WDPS31 PGTW 091500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 12P (GABRIELLE) 
WARNING NR 007//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.6S 154.8E
   INITIAL INTENSITY: 70 KTS
   GEOGRAPHIC REFERENCE: 490 NM NORTH-NORTHEAST OF BRISBANE,
AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTHEASTWARD AT 11 KTS
   SIGNIFICANT WAVE HEIGHT: 30 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
COMPACT SYSTEM WITH A RAGGED AND CLOUD-FILLED EYE FEATURE. HOT
TOWERS CONTINUE TO FIRE UP ON THE PERIPHERY OF THE EYE BUT THE
CONVECTION HAS NOT YET FORMED A CONTINUOUS EYEWALL. THIS IS BACKED
UP BY A 091111Z AMSU-B 89GHZ MICROWAVE IMAGE SHOWING A WEAKLY
DEFINED EYE FEATURE AND A FRAGMENTED EYEWALL, WHICH REMAINS OPEN ON
THE WESTERN SIDE. THE INITIAL POSITION IS ASSESSED WITH HIGH
CONFIDENCE BASED ON THE RAGGED EYE FEATURE IN THE AMSU-B MICROWAVE
IMAGE. THE INITIAL INTENSITY OF 70 KNOTS IS ASSESSED WITH MEDIUM
CONFIDENCE, ROUGHLY BETWEEN THE PGTW T4.0 (65 KTS) AND THE OVERLY
AGGRESSIVE KNES T5.0 (90 KTS) DVORAK CURRENT INTENSITY (CI)
ESTIMATES. THERE REMAINS A WIDE SPREAD IN THE SUBJECTIVE INTENSITY
ESTIMATES AS WELL, WITH THE ADT NEAR 60 KTS, THE AIDT AT 67 KTS
WHILE THE MOST RECENT OPEN-AIIR IS VERY HIGH AT 80 KTS. IT IS CLEAR
THE SYSTEM IS INTENSIFYING, BUT WITHOUT A DISTINCT EYE IN THE EIR
AND A VERY RAGGED EYE IN THE MICROWAVE IMAGERY, IT IS STILL
SOMEWHAT SLOW IN DOING SO. THE ENVIRONMENT REMAINS CONDUCIVE FOR
ADDITIONAL INTENSIFICATION, WITH LOW (10-15 KTS), BUT INCREASING
VWS, WARM SSTS AND ROBUST POLEWARD OUTFLOW. WATER VAPOR IMAGERY
SHOWS AN EXTREMELY DRY MID AND UPPER-LEVEL AIR MASS SITTING OVER
EASTERN AUSTRALIA, ASSOCIATED WITH A ROBUST UPPER-LEVEL TROUGH,
WHICH FOR THE MOMENT REMAINS FAR ENOUGH AWAY TO NOT HAVE MUCH OF AN
IMPACT, BUT WHICH WILL BECOME INCREASINGLY IMPORTANT MOVING
FORWARD.

INITIAL WIND RADII BASIS: PARTIAL SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: SOUTHWESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) TO THE EAST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.0 - 65 KTS
   KNES: T5.0 - 90 KTS
   ABRF: T4.0 - 65 KTS
   CIMSS SATCON: 65 KTS AT 090950Z
   CIMSS ADT: 59 KTS AT 091130Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: STRONG POLEWARD

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TROPICAL CYCLONE 12P (GABRIELLE) CONTINUES TO
ACCELERATE ON A SOUTHEASTWARD HEADING, NOW FULLY UNDER THE STRONG
GRADIENT BETWEEN THE STR TO THE EAST AND AN APPROACHING TROUGH
CURRENTLY OVER EASTERN AUSTRALIA. THE SYSTEM WILL CONTINUE TO
ACCELERATE THROUGH THE NEXT 24 HOURS AS THE TROUGH CONTINUES TO
MOVE IN AND INCREASE THE STEERING GRADIENT. AROUND TAU 36, TC 12P
WILL START TO MOVE UNDER THE 200MB TROUGH AXIS AND AT THE SAME TIME
THE STR TO THE NORTHEAST BEGINS TO WEAKEN AND IN RESPONSE TC 12P
WILL BEGIN TO SLOW AND TURN TO A SLIGHTLY MORE EAST-SOUTHEAST
TRAJECTORY BY THE END OF THE FORECAST PERIOD. THE ENVIRONMENT
REMAINS CONDUCIVE TO FURTHER INTENSIFICATION, THOUGH ONLY FOR ABOUT
ANOTHER 24 HOURS. IN THAT TIME, THE ROBUST POLEWARD OUTFLOW INTO
THE DIVERGENT PORTION OF THE TROUGH MOVING IN FROM THE WEST, THE
WARM SSTS AND LOW TO MODERATE WIND SHEAR, WILL ALLOW THE SYSTEM TO
REACH A PEAK OF 80 KNOTS AT OR BEFORE TAU 24. THE KEY FACTOR WILL
BE IF THE SYSTEM CAN IN FACT DEVELOP AND SUSTAIN AN EYE OR IF THE
SHEAR INCREASES FASTER AND WINS THE RACE. THE SYSTEM CROSSES THE
26C ISOTHERM SHORTLY AFTER TAU 24 AND WILL START TO MOVE UNDER THE
200MB TROUGH AXIS BY TAU 36, MARKING THE ONSET OF SUBTROPICAL
TRANSITION. BEGINNING AROUND TAU 36, THE VERY DRY MID AND
UPPER-LEVEL AIR MASS MENTIONED EARLIER, ACCOMPANIED BY SIGNIFICANT
NORTHWESTERLY SHEAR, MOVES IN AND DECIMATES THE WARM CORE OF TC 12P
BETWEEN TAU 36 AND 48. SUBTROPICAL TRANSITION WILL BE QUICK,
LEAVING A SHALLOW, ASYMMETRIC WARM-CORE SUBTROPICAL, STORM-FORCE
LOW WITH A SIGNIFICANTLY LARGER WIND FIELD BARRELING TOWARDS THE
NORTH ISLAND OF NEW ZEALAND. 

MODEL DISCUSSION: DETERMINISTIC AND ENSEMBLE TRACK GUIDANCE IS IN
VERY GOOD AGREEMENT THROUGH THE FORECAST PERIOD, WITH LITTLE IN THE
WAY OF CROSS-TRACK SPREAD. SOME MARGINAL ALONG-TRACK SPREAD
MANIFESTS ITSELF AFTER TAU 36 BUT IS NOT SIGNIFICANT. INTENSITY
GUIDANCE IS SPLIT, WITH THE HWRF AND GFS INDICATING STEADY
WEAKENING FROM TAU 00, WHILE THE COAMPS-TC (BOTH GFS AND NAVGEM
VERSIONS) AND THE DECAY-SHIPS (BOTH GFS AND NAVGEM) INDICATE AN
ADDITIONAL 10 KNOTS OF INTENSIFICATION THROUGH TAU 24, FOLLOWED BY
SLOW BUT STEADY WEAKENING THEREAFTER. THE JTWC FORECAST CLOSELY
ALIGNS WITH THE LATTER MEMBERS, THOUGH WEAKENS THE SYSTEM AT A
FASTER CLIP THAN THE BULK OF THE GUIDANCE. 

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