WDXS31 PGTW 182100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 26S (TWENTYSIX) 
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 12.7S 96.4E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 42 NM SOUTHWEST OF THE COCOS ISLANDS
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 15 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A TIGHT
VORTEX TILTED WESTWARD WITH HEIGHT, WITH A PARTIALLY EXPOSED LLCC
LOCATED NEAR THE EASTERN EDGE OF THE HEAD OF A CYCLONICALLY CURVED
BAND OF DEEP CONVECTION. MODERATE EASTERLY VERTICAL SHEAR IS
PRESENT DUE TO UPPER-LEVEL EASTERLIES OVERLAYING A BACKGROUND
LOW-LEVEL WESTERLY WIND BURST IN THE EASTERN INDIAN OCEAN. A
181542Z ASCAT-C SCATTEROMETER PASS SHOWED 35 KT WINDS TO THE SOUTH,
WEST, AND NORTH OF A WELL-DEFINED AND CIRCULAR LLCC, WITH THE
RADIUS OF MAXIMUM WINDS CONTRACTING TO ABOUT 25 NM. THE INITIAL
INTENSITY IS SET AT 35 KT BASED PRIMARILY ON THIS DATA, CONSISTENT
WITH THE RANGE OF AGENCY DVORAK INTENSITY ESTIMATES FROM T2.0 (30
KT) TO T2.5 (35 KT). THEREFORE, WARNINGS ARE BEING INITIATED ON THE
SYSTEM AS TROPICAL CYCLONE 26S.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE TO THE SOUTH

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   KNES: T2.0 - 30 KTS
   APRF: T2.5 - 35 KTS
   CIMSS D-PRINT: 42 KTS AT 181800Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 20-25 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE WESTWARD

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TROPICAL CYCLONE 26S HAS FORMED BETWEEN AN
EQUATORIAL WESTERLY WIND BURST TO ITS NORTH AND A SUBTROPICAL RIDGE
TO ITS SOUTH. THE SUBTROPICAL RIDGE IS IMPARTING AN EASTERLY
STEERING FLOW THAT MOSTLY COUNTERS THE INFLUENCE OF THE WESTERLY
WIND BURST, LEADING TO A WEAK OVERALL STEERING ENVIRONMENT.
NUMERICAL MODELS ARE IN GENERAL AGREEMENT ON A SHORT-TERM TURN
SOUTHWARD AT A SLOW FORWARD SPEED. AFTER 24 HOURS, THE SUBTROPICAL
RIDGE TO THE SOUTH WILL WEAKEN, POTENTIALLY ALLOWING THE INFLUENCE
OF THE WESTERLY WIND BURST TO TURN 26S SLIGHTLY EASTWARD BETWEEN 24
AND 72 HOURS. AFTER 72 HOURS, A TURN BACK WESTWARD IS POSSIBLE AS
THE CYCLONE MAKES PROGRESS POLEWARD AND GAINS DISTANCE FROM THE
INFLUENCE OF THE WESTERLY WIND BURST TO THE NORTH. HOWEVER,
STEERING CURRENTS ARE EXPECTED TO REMAIN WEAK THROUGHOUT THE
FORECAST, AND MODELS DISAGREE SUBSTANTIALLY ON THE FINAL POSITION
IN 120 HOURS, WITH CROSS-TRACK SPREAD OF ABOUT 260 NM. THE JTWC
FORECAST SHOWS A NEARLY STALLED MOTION DURING THE 72-120 HOUR
PERIOD, WITH LOW CONFIDENCE IN THE DIRECTION OF TRAVEL AT THAT
TIME. AS FOR THE INTENSITY FORECAST, A SHORT-TERM PERIOD OF
INTENSIFICATION IS EXPECTED AS THE NASCENT INNER CORE WIND FIELD
CONTINUES TO SPIN UP, ASSUMING THE VORTEX DOES NOT IMMEDIATELY
DECOUPLE FULLY FROM THE DEEP CONVECTION. HOWEVER, IN ABOUT 24
HOURS, VERTICAL SHEAR IS EXPECTED TO INCREASE CLOSER TO 30 KT,
LIKELY EXACERBATING THE VORTEX TILT. ADDITIONALLY, GLOBAL MODELS
EXPECT LARGE-SCALE SUBSIDENCE ON THE NORTHERN SIDE OF THE
CIRCULATION AS THE BACKGROUND WESTERLY WIND BURST TURNS
SOUTHEASTWARD AND DESCENDS DOWN A SLIGHTLY SLOPED ISENTROPIC
SURFACE AT THE MID-LEVELS, RESULTING IN SINKING AIR AND SIGNIFICANT
DRYING. THIS DRYING AIR MASS IS EXPECTED TO GET ENTRAINED INTO THE
NORTHERN SIDE OF THE STORM CORE, WORKING WITH MODERATE TO STRONG
VERTICAL SHEAR TO ARREST THE INTENSIFICATION TREND. DYNAMICAL
MODELS ARE IN AGREEMENT ON THIS EVOLUTION, SHOWING NO FURTHER
INTENSIFICATION AFTER 24 HOURS. THE JTWC FORECAST PEAKS AT 45 KT IN
12 TO 24 HOURS, GRADUALLY DECAYING TO 35 KT BY 48 HOURS. BY 72
HOURS, VERTICAL SHEAR IS EXPECTED TO ABATE, BUT A DRIER ENVIRONMENT
AND POTENTIAL COOLING OF SEA SURFACE TEMPERATURES BENEATH THE
SLOW-MOVING STORM IS CURRENTLY EXPECTED TO PREVENT SIGNIFICANT
REINTENSIFICATION. THE JTWC FORECAST EXTENDS TO 120 HOURS, BUT IT
IS POSSIBLE THAT THE CYCLONE COULD WEAKEN BELOW WARNING THRESHOLD A
DAY OR TWO EARLIER.

MODEL DISCUSSION: THE JTWC TRACK FORECAST IS CLOSE TO THE
MULTI-MODEL CONSENSUS UNTIL 96 HOURS, AFTER WHICH IT IS SLIGHTLY
EAST OF THE CONSENSUS. THE JTWC INTENSITY FORECAST IS CLOSE TO THE
MULTI-MODEL CONSENSUS THROUGH 24 HOURS AND THEN IS ABOUT 10 KT
BELOW THE CONSENSUS. THIS IS DUE TO THE STATISTICAL-DYNAMICAL SHIPS
INTENSITY GUIDANCE SHOWING SIGNIFICANTLY HIGHER INTENSITY THAN THE
DYNAMICAL MODELS, WHICH IS LARGELY BEING DISCOUNTED AT THIS TIME
DUE TO THE PROJECTED UNFAVORABLE ENVIRONMENT.

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: LOW
   TRACK 72-120 HR: LOW
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
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