TC 12P(GABRIELLE): CAT 2 US//TC 13S(DINGANI) to reach 65knots by 48h//TC 11S(FREDDY): to peak once again//Invest 90P//1009utc

JTWC IS ISSUING 6HOURLY WARNINGS TC 12P(GABRIELLE) AND 12HOURLY WARNINGS ON TC 11S(FREDDY) AND TC 13S(DINGANI).3HOURLY SATELLITE BULLETINS ARE ISSUED ON TC 11S, TC 12P AND TC 13S.

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: AGAINST ALL ODDS AND IN THE FACE OF SHARPLY INCREASED NORTHWESTERLY SHEAR, ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI), A SERIES OF RECENT MICROWAVE IMAGERY AND RECENT SCATTEROMETER AND SMAP PASSES, INDICATE THAT TC 12P (GABRIELLE) HAS INTENSIFIED. HOWEVER, THE PEAK HAS PROBABLY BEEN REACHED AND THE SYSTEM NOW FACES A RAPID TRANSITION TO A SUBTROPICAL LOW. THE MSI AND ENHANCED INFRARED (EIR) IMAGERY INDICATES THE SYSTEM REMAINS DEVOID OF A DISTINCT EYE FEATURE, EVEN NOW AT ITS PEAK. HOWEVER, MICROWAVE IMAGERY THROUGH THE DAY HAS SHOWN CONSISTENT LOW-LEVEL EYE FEATURES, WITH THE SMALLEST, BEST DEFINED MICROWAVE EYE IN THE MOST RECENT 100527Z SSMIS 37GHZ IMAGE. THE STRONGEST CONVECTION REMAINS DOWNSHEAR, IN THE SOUTHEASTERN QUADRANT, BUT THE EYEWALL REMAINS FIRM ON THE NORTHWEST SIDE, FOR NOW. THE INITIAL INTENSITY HAS BEEN INCREASED TO 90 KNOTS, WHICH IS ABOVE ALL THE OBJECTIVE ESTIMATES BUT IN LINE WITH THE SUBJECTIVE ESTIMATES. THE ADT IS T5.3 (97 KTS), THE AIDT IS 87 KTS AND THE SATCON IS 96 KTS AND A 100331Z AMSR2 WINDSPEED ESTIMATE MEASURED 91 KTS IN THE SOUTHEAST QUADRANT, WHICH COMBINED LENDS HIGH CONFIDENCE TO THE INITIAL INTENSITY. THE SYSTEM IS MOVING INTO AN INCREASINGLY MARGINAL ENVIRONMENT, WITH SHEAR NOW AT OR ABOVE 30 KTS, THOUGH IT IS IN PHASE WITH THE STORM MOTION, HENCE HOW THE SYSTEM HAS BEEN ABLE TO CONTINUE TO INTENSIFY. TC 12P IS IN THE PROCESS OF CROSSING THE 26C ISOTHERM AND WATER VAPOR IMAGERY SHOWS A VERY SHARP UPPER-LEVEL TROUGH, WITH VERY DRY AIR TRAILING THE TROUGH AXIS, JUST TO THE WEST. THE SYSTEM IS ALREADY STARTING THE VERY EARLY STAGES OF SUBTROPICAL TRANSITION AS IT RACES OFF TO THE SOUTHEAST ALONG THE STRENGTHENING GRADIENT BETWEEN THE SUBTROPICAL RIDGE (STR) TO THE EAST AND THE TROUGH TO THE WEST.

FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: HAVING REACHED ITS PEAK AT THE PRESENT ANALYSIS TIME, TC 12P WILL NOW RAPIDLY WEAKEN AND UNDERGO SUBTROPICAL TRANSITION (STT). THE SYSTEM WILL CONTINUE TO VERY RAPIDLY MOVE SOUTHEASTWARD ALONG THE TIGHT GRADIENT BETWEEN THE STR TO THE EAST AND THE TROUGH NOW ENCROACHING FROM THE WEST. TRACK SPEEDS ARE ANTICIPATED TO INCREASE FURTHER THROUGH TAU 12 BUT WILL THEN BEGIN TO SLOW AND TURN A BIT MORE EAST-SOUTHEASTWARD THROUGH THE END OF THE FORECAST AS THE SYSTEM MOVES UNDER THE UPPER-LEVEL TROUGH AND INTO A REDUCED STEERING GRADIENT. THE SYSTEM WILL CROSS THE 26C ISOTHERM IN THE NEXT FEW HOURS AND MOVE INTO STEADILY COOLER WATERS, ROBBING IT OF THE ENERGY SOURCE IT HAS ENJOYED THE PAST FEW DAYS. AT THE SAME TIME, THE TROUGH, WHICH HAS AN AXIS SITTING AT ABOUT 156E WILL RAPIDLY MOVE IN, BRINGING INCREASED SHEAR AND A WEDGE OF VERY DRY AIR OVER TOP OF THE SYSTEM, EFFECTIVELY DECAPITATING IT WITHIN THE NEXT 24 HOURS. THE COMBINATION OF THESE EFFECTS WILL LEAD TO RAPID WEAKENING, PARTICULARLY IN THE FIRST 24 HOURS. THE SYSTEM IS ALREADY STARTING THE VERY EARLY PHASES OF STT, AND TRANSITION TO A STORM-FORCE SUBTROPICAL LOW IS EXPECTED NO LATER THAN TAU 36 AND POTENTIALLY AS EARLY AS TAU 24. AFTER STT, THE SYSTEM WILL CONTINUE TO TRACK TOWARDS THE NORTH ISLAND OF NEW ZEALAND AND SOME MODELS SUGGEST SOME POTENTIAL FOR REINTENSIFICATION AS IT APPROACHES.

MODEL DISCUSSION: TRACK GUIDANCE IS IN VERY GOOD AGREEMENT WITH THE FORECAST TRACK, WITH MINIMAL SPREAD THROUGH THE FORECAST PERIOD, LENDING HIGH CONFIDENCE TO THE JTWC FORECAST TRACK. INTENSITY GUIDANCE IS ALSO IN GOOD AGREEMENT ON THE RAPID WEAKENING TREND, THOUGH AS EXPECTED, THERE IS ABOUT A 15 KNOT SPREAD IN THE GUIDANCE, REFLECTIVE OF THE DIFFERENT PACE OF WEAKENING IN THE VARIOUS MODELS. THE HWRF SHOWS THE FASTEST RATE WHILE THE COAMPS-TC AND DECAY-SHIPS SHOW A SLOWER RATE OF WEAKENING. THE JTWC FORECAST WEAKENS THE SYSTEM FASTER THAN THE BULK OF THE GUIDANCE, IN LINE WITH HWRF.

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A VERY COMPACT SYSTEM WITH A PARTIALLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC) DISPLACED TO THE EAST OF A SMALL REGION OF FLARING CONVECTION. TC 13S (DINGANI) CONTINUES TO STRUGGLE AGAINST PERSISTENT EASTERLY SHEAR BUT HAS BEEN ABLE TO STEADILY INTENSIFY OVER THE PAST 12 HOURS. UNFORTUNATELY, THERE HAS NOT BEEN ANY GOOD MICROWAVE PASSES IN OVER 12 HOURS, AND THE SYSTEM IS CROSSING THROUGH A REGION ON THE LIMBS OF BOTH HIMAWARI AND METEOSAT GEOSTATIONARY IMAGERY, AND THUS THE INITIAL POSITION IS ASSESSED WITH MEDIUM CONFIDENCE, USING AN EXTRAPOLATION OF AN 100324Z ASCAT-B SCATTEROMETER PASS. THE INITIAL INTENSITY IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE T3.0 (45 KTS) CURRENT INTENSITY ESTIMATES FROM PGTW AND DEMS, THE WIND FIELD EVIDENT IN THE PREVIOUSLY MENTIONED ASCAT PASS AND THE CURRENT SATCON ESTIMATE. SHEAR REMAINS IN THE LOW TO MODERATE RANGE (15-20 KTS) BUT HAS STARTED TO COME DOWN SINCE THE PREVIOUS FORECAST. THE ENVIRONMENT IS ASSESSED AS MARGINALLY FAVORABLE, WITH THE SHEAR BEING OFFSET BY GOOD AND IMPROVING POLEWARD OUTFLOW AND WARM SSTS.

FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: THROUGH THE FIRST 36 HOURS OF THE FORECAST, TC 13S IS FORECAST TO TRACK GENERALLY WESTWARD ALONG THE NORTHERN PERIPHERY OF THE DEEP STR TO THE SOUTH. BY TAU 48, AN UPPER-LEVEL TROUGH MOVING IN FROM THE WEST WILL BREAK DOWN THE RIDGE AND PUSH THE CENTER TO THE EAST, RESULTING IN A MORE NORTH-SOUTH ORIENTED AXIS. TC 13S WILL TAKE A SHARP TURN SOUTHWESTWARD ALONG THE NEWLY REORIENTED RIDGE AXIS. A NEWLY FORMED STR WILL MOVE IN TO THE SOUTH BY TAU 96, AND THE FORECAST TRACK FLATTENS SOMEWHAT THROUGH THE REMAINDER OF THE FORECAST. SHEAR HAS ALREADY STARTED TO SLACKEN A BIT BASED ON THE LATEST CIMSS ANALYSIS, SUPPORTED BY THE REFORMATION OF A SMALL CENTRAL DENSE OVERCAST (CDO) OVER THE LLCC. HOWEVER, IT WILL LIKELY TAKE A BIT OF TIME FOR THE INTENSITY TO INCREASE SIGNIFICANTLY, THUS THE FORECAST CALLS FOR A FLAT INTENSITY TREND FOR THE FIRST 12 HOURS. BY TAU 24, SHEAR IS EXPECTED TO RELAX FURTHER, AND THE SYSTEM SHOULD START TO INTENSIFY MORE QUICKLY, REACHING A PEAK OF 65 KNOTS BY TAU 48. INCREASED NORTHWESTERLY SHEAR AHEAD OF THE DEEP TROUGH MOVING IN FROM THE WEST, AN INFLUX OF DRY MID-LEVEL AIR, AND REDUCED SSTS (NEAR 26C) WILL USHER IN A WEAKENING TREND FORM TAU 72 ONWARD.

MODEL DISCUSSION: THROUGH TAU 48, DETERMINISTIC TRACK GUIDANCE IS IN GOOD AGREEMENT, DEPICTING MINIMAL SPREAD IN THE CONSENSUS MEMBERS, LENDING HIGH CONFIDENCE TO THE JTWC FORECAST TRACK, WHICH CLOSELY FOLLOWS THE CONSENSUS MEAN. AFTER TAU 72, THE DETERMINISTIC TRACKERS BEGIN TO DIVERGE, THOUGH EVEN BY TAU 120 THE SPREAD IS A MARGINAL 200NM. THE US MODELS INCLUDING NAVGEM, HWRF, AND GFS TAKE THE SYSTEM ON A MORE SOUTHERLY ROUTE, WHILE THE EUROPEAN MODELS INCLUDING UKMET, GALWEM, ECMWF AND THE THEIR ENSEMBLE MEAN TRACKERS, TAKE A MORE NORTHERN TRACK. THE ENSEMBLES (GEFS, ECENS, MOGREPS) SHOW SIGNIFICANTLY MORE SPREAD IN THE MEMBERS AFTER TAU 72, WITH SPREAD BETWEEN OUTLIE MEMBERS UP TO 600NM. THE JTWC FORECAST LIES ON THE SOUTHERN SIDE OF THE CONSENSUS PACKAGE WITH LOW CONFIDENCE, ESPECIALLY IN LIGHT OF THE INCREASED ENSEMBLE SPREAD. INTENSITY GUIDANCE GENERALLY SUPPORTS THE FORECAST SCENARIO, WITH A RANGE OF PEAK INTENSITIES BETWEEN 55 KNOTS FOR THE COAMPS-TC (GFS) AND 75 KNOTS FOR THE HWRF. THE JTWC FORECAST LIES NEAR THE CONSENSUS PEAK, WITH MEDIUM CONFIDENCE.

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A SLIGHTLY ASYMMETRIC CENTRAL DENSE OVERCAST CLOUD LAYER THAT CONTINUES TO BE SHEARED BY EASTERLY WIND SHEAR, WITH A LOW-LEVEL CIRCULATION THAT HAS BECOME OBSCURED BY DEEP CONVECTION. A 092015Z SSMIS 89GHZ MICROWAVE IMAGE REVEALS A DEEP CURVED CONVECTIVE BAND OVER THE NORTHWEST QUADRANT WRAPPING INTO A WELL-DEFINED LOW-LEVEL CIRCULATION. A 092220Z SAR WINDSPEED PASS SHOWS A WELL-DEFINED SYSTEM THAT HAS STRENGTHENED OVER THE PAST SIX HOURS. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE SAR PASS ALONG WITH THE AFOREMENTIONED SSMIS MICROWAVE IMAGE. THE INITIAL INTENSITY OF 70 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE SAR IMAGE IN COMBINATION WITH PGTW AND KNES DVORAK CURRENT INTENSITY ESTIMATES.

FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: OVER THE PAST 12 HOURS TROPICAL CYCLONE 11S HAS TRACKED GENERALLY WEST-NORTHWESTWARD AT 8 KNOTS. OVER THE NEXT 12 HOURS 11S IS FORECAST TO ACCELERATE NORTHWESTWARD AS THE SYSTEM TRANSITS ALONG THE NORTHWEST PERIPHERY OF THE DEEP-LAYER SUBTROPICAL RIDGE (STR) TO THE SOUTH. THE SYSTEM WILL CONTINUE TO TRANSIT NORTHWESTWARD QUICKLY THROUGH TAU 48, DUE TO THE SYSTEM'S POSITION WITHIN THE STR. AFTER WHICH, TC 11S WILL REACH A POINT NEAR 15S BEFORE TURNING AND TRACKING IN A WEST-SOUTHWESTWARD DIRECTION THROUGH THE REMAINDER OF THE FORECAST. TC 11S WILL STEADILY INTENSIFY OVER THE NEXT 48 HOURS AS DETERMINISTIC MODELS AGREE THAT VERTICAL WIND SHEAR (VWS) IS EXPECTED TO DECREASE. THIS WILL ALLOW THE SYSTEM TO ULTIMATELY ACHIEVE A PEAK INTENSITY OF 85 KNOTS BY TAU 48. AFTER TAU 48, SHEAR LOOKS TO PICK UP AGAIN AS THE SYSTEM MOVES INTO RELATIVELY COOLER (26-27C) WATERS AND UPPER-LEVEL FLOW BECOMES INCREASINGLY CONVERGENT, LEADING TO SLOW BUT STEADY WEAKENING THROUGH THE REMAINDER OF THE FORECAST PERIOD.

MODEL DISCUSSION: THE BULK OF THE DETERMINISTIC GUIDANCE REMAINS IN TIGHT AGREEMENT WITH ONLY A 35 TO 75NM SPREAD IN SOLUTIONS FROM TAU 12 TO TAU 72. WITH THE NAVGEM BEING THE OUTLIER TO THE LEFT OF THE TRACK. BETWEEN TAU 96 AND TAU 120, TRACK GUIDANCE SPREADS FROM 135NM TO 210NM. THEREFORE, THERE IS HIGH CONFIDENCE THROUGH TAU 72 AND LOW CONFIDENCE THEREAFTER AS THE SPREAD INCREASES THERE IS SIGNIFICANT UNCERTAINTY IN THE JTWC INTENSITY FORECAST AS MODEL GUIDANCE. HWRF AND GFS CONTINUE TO INDICATE A GRADUAL INTENSIFICATION BY TAUS 72-96, WHILE COAMPS-TC SHOW A GRADUAL DECREASE BY TAU 72. OVERALL, THE JTWC INTENSITY FORECAST IS PLACED WITH LOW CONFIDENCE GIVEN THE DISAGREEMENT IN THE INTENSITY MODELS.