WDPS31 PGTW 181500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 09P (IRENE) WARNING NR 
003//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 18.3S 165.3E
   INITIAL INTENSITY: 50 KTS
   GEOGRAPHIC REFERENCE: 175 NM WEST-SOUTHWEST OF PORT VILA,
VANUATU
   MOVEMENT PAST 6 HOURS: EASTWARD AT 18 KTS
   SIGNIFICANT WAVE HEIGHT: 23 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE 09P (IRENE) HAS RAPIDLY IMPROVED IN OVERALL
STRUCTURE, DEVELOPING A WELL-DEFINED CENTRAL DENSE OVERCAST (CDO)
OVER THE PAST SIX HOURS. OVERSHOOTING TOPS NEAR THE ASSESSED CENTER
EXHIBIT TEMPERATURES OF AROUND -90C AND AS OF THE 1300Z HOUR AND
LARGE BURST OF LIGHTNING IS OBSERVED NEAR THE CORE OF THE
CIRCULATION. THE INITIAL POSITION IS PLACED BACK A BIT UPSTREAM
FROM THE CENTER OF THE CDO BASED ON A PERSISTENT NORTHWEST SHEAR 
VECTOR, AND AN EXTRAPOLATION OF A LOW LEVEL CIRCULATION CENTER
(LLCC) EVIDENT IN A 180911Z AMSU-B 89GHZ MICROWAVE IMAGE, WHICH WAS
DISPLACED TO THE NORTHWEST OF THE CENTER OF THE CONVECTION. WHILE
THE VORTEX REMAINS TILTED TOWARDS THE SOUTHEAST WITH HEIGHT,
CLEARLY THE SYSTEM HAS BEEN ABLE TO PUSH BACK AGAINST THE MODERATE
NORTHWESTERLY SHEAR, AND THE LATEST ATMOSPHERIC MOTION VECTOR (AMV)
ANALYSIS DEPICTS THE UPPER-LEVEL FLOW DIVERGING AND FLOWING AROUND
THE SYSTEM, WHICH IS CREATING A LOCALIZED REGION OF LOWER ACTUAL
SHEAR OVER THE SYSTEM. DVORAK CURRENT INTENSITY ESTIMATES REMAIN
RATHER LOW (T2.5-T3.0) AND THE INITIAL INTENSITY IS HEDGED A BIT
HIGHER BASED ON THE CIMSS OPEN-AIIR ESTIMATE OF 50 KNOTS AND THE
STEADILY IMPROVING STRUCTURE. ENVIRONMENTAL PARAMETERS ARE
FAVORABLE, WITH WARM SSTS, LOCALIZED LOW SHEAR AND STRONG POLEWARD
OUTFLOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A DEEP-LAYER NEAR
EQUATORIAL RIDGE (NER) POSITIONED TO THE EAST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   KNES: T2.5 - 35 KTS
   PHFO: T2.5 - 35 KTS
   NFFN: T3.0 - 45 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 15-20 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: STRONG POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   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 09P IS VERY SLOWLY STARTING TO TURN ONTO A
MORE SOUTHEASTWARD TRACK, THOUGH THE PAST SIX-HOUR MOTION VECTOR
WAS JUST SOUTH OF DUE EAST. THE STEERING NER IS CURRENTLY DUE EAST
OF THE STORM AND QUICKLY SLIDING SOUTH. IT IS EXPECTED TO ELONGATE
AND ULTIMATE MERGE WITH THE SUBTROPICAL RIDGE (STR) CURRENTLY
SITTING FAR TO THE SOUTHEAST. THE RESULT OF THIS WILL BE A
STRENGTHENING OF THE STEERING GRADIENT, LEADING TO AN INCREASE IN
TRACK SPEED, WITH A CONTINUED GRADUAL TURN TOWARDS THE SOUTHEAST.
BY THE END OF THE FORECAST, TC 09P WILL SLOW AND TURN MORE SHARPLY
POLEWARD AS THE REMNANT VORTEX COMES UNDER THE STEERING INFLUENCE
OF THE LOWER LEVELS OF THE STR. WHILE CONDITIONS ARE CURRENTLY
FAVORABLE FOR ADDITIONAL INTENSIFICATION, THE WINDOW OF OPPORTUNITY
IS QUICKLY CLOSING, WITH THE PEAK INTENSITY LIKELY TO BE REACHED
WITHIN THE NEXT 12 TO 18 HOURS. THE KEY WILL BE HOW LONG THE DEEP
CONVECTION, WHICH IS PUSHING BACK AGAINST THE PERSISTENT SHEAR, CAN
BE SUSTAINED. IF THIS CAN CONTINUE TO PROVIDE A COCOON OF LOCALIZED
LOW SHEAR, THE SYSTEM WILL LIKELY CONTINUE INTENSIFY, BUT IF IT
BREAKS DOWN FOR EVEN A SHORT PERIOD OF TIME, THE SHEAR WILL QUICKLY
OVERPOWER IT. THE FORECAST CALLS FOR A PEAK OF 55 KNOTS BY TAU 12
BUT IT IS POSSIBLE THE SYSTEM WILL REACH A SLIGHTLY HIGHER PEAK
AROUND TAU 18. BY TAU 24 THE SYSTEM WILL CROSS THE 26C SST
ISOTHERM, BEGIN TO BE ENVELOPED BY MID-LEVEL DRY AIR AND FEEL THE
EFFECTS TO RAPIDLY INCREASING SHEAR, MARKING THE BEGINNING OF
SUBTROPICAL TRANSITION (STT). COMPLETELY CHOKED BY DRY AIR AND
UNDER VERY HIGH SHEAR, TRANSITION TO A SUBTROPICAL LOW IS EXPECTED
NO LATER THAN TAU 72, AND POTENTIALLY AS EARLY AS TAU 48. 

MODEL DISCUSSION: TRACK GUIDANCE IS IN VERY GOOD AGREEMENT THROUGH
THE FORECAST PERIOD, WITH LITTLE IN THE WAY OF CROSS-TRACK SPREAD
IN THE CONSENSUS MEMBERS. THE MEMBERS BEGIN TO SPREAD OUT
ALONG-TRACK AFTER TAU 24, REACHING NEARLY 300NM BY TAU 72. OVERALL
CONFIDENCE IN THE JTWC TRACK IS THUS REDUCED TO MEDIUM. INTENSITY
GUIDANCE IS FOR THE MOST PART IN GOOD AGREEMENT, WITH THE NOTABLE
EXCEPTION OF THE DECAY-SHIPS (GFS) WHICH INTENSIFIES THE SYSTEM UP
TO ABOUT 65 KNOTS BY TAU 24 AND THEN STAYS ABOVE 60 KNOTS THROUGH
THE FORECAST PERIOD. ON THE OTHER END OF THE SPECTRUM IS THE HWRF,
WHICH CONTINUES TO BE A NON-BELIEVER, SHOWING NO FURTHER
INTENSIFICATION AND RAPID WEAKENING AFTER TAU 12. THE REMAINDER OF
THE CONSENSUS MEMBERS CONCUR ON A 55 KNOT PEAK FOLLOWED BY A FAIRLY
RAPID WEAKENING. THE JTWC FORECAST CLOSELY TRACKS THE BULK OF THE
GUIDANCE AND THE CONSENSUS MEAN WITH MEDIUM CONFIDENCE. 

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