WDXS31 PGTW 202100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 18S (NEVILLE) WARNING 
NR 033//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.9S 107.9E
   INITIAL INTENSITY: 60 KTS
   GEOGRAPHIC REFERENCE: 438 NM NORTHWEST OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: WESTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 19 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
CYCLONE(TC) 18S WITH A RAGGED, PINHOLE EYE FEATURE AND WELL-DEFINED
CONVECTIVE CLOUD BANDS WRAPPING INTO THE CENTER. BASED ON MICROWAVE
IMAGERY, IT APPEARS THAT THE VORTEX IS WELL ORGANIZED IN THE
UPPER-LEVELS BUT DOES NOT FULLY EXTEND TO THE LOW LEVELS AS
DEPICTED IN AN EARLIER 201124Z 37GHZ SSMIS IMAGE. THE SYSTEM IS
CURRENTLY BEING STEERED ON THE NORTHERN PERIPHERY OF A SUBTROPICAL
RIDGE (STR) TO THE SOUTH. CURRENTLY THE SYSTEM IS IN FAVORABLE SEA
SURFACE TEMPERATURES (SST) OF 26C. MILD AMOUNTS OF VERTICAL WIND
SHEAR (VWS) FROM THE NORTHEAST SLIGHTLY CHALLENGE THE VORTEX. THE
MOIST CORE IS CURRENTLY SELF-SUSTAINING WHILE SWADDLED BY DRY AIR
THAT HAS YET TO PENETRATE THE CORE. UPPER-LEVEL OUTFLOW IS RADIAL,
BUT RELATIVELY CONSTRAINED TO THE IMMEDIATE VICINITY OF THE SYSTEM,
WITH NO OUTFLOW CHANNELS. THE INITIAL POSITION WAS ASSESSED WITH
HIGH CONFIDENCE BASED ON ANIMATED EIR DEPICTING A CLEAR UPPER-LEVEL
CORE SURROUNDED BY BANDING. THE INITIAL INTENSITY WAS ASSESSED WITH
HIGH CONFIDENCE BASED ON A 201454Z ULTRA-HIGH RESOLUTION (UHR)
METOP-B WIND SPEED PRODUCT DEPICTING 50-60KT WINDS IN THE NORTHERN
HEMISPHERE.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: THE NORTHERN PERIPHERY OF A STR TO THE
SOUTH.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.0 - 65 KTS
   KNES: T4.5 - 77 KTS
   DEMS: T3.5 - 55 KTS
   APRF: T4.0 - 65 KTS
   CIMSS SATCON: 58 KTS AT 201900Z
   CIMSS ADT: 51 KTS AT 201800Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 26-27 CELSIUS
   OUTFLOW: MODERATE RADIAL

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TC 18S IS FORECAST TO CONTINUE TO TRACK WEST
THROUGH TAU 36 BEFORE CURVING TO THE WEST-SOUTHWEST AS IT BEGINS TO
ROUND THE STR THROUGH THE END OF THE FORECAST PERIOD. IN TERMS OF
INTENSITY, THE SYSTEM IS FORECAST TO REMAIN IN A SELF-SUSTAINING
MOIST POCKET SURROUNDED BY DRY AIR, ALLOWING THE SYSTEM TO SUSTAIN
GRADUAL INTENSIFICATION FOR THE FIRST 24 HOURS. BY TAU 24, THE JET
MAXIMUM TO THE SOUTH IS FORECAST TO AMPLIFY UPPER-LEVEL DIVERGENCE
AND SIGNIFICANTLY IMPROVE POLEWARD OUTFLOW, LEADING TO MORE RAPID
INTENSIFICATION THROUGH TAU 48. AFTER 48 HOURS, VWS IS FORECAST TO
INCREASE TO MODERATE AND HIGH LEVELS, COUNTERING THE IMPROVEMENT OF
POLEWARD OUTFLOW, AND CAUSING DRY AIR TO SMOTHER THE VORTEX. THERE
IS A POTENTIAL FOR A BRIEF PERIOD OF SUBTROPICAL INFLUENCE ON TC
18S, AS IT INTERACTS WITH A MID-LATITUDE UPPER-LEVEL TROUGH. THE
SYSTEM WILL LIKELY BEGIN THE EARLY PHASES OF SUBTROPICAL TRANSITION
(STT) BETWEEN TAU 96 AND TAU 120, BUT IT IS EXPECTED TO COMPLETELY
DISSIPATE BEFORE STT CAN COMPLETE. 

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE IS IN GOOD AGREEMENT
THROUGH TAU 72 THAT TC 18S WILL CONTINUE TO CURVE WEST AND THEN
SLOWLY WEST-SOUTHWEST AS IT ROUNDS A STR TO THE SOUTH. HOWEVER, IN
THE LATER PORTION OF THE FORECAST, THE MODEL GUIDANCE DIVERGES
SHARPLY, WITH THE GALWEM AND UKMET TRACKING THE SYSTEM ALMOST DUE
WEST THROUGH THE END OF THE FORECAST, WHILE THE GFS DIVES SHARPLY
SOUTHWARD, LEADING TO A 630NM SPREAD BY TAU 120. THE JTWC FORECAST
LIES JUST SOUTH OF THE MULTI-MODEL CONSENSUS MEAN, WITH HIGH
CONFIDENCE THROUGH TAU 72, TRENDING TO LOW CONFIDENCE IN THE
EXTENDED FORECAST. THE JTWC INTENSITY GUIDANCE IS IN GOOD AGREEMENT
AS WELL THAT THERE WILL BE INTENSIFICATION THROUGH TAU 48 BEFORE
WEAKENING, BUT THERE IS DISAGREEMENT IN TERMS OF EXACTLY HOW MUCH.
HWRF REFLECTS A PEAK OF 105KTS, BUT GFS ONLY FORECASTS A PEAK OF
70KTS. COAMPS-TC IS THE ONLY STARK OUTLIER AND FORECASTS THAT THE
SYSTEM WILL NOT INTENSIFY AT ALL AND COMPLETELY DISSIPATE BY TAU
96. 

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