WDXS31 PGTW 281500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 11S (ELEVEN) WARNING 
NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 11.7S 76.2E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 352 NM SOUTHEAST OF DIEGO GARCIA
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHEASTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 15 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
CYCLONE 11S WITH A WELL-DEFINED, EXPOSED LOW-LEVEL CIRCULATION
CENTER (LLCC). PERSISTENT CONVECTION IS PLACED OVER THE
SOUTHWESTERN PERIPHERY OF THE CIRCULATION, REVEALING THE TILTED
NATURE OF THE VORTEX. ENVIRONMENTAL ANALYSIS INDICATES THAT 11S IS
IN A MARGINALLY FAVORABLE ENVIRONMENT FOR DEVELOPMENT CHARACTERIZED
BY MODERATE POLEWARD OUTFLOW ALOFT AND WARM (28-29 C) SEA SURFACE
TEMPERATURES, OFFSET BY DRY AIR TO THE WEST OF THE LLCC AND
MODERATE (15-20 KTS) EASTERLY VERTICAL WIND SHEAR. THE INITIAL
POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE EXPOSED LLCC
IN THE ANIMATED MSI. THE INITIAL INTENSITY OF 35 KTS IS ASSESSED
WITH LOW CONFIDENCE BASED ON A 280845Z AMSR2 WIND SPEED IMAGE
SHOWING 32-37 KTS WITHIN THE SOUTHWESTERN QUADRANT BUT COULD BE
ELEVATED DUE TO THE PRESENCE OF CONVECTION. AGENCY DVORAK ESTIMATES
ARE INTERPRETED TO BE SLIGHTLY LOW DUE TO THE EXPOSED NATURE OF THE
LLCC.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: NORTHWESTERN PERIPHERY OF A SUBTROPICAL
RIDGE (STR) CENTERED FAR TO THE EAST-SOUTHEAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   KNES: T2.0 - 30 KTS
   FMEE: T1.5 - 25 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE POLEWARD
   OTHER FACTORS: MILD DRY AIR ENTRAINMENT WITHIN THE WESTERN
SECTOR OF THE SYSTEM

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: 11S IS FORECAST TO TRACK SOUTHWESTWARD ALONG
THE NORTHWESTERN PERIPHERY OF THE STR THROUGH TAU 36. AFTER TAU 36,
A SECOND RIDGE IS EXPECTED TO BUILD SOUTH OF THE SYSTEM AND DRIVE
IT MORE WESTWARD. AROUND TAU 96, 11S WILL BEGIN TO ROUND THE
NORTHWESTERN PERIPHERY OF THAT RIDGE AND TRACK SOUTHWESTWARD
THROUGH THE REMAINDER OF THE FORECAST PERIOD. REGARDING INTENSITY,
11S IS FORECAST TO SLIGHTLY INTENSIFY OVER THE NEXT 12 HOURS TO
AROUND 40 KTS. AFTER TAU 12, INTENSITY IS FORECAST TO MAINTAIN
THROUGH TAU 48. THE PRIMARY LIMITING FACTOR IS THE TILTED NATURE OF
THE VORTEX AND PERSISTENT MODERATE EASTERLY SHEAR. THESE TWO
FACTORS WILL INHIBIT ANY FURTHER INTENSIFICATION DURING THIS TIME.
AROUND TAU 72, MODELS VARY SIGNIFICANTLY ON THE NATURE OF THE
VORTEX ALIGNMENT. DEPENDING ON HOW MUCH THE SYSTEM CAN ALIGN, AN
INTENSIFICATION TREND COULD ENSUE. IF IT REMAINS TILTED IT COULD
SIMPLY FALL APART. AS FOR NOW, THE JTWC FORECAST CALLS FOR MODERATE
INTENSIFICATION AFTER TAU 72 DUE TO THE FACT THAT EASTERLY SHEAR
WILL CONTINUE TO BE A FACTOR. 

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN MODERATE AGREEMENT
ON THE TRACK OF 11S WITH A 210 NM CROSS-TRACK SPREAD AT TAU 72
WHICH OPENS TO 320 NM AT TAU 120. ECMWF IS THE EASTERNMOST MEMBER
OF CONSENSUS WHILE GFS IS THE WESTERNMOST MEMBER. THE JTWC TRACK
FORECAST IS PLACED CLOSE TO CONSENSUS THROUGHOUT THE FORECAST
PERIOD WITH MEDIUM CONFIDENCE THROUGH TAU 72 AND LOW THEREAFTER.
INTENSITY GUIDANCE IS IN POOR AGREEMENT WITH HIGH VARIANCE BETWEEN
MODELS. AFTER TAU 24, GFS SUGGESTS A CONTINUAL GRADUAL WEAKENING
TREND WHILE HAFS-A, HWRF, AND COAMPS-TC SUGGEST SLIGHT
INTENSIFICATION THOUGH TAU 48. AFTER TAU 48, THERE IS AN EXTREME
DISPARITY BETWEEN GUIDANCE. HAFS-A AND COAMPS-TC SUGGEST RAPID
INTENSIFICATION THROUGH TAU 120 TO A PEAK OF NEAR 115 KTS WHILE
HWRF SUGGESTS A CONSTANT INTENSITY OF AROUND 40 KTS. THE JTWC
INTENSITY FORECAST SHOOTS THE DIFFERENCE BETWEEN THE TWO
SCENARIOS, DUE TO THE EXPECTED MARGINALLY FAVORABLE ENVIRONMENT.

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