WDPS31 PGTW 101500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 12P (GABRIELLE) 
WARNING NR 011//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 24.8S 162.4E
   INITIAL INTENSITY: 75 KTS
   GEOGRAPHIC REFERENCE: 260 NM SOUTHWEST OF NOUMEA, NEW CALEDONIA
   MOVEMENT PAST 6 HOURS: EAST-SOUTHEASTWARD AT 30 KTS
   SIGNIFICANT WAVE HEIGHT: 35 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY INDICATES THAT
TC 12P (GABRIELLE) HAS ALREADY STARTED TO SHEAR APART AND BECOME
DECAPITATED BY SHARPLY INCREASING NORTHWESTERLY VERTICAL WIND SHEAR
(VWS). THE UPPER-LEVEL CIRCULATION WHICH IS CLEARLY EVIDENT IN THE
EIR IS ALREADY PULLING AWAY TO THE SOUTHEAST OF THE NOW PARTIALLY
EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC). EXTRAPOLATION OF A
WELL-DEFINED CIRCULATION CENTER IN THE 101049Z AMSU-B 89GHZ
MICROWAVE IMAGE, THE PARTIALLY EXPOSED LLCC IN THE EIR AND A CLOSE
CLUSTERING OF THE AGENCY POSITIONS PROVIDED HIGH CONFIDENCE TO THE
INITIAL POSITION. THE INITIAL INTENSITY IS SET AT 75 KNOTS WITH
MEDIUM CONFIDENCE. THE AGENCY CURRENT INTENSITY ESTIMATES RANGE
BETWEEN T4.0 TO T4.5 (65-77 KTS), WHILE THE ADT AND AIDT ARE
SIMILAR AT 77 KTS AND 68 KTS RESPECTIVELY. THE JTWC INTENSITY IS
PLACED ON THE HIGHER END OF THE INTENSITY ESTIMATES, THOUGH IT IS
COMING DOWN EXTREMELY QUICKLY. THE ENVIRONMENT HAS QUICKLY TURNED
UNFAVORABLE, AS SHEAR IS NOW ESTIMATED AT OVER 40 KNOTS, AND SSTS
HAVE DROPPED TO BELOW 26C.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK AND PARTIAL
SCATTEROMETER DATA.

CURRENT STEERING MECHANISM: STRONG GRADIENT BETWEEN STR TO THE EAST
AND STRONG UPPER-LEVEL TROUGH TO THE WEST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.5 - 77 KTS
   ABRF: T4.0 - 65 KTS
   CIMSS ADT: 77 KTS AT 101130Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 30+ KTS
   SST: 26-27 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: INFLUX OF VERY DRY AIR IN THE MID TO UPPER-LEVELS

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: TC 12P IS NOW RACING SOUTHEASTWARD AT OVER 30
KNOTS WITHIN THE VERY TIGHT GRADIENT BETWEEN THE STRONG DEEP-LAYER
STR TO THE EAST AND A DEEP UPPER-LEVEL TROUGH RAPIDLY MOVING IN
FROM THE WEST. ANIMATED WATER VAPOR IMAGERY SHOWS THE TROUGH AXIS
LIES ABOUT FOUR DEGREES TO THE WEST OF TC 12P BUT IS MOVING IN
QUICKLY. HENCE, THE GRADIENT IS LIKELY ABOUT AS TIGHT AS ITS GOING
TO BE AT THE CURRENT TIME, SO TRACK SPEED GOING FORWARD SHOULD
START TO SLOW STEADILY THROUGH THE SHORT DURATION OF THE FORECAST.
AT THE SAME TIME, SHEAR IS LIKELY PEAKING AT THE CURRENT TIME AND
AS THE TROUGH MOVES EVER CLOSER IT WILL USHER FORTH A MASSIVE
INFLUX OF VERY DRY AIR AND TC 12P WILL LIKELY BE FULLY DECAPITATED
WITHIN THE NEXT 12 HOURS, LEAVING BEHIND A NAKED LLCC WITH A
SHALLOW WARM CORE. THE SYSTEM HAS ALREADY BEGUN SUBTROPICAL
TRANSITION (STT) AND WILL COMPLETE STT BY TAU 24 AS IT MOVES
DIRECTLY UNDER THE UPPER-LEVEL TROUGH AXIS. WHILE THE SYSTEM IS
RAPIDLY WEAKENING, IT WILL REMAIN A STRONG GALE-FORCE TO
STORM-FORCE SUBTROPICAL LOW EVEN AFTER THE END OF THE FORECAST
PERIOD, AND TRACK GENERALLY TOWARDS THE NORTH ISLAND OF NEW
ZEALAND. 

MODEL DISCUSSION: TRACK GUIDANCE IS IN VERY TIGHT AGREEMENT THROUGH
THE SHORT DURATION OF THE FORECAST PERIOD, LENDING HIGH CONFIDENCE
TO THE JTWC FORECAST TRACK. INTENSITY GUIDANCE IS IN GOOD AGREEMENT
ON THE RAPID WEAKENING SCENARIO, WITH SOME EXPECTED DIFFERENCES IN
THE RATE OF WEAKENING. THE JTWC FOLLOWS THE MOST AGGRESSIVE
WEAKENING DEPICTED BY THE DECAY-SHIPS (GFS VERSION), ABOUT 10-15
KNOTS BELOW THE REMAINDER OF THE CONSENSUS MEMBERS AT TAU 24.
CONFIDENCE IN THE INTENSITY FORECAST IS MEDIUM DUE TO THE
RELATIVELY LARGE SPREAD IN THE GUIDANCE AND THE LOW CONFIDENCE
INITIAL INTENSITY. 

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