TD 17W weak and ill-defined for now// 02P(MAL) intensifying while approaching FIJI//1309utc

JTWC IS ISSUING 6 HOURLY WARNINGS AND 3HOURLY SATELLITE BULLETINS ON 17W AND 02P.

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: TROPICAL DEPRESSION (TD) 17W (SEVENTEEN) HAS BEEN REPOSITIONED. REANALYSIS OF ANIMATED VISIBLE IMAGERY AND 122345Z AND 130039Z ASCAT PASSES RESULTED IN A REPOSITIONING OF THE 130000Z BEST TRACK POSITION APPROXIMATELY 99NM TO THE NORTHEAST OF THE ORIGINAL POSITION. TD 17W REMAINS DISORGANIZED AND HIGHLY TILTED, WITH MULTIPLE VORTICES LYING ALONG AN ELONGATED, EAST-WEST ORIENTED TROUGH THAT EXTENDS FROM THE INITIAL POSITION FAR TO THE EAST. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS MULTIPLE POCKETS OF FLARING CONVECTION, WHICH SUBSEQUENTLY DISSIPATE IN SHORT ORDER, LEAVING BEHIND A COMPLEX PATTERN OF OUTFLOW BOUNDARIES, COMPLICATING ASSESSMENT OF THE INITIAL POSITION. EXTRAPOLATION OF THE EARLIER ASCAT DATA ALONG WITH ANALYSIS OF THE LOW-LEVEL CLOUD MOVEMENT SUGGESTS THE LOW LEVEL CIRCULATION CENTER (LLCC) IS SOUTHEAST OF YAP, AND EAST OF NGULU ATOLL AT THE SYNOPTIC HOUR. HOWEVER, THIS ASSESSMENT IS MADE WITH EXTREMELY LOW CONFIDENCE BASED ON THE REASONS NOTED ABOVE. THE INITIAL INTENSITY REMAINS SET AT 25 KNOTS BASED ON THE EARLIER ASCAT DATA WHICH SHOWED WINDS OF JUST 20-25 KNOTS AROUND THE WESTERN HALF OF THE CIRCULATION, BUT ONLY 10 KNOTS OR LESS ELSEWHERE. THE SYSTEM IS DRIFTING WEST IN ALONG A WEAK LOW-LEVEL GRADIENT ON THE SOUTHERN SIDE OF THE SUBTROPICAL RIDGE (STR). ANALYSIS REVEALS A MARGINALLY UNFAVORABLE ENVIRONMENT, WITH PERSISTENT EAST-NORTHEASTERLY SHEAR AND DRY MID-LEVEL AIR BEING THE PRIMARY NEGATIVE FACTORS. UPPER-LEVEL FLOW IS DIVERGENT BUT TRUCKING ALONG AT ABOUT 35 KNOTS BASED ON THE MOST RECENT KOROR AND YAP SOUNDINGS, WHICH IS PROHIBITING THE CONVECTION FROM ORGANIZING AROUND THE LOW-LEVEL VORTEX.

FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST, THOUGH THE INITIAL POSITION HAS BEEN RELOCATED APPROXIMATELY 100NM NORTHEAST OF WHERE IT WAS FORECAST TO BE AT 130600Z. FORECAST DISCUSSION: DUE TO THE RELOCATION OF THE EARLIER POSITIONS, TD 17W APPEARS TO BE MOVING AT A MODEST CLIP TOWARDS THE WEST. HOWEVER, THE MOST RECENT IMAGERY SUGGESTS A DRAMATIC SLOW-DOWN, AND THE SYSTEM IS NOW ASSESSED TO BE DRIFTING WESTWARD AT ABOUT 3 OR 4 KNOTS AT BEST. A WEAKNESS IN THE STR SITTING DUE NORTH OF THE SYSTEM IS LEADING TO A RELAXED STEERING GRADIENT IN THE LOW- TO MID-LEVELS, ALLOWING FOR THE SYSTEM TO SLOW DOWN SIGNIFICANTLY. THE WEAKNESS IN THE RIDGE PERSISTS FOR THE NEXT 24 TO 36 HOURS, AND TD 17W WILL DRIFT RATHER AIMLESSLY FOR THE NEXT DAY OR SO AS A RESULT. IN FACT, ERRATIC MOTION OF THE VORTEX, ENHANCED BY THE PRESENCE OF MULTIPLE AREAS OF VORTICITY, IS LIKELY THROUGH THE NEXT 24 HOURS. AFTER TAU 36, THE RIDGE IS EXPECTED TO STRENGTHEN AND TD 17W WILL ACCELERATE OUT TO THE WEST-NORTHWEST, CONTINUING TO RIDE THE SOUTH SIDE OF THE RIDGE THROUGH THE REMAINDER OF THE FORECAST PERIOD. TD 17W IS CLEARLY STRUGGLING IN THE FACE OF PERSISTENT, MODERATELY STRONG, DEEP-LAYER EASTERLY FLOW ALOFT. THE 13000Z SOUNDING FROM YAP SHOWED 35 KNOT OR GREATER WINDS EXTENDING FROM ABOUT 400MB ALL THE WAY TO THE TOP OF THE COLUMN AND 25-30 KNOT NORTHEASTERLIES BELOW THAT. INTERESTINGLY THE KOROR SOUNDING HIGHLIGHTS THE STRONGLY DIVERGENT NATURE OF THE UPPER-LEVEL FLOW, WITH NORTH-NORTHWESTERLIES AT 500MB, LIGHT AND VARIABLE WINDS FROM THERE TO 250MB AND EASTERLIES ABOVE THAT. THIS STRONGLY DIVERGENT FLOW DOWNSTREAM IS ALLOWING FOR THE DEVELOPMENT OF THE FLARING CONVECTION BUT UNTIL THIS CONVECTION CAN ROTATE OR MOVE UPSHEAR OF THE LOW-LEVEL VORTEX, THE SYSTEM WILL BE UNABLE TO INTENSIFY. ADDITIONALLY, AN AREA OF LOW OCEAN HEAT CONTENT LURKS TO THE WEST, NORTH OF PALAU. AS THE SYSTEM TRACKS SLOWLY THROUGH THIS REGION, THE LACK OF ENERGY FLUX WILL COMPOUND THE OTHER FACTORS, GENERATING LITTLE TO NO INTENSIFICATION THROUGH TAU 48. AFTER TAU 72, THE SYSTEM MOVES INTO A HIGH OHC REGION, SHEAR IS EXPECTED TO DECREASE (THOUGH MODERATE EASTERLY FLOW ALOFT PERSISTS) AND THE MID-LEVELS OF THE ATMOSPHERE WILL MOISTEN, ALLOWING FOR A SMALL AMOUNT OF INTENSIFICATION PRIOR TO LANDFALL IN THE EASTERN PHILIPPINES BY THE END OF THE FORECAST.

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN MODEST AGREEMENT THROUGH TAU 72, WITH ALL OF THE CONSENSUS MEMBERS AGREEING ON A SLOW, ERRATIC TRACK FOR THE FIRST 24 TO 36 HOURS, FOLLOWED BY A KICK OUT TO THE WEST AFTER TAU 36. SIGNIFICANT DISAGREEMENT DEVELOPS BY TAU 72, WITH THE GFS DIVERGING FROM THE REST OF THE MODELS, TRACKING THE SYSTEM DUE WEST AT A MUCH HIGHER SPEED, WHILE THE NAVGEM AND UKMET TAKE THE SYSTEM ON A MORE NORTHWEST TRACK. BY TAU 72, ALONG-TRACK SPREAD INCREASES TO 275NM WHILE CROSS-TRACK SPREAD INCREASES TO 330NM BETWEEN THE GFS AND GALWEM. BY TAU 120, THE MODEL GUIDANCE IS IN DISARRAY, WITH THE GFS RACING ACROSS THE PHILIPPINES AND MOVING INTO THE SOUTH CHINA SEA, WHILE THE BULK OF THE OTHER GUIDANCE REMAINS EAST OF SAMAR. ALONG AND CROSS-TRACK SPREAD INCREASE TO BEYOND 650NM BY THE END OF THE FORECAST. GEFS AND ECENS ENSEMBLE SPREAD IS LESS, AND BOTH MEANS ARE CONSISTENT WITH THE DETERMINISTIC CONSENSUS MEAN. THE JTWC FORECAST IS SHIFTED SOUTH FROM THE PREVIOUS FORECAST, BUT REMAINS CONSISTENT WITH, IF SLIGHTLY SOUTH OF, THE CONSENSUS MEAN, WITH LOW CONFIDENCE. INTENSITY GUIDANCE IS RELATIVELY GOOD AGREEMENT, SHOWING SLIGHT INTENSIFICATION THROUGH TAU 36 THEN ADDITIONAL INTENSIFICATION THEREAFTER, WITH THE MAJORITY OF THE GUIDANCE KEEPING THE SYSTEM BELOW 45 KNOTS THROUGH THE FORECAST. THE NOTABLE EXCEPTION CONTINUES TO BE COAMPS-TC (BOTH VERSIONS), WHICH CONTINUES TO SHOW RAPID INTENSIFICATION AFTER TAU 72, WITH A PEAK APPROACHING 80 KNOTS BY THE TAU 120. THE CTCXENS INTENSITY PROBABILITIES ALSO SHOW A 40 PERCENT CHANCE OF RI IN THE SAME TIME FRAME. WHILE THIS IS CURRENTLY ASSESSED AS A LOW PROBABILITY SCENARIO, THE CHANCE OF RI IN A MODERATE EASTERLY SHEAR ENVIRONMENT CANNOT BE DISCOUNTED AND SIGNS OF THIS OCCURRING WILL BE CLOSELY MONITORED.

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A RAPIDLY CONSOLIDATING TROPICAL CYCLONE, WITH CONVECTIVE HOT TOWERS FIRING OFF AROUND THE ASSESSED LOW LEVEL CIRCULATION CENTER (LLCC), WHICH REMAINS OBSCURED IN THE MSI AND INFRARED IMAGERY. A FORTUITOUS 130553Z SSMIS 89GHZ MICROWAVE IMAGE REVEALED A NASCENT MICROWAVE EYE FEATURE AND WELL-DEFINED CONVECTIVE BANDING FEATURES TO THE NORTHEAST AND SOUTHWEST, WRAPPING INTO THE DEVELOPING CORE. CONFIDENCE IN THE INITIAL POSITION IS ASSESSED AS MEDIUM DUE TO LIKELY VORTEX TILT DUE TO MODERATE MID- TO UPPER-LEVEL SHEAR FROM THE NORTHEAST. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE, PUSHED HIGHER THAN THE BULK OF THE AGENCY DVORAK INTENSITY ESTIMATES IN LIGHT OF THE HIGHER KNES ESTIMATE WHICH LINES UP EXACTLY WITH THE INITIAL POSITION, THE CIMSS ADT, DPRINT AND DMINT ESTIMATES AS WELL AS THE NASCENT MICROWAVE EYE FEATURE. THE SYSTEM HAS BEGUN TO ACCELERATE SOUTHEASTWARD ALONG THE WESTERN SIDE OF A DEEP-LAYER SUBTROPICAL RIDGE (STR) CENTERED TO THE EAST. ENVIRONMENTAL CONDITIONS ARE FAVORABLE FOR CONTINUED DEVELOPMENT, WITH WARM SSTS, HIGH OCEAN HEAT CONTENT (OHC), STRONG POLEWARD OUTFLOW AND LOW DEEP-LAYER VERTICAL WIND SHEAR.

FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: TROPICAL CYCLONE (TC) 02P (MAL) IS RAPIDLY CONSOLIDATING AND INTENSIFYING AS IT BEGINS TO TRACK SOUTHEAST TOWARDS A RENDEZVOUS WITH FIJI. THE SYSTEM IS FORECAST TO TRACK SOUTHEASTWARD ALONG THE WEST-SOUTHWEST SIDE OF THE STEERING RIDGE THROUGH THE ENTIRETY OF THE FORECAST PERIOD, PROVIDING FOR A HIGH CONFIDENCE TRACK FORECAST WHICH WILL SKIRT THE WEST SIDE OF THE MAIN ISLAND OF VITI LEVU BETWEEN TAU 24 AND TAU 36. THE SYSTEM THEN CONTINUES SOUTHEASTWARD INTO THE OPEN WATERS OF THE SOUTH PACIFIC THROUGH THE REMAINDER OF THE FORECAST PERIOD. IN TERMS OF INTENSITY, THE SYSTEM HAS ALREADY INTENSIFIED RATHER QUICKLY, HAVING INCREASED IN INTENSITY FROM 30 KNOTS TO AT LEAST 45 KNOTS IN THE PAST 12 HOURS. CONDITIONS ARE EXPECTED TO REMAIN FAVORABLE FOR THE NEXT 36 HOURS, ALLOWING FOR THE SYSTEM TO CONTINUE TO STRENGTHEN UP TO AT LEAST 65 KNOTS BY TAU 24. SOME MODEL GUIDANCE SUGGESTS THE SYSTEM COULD UNDERGO RAPID INTENSIFICATION (RI) OVER THE NEXT 24 HOURS. THIS IS A LOW-PROBABILITY SCENARIO AT PRESENT, BUT IF THE VORTICAL HOT TOWERS (VHTS) CURRENTLY FIRING NEAR THE CENTER ARE ABLE TO WRAP UPSHEAR AND FURTHER CONSOLIDATE AND ALIGN THE INNER CORE OF THE SYSTEM, THE SYSTEM COULD CERTAINLY UNDERGO RI AND REACH A HIGHER PEAK INTENSITY. REGARDLESS, THE PEAK INTENSITY WILL OCCUR AS THE SYSTEM IS PASSING WEST OF FIJI. AFTER PASSING 20S LATITUDE, AROUND TAU 48, THE SYSTEM MOVES ACROSS THE 26C ISOTHERM INTO MUCH COOLER WATERS, IS ENVELOPED IN EXTREMELY DRY MID-LEVEL AIR, AND EXPERIENCES A DRASTIC INCREASE IN WESTERLY SHEAR WHICH WILL DECAPITATE THE CYCLONE. THIS ALSO MARKS THE ONSET OF A RAPID SUBTROPICAL TRANSITION (STT), WHICH IS FORECAST TO BE COMPLETE NO LATER THAN TAU 96.

MODEL DISCUSSION: DETERMINISTIC AND ENSEMBLE TRACK GUIDANCE IS IN VERY GOOD AGREEMENT, WITH THE ONE NOTABLE EXCEPTION OF NAVGEM WHICH IS A SIGNIFICANT OUTLIER TO THE WEST. DISCOUNTING THE NAVGEM, THE REMAINDER OF THE TRACK GUIDANCE IS CONSTRAINED TO A VERY TIGHT ENVELOPE OF JUST 105NM AT TAU 72, WIDENING SLIGHTLY TO 150NM BY THE END OF THE FORECAST AT TAU 96. THUS THE TRACK IS LAID WITH HIGH CONFIDENCE AND REMAINS CONSISTENT WITH THE PREVIOUS FORECAST AND THE CONSENSUS MEAN. INTENSITY GUIDANCE IS MIXED AND SHOWS A FAIRLY LARGE AMOUNT OF SPREAD. ON THE HIGH END OF THE ENVELOPE ARE THE DECAY SHIPS (GFS AND NAVGEM), WHICH PEAK THE SYSTEM BETWEEN 65-75 KNOTS, AS WELL AS THE RIPA RI AID WHICH PEAKS AT 85 KNOTS IN 24 HOURS. THE CONSENSUS MEAN, HWRF, HAFS-A AND COAMPS-TC MEANWHILE SHOW PEAK INTENSITY BETWEEN 50-60 KNOTS. THE JTWC FORECAST LIES SLIGHTLY BELOW THE MOST AGGRESSIVE MODELS BUT ABOUT 10 KNOTS ABOVE THE MEAN. AS NOTED ABOVE, THE POSSIBILITY OF RI CANNOT BE DISCOUNTED, THOUGH IT REMAINS A LOW PROBABILITY SCENARIO AT THE CURRENT TIME.