRECTIONS DUE
TO DIFFERING PREDICTIONS OF THE EVOLUTION OF THE SUBTROPICAL
STEERING RIDGE TO THE SOUTHEAST. THE MULTI-MODEL CONSENSUS HAS BEEN
FAIRLY CONSISTENT IN ITS ORIENTATION OVER THE PAST SEVERAL RUNS,
AND THE INITIAL FORECAST TRACK IS CONSISTENT WITH THIS GUIDANCE.
INTENSITY GUIDANCE IS IN VERY GOOD AGREEMENT ACROSS THE BOARD,
INDICATING LIMITED NEAR-TERM INTENSIFICATION FOLLOWED BY STEADY 
DISSIPATION AS TC 12S ENCOUNTERS INCREASING VERTICAL WIND SHEAR.

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