WDXS33 PGTW 021500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 17S (SEVENTEEN) 
WARNING NR 002//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 10.2S 111.4E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 742 NM NORTH-NORTHWEST OF LEARMONTH,
AUSTRALIA
   MOVEMENT PAST 6 HOURS: NORTHEASTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
EXTREMELY SMALL, FULLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC)
FULLY EXPOSED, WHICH WAS DEVOID OF SIGNIFICANT CONVECTION AT THE
021200Z HOUR. HOWEVER, SUBSEQUENT IMAGERY INDICATES THAT WHILE THE
LLCC WAS EXPOSED, DEEP CONVECTION AGAIN BEGAN TO FLARE UP
IMMEDIATELY IN THE VICINITY OF THE LLCC, AND PERSISTED THROUGH THE
PRESENT TIME. THE MOST RECENT MICROWAVE IMAGERY FROM 120854Z SHOWED
THE LLCC BEGINNING TO BE EJECTED SHARPLY NORTHEASTWARD, WITHOUT
CONVECTION. THE INITIAL POSITION WAS ASSESSED WITH HIGH CONFIDENCE
BASED ON THE EXPOSED LLCC. THE INITIAL INTENSITY IS ASSESSED AT A
GENEROUS 35 KNOTS, IN LINE WITH THE ADT AND AIDT ESTIMATES OF 35-38
KNOTS RESPECTIVELY. UNFORTUNATELY THERE HAS BEEN NO RECENT
SCATTEROMETER DATA TO PROVIDE A MORE NUANCED INTENSITY MEASUREMENT.
THE ENVIRONMENT SURROUNDING TC 17S IS MARGINAL AT BEST, WITH VERY
WARM SSTS (29-30C) BEING OFFSET BY MODERATE TO STRONG (20-25 KNOTS)
EASTERLY VWS. THE RECENT FLAREUP IN CONVECTION IS LIKELY IN
RESPONSE TO SOME MODERATE MESOSCALE DIVERGENCE IN THE UPPER-LEVEL
FLOW AS INDICATED BY THE CIMSS ATMOSPHERIC MOTION VECTORS (AMVS).

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: LOW TO MID-LEVEL SOUTHERLY FLOW ON THE
NORTHEASTERN PERIPHERY OF A SUBTROPICAL RIDGE (STR) FAR TO THE
SOUTHEAST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS
   CIMSS ADT: 35 KTS AT 021150Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 20-25 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: WEAK WESTWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: WHILE TC 17S HAS MOVED NORTHEASTWARD OVER THE
PAST FEW HOURS, THIS IS LIKELY IN RESPONSE TO A SURGE IN THE
LOW-LEVEL SOUTHERLY FLOW EMANATING FROM A STRONG STR TO THE
SOUTH-SOUTHWEST. HOWEVER, IN THE NEXT 12 HOURS THE SYSTEM SHOULD
START TO MOVE EASTWARD THEN TURN GRADUALLY SOUTHEASTWARD AS A
NEAR-EQUATORIAL RIDGE (NER) RIDGES IN TO THE EAST OF THE SYSTEM.
THE SYSTEM IS EXPECTED TO CONTINUE TO STRUGGLE AGAINST THE
PERSISTENT EASTERLY SHEAR, AND IS UNLIKELY TO INTENSIFY THROUGH THE
FORECAST PERIOD. DUE TO THE SMALL NATURE OF THE CIRCULATION,
MESOSCALE CHANGES IN THE UPPER-LEVEL FLOW PATTERN WILL ALLOW FOR
CYCLES OF INTENSIFICATION AND WEAKENING ABOUT A MEAN OF 35 KNOTS
FOR ANOTHER 24 HOURS. BY TAU 36 HOWEVER, SHEAR IS EXPECTED TO
INCREASE TO THE POINT WHERE THE SYSTEMS CORE CANNOT SUSTAIN ITSELF,
 DRY AIR WILL COMPLETELY ENVELOPE THE SYSTEM AND TC 17S WILL
DISSIPATE AS A TROPICAL CYCLONE. 

MODEL DISCUSSION: THE MAJORITY OF THE TRACK GUIDANCE IS IN
RELATIVELY GOOD AGREEMENT IN SHOWING A GRADUAL TURN TOWARDS THE
SOUTH AFTER TAU 24. THE SINGLE SIGNIFICANT OUTLIER IS THE ECMWF
ENSEMBLE, WHICH AT LEAST THROUGH TAU 24 IS IN LINE WITH THE OTHER
MODELS, THEN SHARPLY TURNS EQUATORWARD AND LOOPS THE SYSTEM BACK TO
THE WEST. THE JTWC TRACK LIES ON THE NORTH AND EAST SIDE OF THE
TRACK ENVELOPE TO ACCOUNT FOR THE RECENT NORTHEASTWARD MOTION, WITH
MEDIUM CONFIDENCE. CONFIDENCE IN THE INTENSITY FORECAST IS LOW,
WITH SIGNIFICANT SPREAD IN THE MODEL GUIDANCE. THE MESOSCALE MODELS
SUPPORT STEADY WEAKENING FROM THE START OF THE FORECAST, WHILE THE
DECAY SHIPS GUIDANCE SUGGESTS A SLIGHT WEAKENING IN THE NEAR-TERM
FOLLOWED BY INTENSIFICATION TO A PEAK OF 45 KNOTS BY TAU 120. DUE
TO THE CONSISTENT SHEAR, AND DRY AIR ENTRAINMENT, THIS SOLUTION IS
CONSIDERED HIGHLY UNLIKELY, BUT DOES INSERT SIGNIFICANT UNCERTAINTY
INTO THE FORECAST.  

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