WDXS31 PGTW 052100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21S (TWENTYONE) WARNING 
NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 13.3S 120.2E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 642 NM NORTHEAST OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTHWARD AT 05 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY SHOWS A
MEDIUM-SIZED SYSTEM THAT CONTINUED TO CONSOLIDATE AND EXPAND WITH
FEEDER BANDS WRAPPING TIGHTER TOWARD AN OBSCURED LOW LEVEL
CIRCULATION CENTER (LLCC). A STRONG POLEWARD OUTFLOW IS ALSO EVIDENT
WITH EXPANSIVE CIRRUS STREAKS ALONG THE SOUTHERN PERIPHERY. THE
INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON A
MICROWAVE LLCC FEATURE IN THE 051719Z AMSR2 IMAGE SUITE. THE
INITIAL INTENSITY IS PLACED WITH MEDIUM CONFIDENCE BASED ON THE
PGTW AND APRF DVORAK ESTIMATES OF T2.5 AND REFLECTS THE 6-HR
CONVECTIVE AND WRAP IMPROVEMENT. ANALYSIS INDICATES A FAVORABLE
ENVIRONMENT WITH LOW VWS, STRONG OUTFLOW, AND WARM SST SLIGHTLY
OFFSET BY DRY AIR INTRUSION AT THE LOW LEVELS ALONG THE WESTERN
PERIPHERIES.

INITIAL WIND RADII BASIS: PERSISTENCE.

CURRENT STEERING MECHANISM: PERIPHERY OF NEAR EQUATORIAL RIDGE
(NER) TO THE EAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   KNES: T1.5 - 25 KTS
   APRF: T2.5 - 35 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TC 21S WILL CONTINUE TO TRACK SOUTHWARD THEN
SOUTH-SOUTHWESTWARD UNDER THE STEERING INFLUENCE OF THE NER AND ITS
SOUTHWEST EXTENSION. AFTERWARD, AS THE CYCLONE GRADUALLY WEAKENS, A
LOW LEVER REFLECTION OF THE SUBTROPICAL RIDGE TO THE SOUTHEAST WILL
ASSUME STEERING AND DRIVE A WEAKER AND SHALLOWER 21S
WEST-SOUTHWESTWARD. THE FAVORABLE ENVIRONMENT WILL FUEL A FASTER
THAN NORMAL RATE OF INTENSIFICATION TO A PEAK OF 65KTS BY TAU 36.
AFTERWARD, INCREASING COLD DRY AIR INTRUSION AT THE LOW LEVELS, THEN
LATER ON, INCREASING VWS AS THE CYCLONE APPROACHES THE JET LEVEL
WINDS ASSOCIATED WITH THE PREVAILING WESTERLIES, AND COOLING SST
WILL GRADUALLY WEAKEN THE SYSTEM DOWN TO 35KTS BY TAU 120.  

MODEL DISCUSSION: NUMERICAL WEATHER PREDICTION MODELS ARE IN GOOD
AGREEMENT WITH A GRADUAL AND EVEN SPREAD ACROSS- AND ALONG-TRACK
WITH THE EXCEPTION OF NAVGEM THAT IS ON THE LEFT MARGIN OF THE
MODEL ENVELOPE. THE MAIN PACK THAT SPREADS OUT TO A MERE 75NM AT
TAU 72 AND 95NM BY TAU 120. IN VIEW OF THESE, THERE IS MEDIUM
CONFIDENCE IN THE JTWC TRACK FORECAST THAT IS LAID JUST TO THE
RIGHT OF CONSENSUS TO OFFSET NAVGEM. 

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