CAT 2 US 19S(FABIEN) approaching DIEGO GARCIA and intensifying// 1515utc

JTWC IS ISSUING 6HOURLY WARNINGS AND 3HOURLY SATELLITE BULLETINS ON TC 19S(FABIEN). 3HOURLY SATELLITE BULLETINS WERE DISCONTINUED ON THE REMNANTS OF TC 01B(MOCHA) AT 15/0530UTC.

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A COMPACT CENTRAL DENSE OVERCAST (CDO) WITH DEEP FLARING CONVECTION DEVELOPING ON THE WEST SIDE OF THE ASSESSED LOW LEVEL CIRCULATION CENTER (LLCC) AND WRAPPING UPSHEAR TOWARDS THE EAST. ISOLATED CONVECTIVE HOT TOWERS ARE SHOWING CLOUD TOP TEMPERATURE APPROACHING -90C. MULTIPLE GRAVITY WAVE PULSES MARKING THE EASTWARD LIMIT OF THE OUTFLOW ENVELOPE ARE EVIDENT IN ANIMATED WATER VAPOR IMAGERY, WHICH HAS ALLOWED FOR AN EXPANSION OF THE CDO AND REDUCTION IN THE DEEP-LAYER SHEAR VALUES. A 151211Z GMI 89GHZ IMAGE SHOWS A SOLID EYEWALL OF DEEP CONVECTION THAT NEARLY SURROUNDS THE EYE, THOUGH THE EASTERN SIDE OF THE EYEWALL REMAINS OPEN DUE TO THE PERSISTENT MODERATE EASTERLY SHEAR, PARTICULARLY IN THE MID-LEVELS. COMPARISON WITH THE 37GHZ IMAGE SUGGESTS A SMALL AMOUNT OF WESTWARD TILT WITH HEIGHT, SUPPORTING THE ASSESSMENT THAT MODERATE EASTERLY MID-LEVEL SHEAR IS STILL PRESENT. OVERALL, THE ENVIRONMENT CONTINUES TO IMPROVE WITH DEEP-LAYER AVERAGED SHEAR NOW BELOW 10 KNOTS, WARM SSTS AND IMPROVING RADIAL OUTFLOW PROVIDED BY A STRENGTHENING ANTICYCLONE OVER TOP OF THE SYSTEM. THE DEEP-LAYER SUBTROPICAL RIDGE (STR) CENTERED TO THE SOUTHEAST CONTINUES TO BUILD IN TO THE WEST ALONG THE 15S LATITUDE, RESULTING IN A TRACK WHICH HAS TURNED TO 260 DEG, OR NEARLY DUE WEST OVER THE PAST SIX HOURS.

FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: DUE TO THE STR BUILDING TOWARDS THE WEST ALONG THE 15S LATITUDE, THE TRACK OF TC 19S HAS FLATTENED OUT OVER THE LAST 6 TO 12 HOURS, NOW MOVING ALMOST DUE WEST. BOTH THE GFS AND ECMWF GLOBAL DEEP LAYER STEERING FIELDS AGREE THAT THIS RIDGE WILL REMAIN FIRMLY IN PLACE ALONG THE 15S LATITUDE LINE, BUT WILL SLOWLY ERODE FROM THE WEST BEGINNING AROUND TAU 24. A TURN TOWARDS THE WEST-SOUTHWEST IS EXPECTED WITHIN THE NEXT 12 TO 24 HOURS AS THE RIDGE WEAKENS DUE SOUTH OF TC 19S. BY TAU 48, AN APPROACHING MID-LATITUDE TROUGH WILL INDUCE A BREAK IN THE STEERING RIDGE TO THE SOUTH BUT WILL NOT BE STRONG ENOUGH TO PULL TC 19S POLEWARD. IN THE WAKE OF THE TROUGH, THE DEEP STR CENTERED OVER NORTHERN MADAGASCAR WILL BUILD IN TO THE EAST, WHILE A NEAR EQUATORIAL RIDGE (NER) WILL DEVELOP AND STRENGTHEN JUST SOUTH OF SRI LANKA. TC 19S WILL BE TRAPPED BETWEEN THESE TWO RIDGE SYSTEMS, EFFECTIVELY COCOONED IN A WEAK STEERING PATTERN THROUGH THE REMAINDER OF THE FORECAST PERIOD. THUS, AFTER TAU 48, TC 19S WILL SLOW SIGNIFICANTLY, DRIFTING TOWARDS THE WEST AT JUST A COUPLE OF KNOTS AT BEST. ERRATIC MOTION INCLUDING QUASI-STATIONARY PERIODS, LOOPING OR OTHERWISE HIGHLY UNCERTAIN TRACK MOTION CAN BE EXPECTED BETWEEN TAU 48 TO TAU 120. IN TERMS OF INTENSITY, NOW THAT THE LOW LEVEL VORTEX HAS BEEN FIRMLY ESTABLISHED AND ENVIRONMENTAL SHEAR HAS LOWERED, THE STAGE IS SET. THROW IN AN ANTICIPATED DEVELOPMENT OF STRONG POLEWARD OUTFLOW CHANNEL WITHIN THE NEXT 12-24 HOURS, THE SYSTEM IS RIPE FOR RAPID INTENSIFICATION (RI). THUS, PEAK INTENSITY HAS BEEN INCREASED TO 115 KNOTS WITH THIS FORECAST, EXPECTED BY TAU 36 AS THE SYSTEM PASSES DIEGO GARCIA TO THE EAST. AFTER PEAKING, THE COMBINATION OF UPWELLING INDUCED SST COOLING AND A CHANGE IN THE UPPER-LEVEL PATTERN WHICH WILL INDUCE CONVERGENT FLOW ALOFT, WILL LEAD TO A SLOW WEAKENING TREND THROUGH THE REMAINDER OF THE FORECAST PERIOD.

MODEL DISCUSSION: OVERALL, TRACK GUIDANCE IS IN MODEST AGREEMENT, THOUGH THERE REMAIN SIGNIFICANT DIFFERENCES IN HOW THE MODELS HANDLE THE TRACK THROUGH THE PERIOD OF WEAK STEERING INFLUENCE. THE BULK OF THE CONSENSUS MEMBERS HAVE COME INTO AGREEMENT THAT THE SYSTEM WILL TRACK WEST-SOUTHWEST THROUGH THE FORECAST PERIOD, STEADILY SLOWING TO NEAR QUASI-STATIONARY SPEEDS AFTER TAU 48, AND MEANDERING SOUTH OF DIEGO GARCIA. THE JTWC FORECAST REMAINS CONSISTENT WITH THE PREVIOUS FORECAST, THOUGH SHIFTED WEST DUE TO THE RECENT WESTWARD TRACK MOTION, AND CLOSELY TRACKS THE GFS AND CONSENSUS MEAN. HOWEVER, THE COAMPS-TC (GFS AND NAVGEM) AND THE HWRF SHOW THE SYSTEM SLOWING AT A FASTER RATE THAN THE OTHER GLOBAL MODELS, AND THEN TURN THE SYSTEM TO THE WEST BEFORE LOOPING CLOCKWISE IN CLOSE PROXIMITY AROUND DIEGO GARCIA. WHILE THIS IS CONSIDERED A LOW PROBABILITY SCENARIO AT THE CURRENT TIME, IT CANNOT BE RULED OUT. MULTIPLE RI AIDS INCLUDING RIPA, RICN, DTOPS AND RIDE CONTINUE TO BE TRIGGERED WITH THE RIPA AND DTOPS INDICATING A PEAK INTENSITY OF 130 KNOTS AT TAU 36. THE REMAINDER OF THE GUIDANCE SHOWS A PEAK BETWEEN 100-105 KNOTS AROUND TAU 24. THE JTWC FORECAST FOLLOWS THE RI30 TREND LINE WHICH PEAKS AT 115 KNOTS AT TAU 36, THEN JOINS THE DECAY SHIPS GUIDANCE THEREAFTER. CONFIDENCE IN BOTH THE TRACK AND INTENSITY FORECASTS IS MEDIUM TRENDING TO LOW DUE TO THE HIGH UNCERTAINTY SURROUNDING THE TRACK AFTER TAU 48 AS THE SYSTEM ENTERS THE WEAK STEERING PATTERN.