Typhoon 11W(SHANSHAN) interacting with JAPAN rapidly weakening// Tropical Cyclone Formation Alert of INVEST 91A//2903utc

JTWC IS ISSUING 6HOURLY WARNINGS ON 11W, ON 01C ON 07E AND ON 08E. 3HOURLY SATELLITE BULLETINS ARE ISSUED ON 11W ON 01C ON 07E AND ON INVEST 91A

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS DEEP CONVECTION OBSCURING A LOW-LEVEL CIRCULATION (LLCC) SYSTEM. WEST-SOUTHWESTERLY SHEAR ERODES THE NORTHWESTERN SIDE OF THE TYPHOON AND INTRODUCES DRY AIR; HOWEVER, THE VORTEX REMAINS VERTICALLY STACKED AND THE ENVIRONMENT REMAINS MOIST. WATER VAPOR IMAGERY REVEALS STRONG POLEWARD OUTFLOW FED INTO AN UPPER-LEVEL JET LOCATED TO THE NORTH. A 292119Z RCM-2 SYNTHETIC APERTURE RADAR (SAR) IMAGE REVEALS LARGER WIND RADII THAN REFLECTED IN THE PREVIOUS WARNING AND A LARGE SWATH OF 75-80KT WINDS TO THE WEST. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON RADAR AND ANIMATED MSI. THE INITIAL INTENSITY OF 75 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE AFOREMENTIONED SAR IMAGERY SUPPLEMENTED BY THE AGENCY AND OBJECTIVE FIXES LISTED BELOW.

FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THE WIND RADII ARE LARGER THAN PREVIOUSLY FORECASTED BASED ON AN RCM-2 292119Z SAR PASS. AS A RESULT, SPECIFICALLY THE NORTHWESTERN AND SOUTHWESTERN WIND RADII ARE LARGER IN THE NEAR-TERM FORECAST. FORECAST DISCUSSION: TYPHOON (TY 11W) IS FORECAST TO REMAIN IN A COMPETING STEERING ENVIRONMENT BETWEEN TWO RIDGES. FROM TAU 24-36, TY 11W WILL DRIFT EASTWARD WHILE CAPTURED BY THE COL. WHILE IN A WEAK ENVIRONMENT, THE SYSTEM HAS THE POTENTIAL TO MOVE ERRATICALLY DURING A QUASI-STATIONARY PERIOD WITH POSSIBLE LOOPS AND BACKTRACKING NOT REFLECTED ON THE WARNING GRAPHIC BETWEEN TAU 36-72. AN UPPER-LEVEL TROUGH WILL BREAK DOWN THE RIDGE TO THE EAST, ALLOWING THE TYPHOON TO TRACK NORTHEASTWARD OVER LAND BETWEEN TAU 96-120. AS THE SYSTEM INTERMITTENTLY INTERACTS WITH THE TERRAIN OF SOUTHERN JAPAN, IT WILL WEAKEN SIGNIFICANTLY OVER THE FIRST 36 HOURS TO 40KTS, WHICH IT IS EXPECTED TO STAGNATE BETWEEN TAU 36-72 WHILE IN A QUASI-STATIONARY PERIOD. AS THE SYSTEM TRACKS BACK OVER LAND BETWEEN TAU 96-120, IT IS EXPECTED TO CONTINUE WEAKENING TO 30KTS.

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE VARIES GREATLY WITHIN MEMBERS OF THE JTWC MULTI-MODEL CONSENSUS. GFS TRACKS IN A SOUTHWESTERLY DIRECTION WHILE ALL OF THE OTHER GUIDANCE TRACKS NORTHEASTWARD. THE MAJORITY OF THE CONSENSUS MEMBERS ALSO HAVE A SLOW 5-10KT TRACK SPEED WITH THE EXCEPTION OF ECMWF, GSM, AND GALWEM. THE SLOWER TRACK SOLUTIONS REPRESENT MULTIPLE DIFFERENT TIMES AND POSITIONS WHERE LOOPING AND BACKTRACKING IS POSSIBLE BETWEEN TAU 36-72, WHICH IS WHY THE JTWC FORECAST SLOWS SIGNIFICANTLY DURING THAT PERIOD, BUT MAY NOT CAPTURE THE MINUTE MOTION WITHIN THE FORECAST. THE INTENSITY GUIDANCE IS IN GOOD AGREEMENT FOR A FAST RATE OF WEAKENING WITHIN THE FIRST 24-36 HOURS AND THEN A MORE GRADUAL WEAKENING AS THE SYSTEM INTERACTS WITH LAND AND WATER THROUGH THE END OF THE FORECAST. HOWEVER, THE TRACK VARIABILITY AND MEDIUM TRACK CONFIDENCE WILL IMPACT THE RATE OF WEAKENING, THEREBY DECREASING THE FAR-TERM INTENSITY FORECAST CONFIDENCE AS WELL.

THE AREA OF CONVECTION (INVEST 91A) PREVIOUSLY LOCATED NEAR 24.3N 70.5E IS NOW LOCATED NEAR 23.9N 69.1E, APPROXIMATELY 126 NM EAST- SOUTHEAST OF KARACHI. ANIMATED MULTISPECTRAL SATELLITE IMAGERY DEPICT FORMATIVE BANDING WITH EMBEDDED CONVECTION OVER THE NORTHWESTERN COAST OF INDIA. ENVIRONMENTAL ANALYSIS REVEALS FAVORABLE CONDITIONS FOR DEVELOPMENT, INCLUDING HIGH (20-25KTS) VERTICAL WIND SHEAR, STRONG UPPER-LEVEL OUTFLOW EQUATORWARD, WARM (28-29C) SEA SURFACE TEMPERATURE, AND MINIMUM SEA LEVEL PRESSURE IS ESTIMATED TO BE NEAR 992 MB. GLOBAL DETERMINISTIC MODELS INDICATE THAT 91A WILL CONTINUE TO DEVELOP WITH A WESTWARD TRACK OVER THE NEXT 24 HOURS. MAXIMUM SUSTAINED SURFACE WINDS ARE ESTIMATED AT 28 TO 33 KNOTS. MAXIMUM SUSTAINED SURFACE WINDS ARE ESTIMATED AT 28 TO 33 KNOTS. MINIMUM SEA LEVEL PRESSURE IS ESTIMATED TO BE NEAR 998 MB. MAXIMUM SUSTAINED SURFACE WINDS ARE ESTIMATED AT 28 TO 33 KNOTS. MINIMUM SEA LEVEL PRESSURE IS ESTIMATED TO BE NEAR 994 MB. THE POTENTIAL FOR THE DEVELOPMENT OF A SIGNIFICANT TROPICAL CYCLONE WITHIN THE NEXT 24 HOURS IS UPGRADED TO HIGH.

GLOBAL DETERMINISTIC MODELS INDICATE THAT 91A WILL CONTINUE TO DEVELOP WITH A WESTWARD TRACK OVER THE NEXT 24 HOURS.