WDXS31 PGTW 020900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 11S (IGGY) WARNING NR 
007//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 16.1S 109.4E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 457 NM NORTHWEST OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTHWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 16 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE (TC) 11S (IGGY) HAS WEAKENED SUBSTANTIALLY
OVER THE COURSE OF THE PREVIOUS 12 HOURS. DEEP CONVECTION BRIEFLY
REFORMED OVER THE LOW LEVEL CIRCULATION CENTER (LLCC) AROUND 0000Z,
BUT WAS RAPIDLY DISPLACED TO THE EAST DUE TO HIGH WESTERLY VERTICAL 
WIND SHEAR. CURRENT ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) 
DEPICTS A FULLY EXPOSED, AND INCREASINGLY ILL-DEFINED LLCC WITH 
REMNANT CONVECTION IN THE NORTHEASTERN QUADRANT. MODEL CROSS-
SECTIONS, AND A 020602Z NOAA-21 ATMS 89GHZ IMAGE REVEALED SIGNIFICANT 
DRY MID-LEVEL AIR INTRUSION FROM THE WEST AND OVER TOP OF THE NOW 
EXPOSED LLCC. ANIMATED WATER VAPOR IMAGERY SHOWS A DEEP TROF AXIS 
POSITIONED TO THE SOUTHWEST, PROVIDING THE SYSTEM WITH STRONG 
POLEWARD OUTFLOW.  HOWEVER, THIS IS INSUFFICIENT TO COUNTERACT THE 
EFFECTS OF HIGH VWS AND DRY AIR INTRUSION. THE INITIAL POSITION IS 
ASSESSED WITH HIGH CONFIDENCE BASED ON THE EXPOSED LLCC IN THE MSI, AS 
WELL AS AN IDENTIFIABLE LLCC IN A 020556Z AMSR2 36GHZ MICROWAVE IMAGE. 
THE INITIAL INTENSITY IS ASSESSED WITH HIGH CONFIDENCE, HEDGED 
SLIGHTLY HIGHER THAN THE MAJORITY OF THE SUBJECTIVE AND OBJECTIVE 
INTENSITY ESTIMATES NOTED BELOW, GIVEN A 020409Z OSCAT-3 PASS WHICH
SHOWED 40 KNOT WINDS IN THE SOUTHERN QUADRANT OF THE CIRCULATION.
ENVIRONMENTAL CONDITIONS ARE UNFAVORABLE, WITH HIGH WESTERLY SHEAR,
SIGNIFICANT DRY AIR INTRUSION, WARM SSTS AND STRONG POLEWARD
OUTFLOW.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: THE SYSTEM IS CURRENTLY UNDERGOING A 
TRANSITION IN STEERING INFLUENCE, FROM THE NEAR EQUATORIAL RIDGE 
(NER) TO THE NORTH AND A BUILDING SUBTROPICAL RIDGE (STR) TO THE 
SOUTH. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   APRF: T2.5 - 35 KTS
   CIMSS SATCON: 39 KTS AT 020602Z
   CIMSS ADT: 35 KTS AT 020540Z
   CIMSS AIDT: 32 KTS AT 020540Z
   CIMSS D-MINT: 33 KTS AT 020602Z
   CIMSS D-PRINT: 30 KTS AT 020610Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 20-25 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: SIGNIFICANT MID-LEVEL DRY AIR MOVING IN FROM THE
WEST. 

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE FORECAST HAS BEEN TRUNCATED TO 48
HOURS. 

FORECAST DISCUSSION: TROPICAL CYCLONE (TC) 11S HAS TURNED SOUTHWARD
OVER THE PAST SIX HOURS, AND THE MOST RECENT FRAMES OF VISIBLE
IMAGERY SUGGEST A SOUTHWESTWARD TRAJECTORY. IN THE LOWER-LEVELS,
THE STR TO THE SOUTHWEST IS BUILDING IN RAPIDLY, PUSHING THE RIDGE
NORTHWARD ALONG THE EASTERN FLANK OF TC 11S, FACILITATING A MORE
RAPID TRANSITION TO A SOUTHWESTWARD TRACK THAN PREVIOUSLY 
ANTICIPATED. OVER THE NEXT 12 TO 18 HOURS, THE RIDGE WILL CONTINUE TO 
BUILD EASTWARD, POSITIONING TC 11S ON ITS NORTHERN PERIPHERY, 
RESULTING IN A WESTWARD TRACK THROUGH THE REMAINDER OF THE FORECAST 
PERIOD. WHILE THE INCREASE IN DRY AIR AND SHEAR WAS ANTICIPATED, THE 
INCREASE IN BOTH PARAMETERS OCCURRED EARLIER THAN ANTICIPATED 
RESULTING IN LOWER PEAK INTENSITY AND MORE RAPID WEAKENING THAN 
PREVIOUSLY FORECAST. OVER THE NEXT 48 HOURS, THE SYSTEM WILL CONTINUE 
TO ENCOUNTER HIGH VWS AND INCREASING DRY AIR INTRUSION INTO AND OVER 
TOP OF THE LLCC. EPISODIC AND BRIEF BURSTS OF CONVECTION WILL 
CONTINUE TO OCCUR OVER THE NEXT 24 TO 36 HOURS, ALLOWING THE SYSTEM 
TO MAINTAIN A 35 KNOT INTENSITY. HOWEVER, BY TAU 48 (IF NOT EARLIER), 
TC 11S WILL DISSIPATE AS SHEAR AND DRY AIR ENTRAINMENT REACH THEIR 
MAXIMUM VALUES.  

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN GOOD
AGREEMENT, ALL MODELS BEING CONSTRAINED TO A 115NM ENVELOPE DEFINED
BY GFS ON THE NORTHERN EDGE AND NAVGEM ON THE SOUTHERN SIDE. THE
REMAINDER OF THE CONSENSUS MEMBERS ARE TIGHTLY CLUSTERED AROUND THE
CONSENSUS MEAN. THE JTWC FORECAST IS POSITIONED NEAR THE ECMWF AND
GDM MEAN THROUGH THE FORECAST PERIOD WITH HIGH CONFIDENCE.
INTENSITY GUIDANCE IS ALSO IN GOOD AGREEMENT, WITH ALL MODELS
EXCEPT THE HAFS-A INDICATING A GRADUAL WEAKENING TREND CULMINATING IN 
DISSIPATION AT TAU 48. THE HAFS-A IS A NOTABLE OUTLIER, FORECASTING A 
HIGHER INTENSITY AND MAINTAINING IT AT OR ABOVE 35 KNOTS. THE JTWC 
FORECAST IS PLACED NEAR THE CONSENSUS MEAN WITH HIGH CONFIDENCE. 

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