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// NNNN