WDXS33 PGTW 020900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 17S (SEVENTEEN) 
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 10.8S 111.1E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 711 NM NORTH-NORTHWEST OF LEARMONTH,
AUSTRALIA
   MOVEMENT PAST 6 HOURS: NORTHEASTWARD AT 01 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A
DISORGANIZED SYSTEM, WITH RELATIVELY WEAK, FLARING CONVECTION
SHEARED OFF TO THE WEST OF A PARTIALLY EXPOSED, ELONGATED LOW LEVEL
CIRCULATION CENTER (LLCC). WHILE THE LLCC IS DISORGANIZED IN THE
ANIMATED MSI, A 020223Z ASCAT-B BULLSEYE PASS INDICATED A VERY WELL
ORGANIZED STRUCTURE, WITH 30 KNOT WINDS SURROUNDING THE CENTER AND
AN AREA OF 35 KNOT WINDS IN THE SOUTHERN HEMISPHERE, MOSTLY IN THE
SOUTHWESTERN QUADRANT. THE INITIAL POSITION IS ASSESSED WITH MEDIUM
CONFIDENCE BASED ON AN EXTRAPOLATION OF THE PREVIOUSLY MENTIONED
ASCAT DATA AND ANALYSIS OF THE EXPOSED LOW LEVEL BANDING FEATURES.
A 020705Z GPM COLORIZED 89GHZ MICROWAVE IMAGE PROVIDED ADDITIONAL
CLARITY TO THE INITIAL POSITION AND VERIFIED THE DISORGANIZED LOW
LEVEL STRUCTURE OF THE SYSTEM. THE INITIAL INTENSITY OF 35 KNOTS IS
ASSESSED WITH HIGH CONFIDENCE BASED ON THE AFOREMENTIONED ASCAT
DATA. THE SYSTEM IS QUASI-STATIONARY FOR THE TIME BEING, IN A WEAK
STEERING ENVIRONMENT, WITH MARGINALLY FAVORABLE CONDITIONS
SUPPORTED TROPICAL CYCLONE DEVELOPMENT. SSTS ARE WARM (29C), BUT
OUTFLOW IS LIMITED AND PERSISTENT EASTERLY SHEAR IS RUNNING IN THE
15-25 KNOT RANGE.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: WEAK STEERING PATTERN BETWEEN A
NEAR-EQUATORIAL RIDGE (NER) TO THE NORTH AND A SUBTROPICAL RIDGE
(STR) TO THE SOUTH. THE SYSTEM IS TRAPPED IN A LOW TO MID-LEVEL
TROUGH EXTENDING SOUTHEAST TO TC 15S. 

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

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: WEAK WESTWARD
   OTHER FACTORS: MID-LEVEL DRY AIR

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: TC 17S IS A VERY SMALL, MARGINAL TROPICAL
CYCLONE AND WILL NOT LIKELY BE LONG IN THE TOOTH. AS DISCUSSED
EARLIER, THE 020233Z ASCAT PASS INDICATES SUPPORTS A REANALYSIS OF
THE 020000Z BEST TRACK TO 35 KNOTS AND SUPPORTS THE CURRENT BEST
TRACK ANALYSIS OF 35 KNOTS. THE SYSTEM HAS BEEN DRIFTING GENERALLY
EASTWARD OVER THE PAST COUPLE OF DAYS AND REMAINS QUASI-STATIONARY
AT THIS POINT IN A WEAK STEERING PATTERN. OVER THE NEXT 12 HOURS
THE NER TO THE NORTH WILL BUILD, WHILE THE TROUGH TO THE SOUTH WILL
STRENGTHEN, AND TC 17S WILL BE EJECTED OUT OF ITS LUMBERING
MOVEMENT TOWARDS THE EAST. AS THE TROUGH TO THE SOUTH CONTINUES TO
DEEPEN AND MOVE TOWARDS THE EAST, TC 17S WILL TURN SOUTHEAST WITH
IT. IN TERMS OF INTENSITY, THE PERSISTENT SHEAR WILL BE THE PRIMARY
INHIBITOR TO SIGNIFICANT DEVELOPMENT AND THE SYSTEM IS NOT EXPECTED
TO INTENSIFY, AS THE SHEAR OFFSETS MARGINAL DIFFLUENT OUTFLOW AND
WARM SSTS. DRY AIR IN THE MID-LEVELS AND SHEAR ARE BOTH EXPECTED TO
INCREASE AFTER TAU 24, LEADING TO DISSIPATION OVER WATER BY TAU 36.
DUE TO THE VERY SMALL AND DISORGANIZED STRUCTURE OF THE SYSTEM AT
PRESENT, IT IS POSSIBLE IT MAY REACT MORE STRONGLY TO THE
PERSISTENT SHEAR AND BE UNABLE TO MAINTAIN TROPICAL STORM STRENGTH
FOR MUCH LONGER THAN 12 HOURS. 

MODEL DISCUSSION: TRACK GUIDANCE IS IN POOR AGREEMENT OVERALL, WITH
THE GFS TURNING THE SYSTEM SHARPLY SOUTH THE SOUTHWEST, WHILE
THE GFS AND ECMWF ENSEMBLE MEANS TURN THE TRACK IN AN EQUATORWARD
LOOP AFTER TAU 24. THE JTWC FORECAST LIES NORTH OF THE CONSENSUS
MEAN, FAVORING THE ECMWF TRACKER MOST CLOSELY. CONFIDENCE IN THE
TRACK IS LOW CONSIDERING THE HIGH UNCERTAINTY IN THE GUIDANCE.
INTENSITY GUIDANCE IS IN GOOD AGREEMENT, WITH ONLY THE NAVGEM DECAY
SHIPS INDICATING ANY SIGNIFICANT DEVELOPMENT. THE JTWC FORECAST
LIES CLOSE TO THE MEAN THROUGH THE FORECAST WITH MEDIUM CONFIDENCE.


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