Western Pacific: Tropical Storms 09W(IN-FA) and 10W are both intensifying, 18/21utc updates

JTWC HAS BEEN ISSUING 6HOURLY WARNINGS ON 09W AND 10W. 3HOURLY SATELLITE BULLETINS ARE ISSUED FOR BOTH SYSTEMS.

TS 09W(IN-FA). WARNING 11 ISSUED AT 18/21UTC.THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: UNCERTAINTY IN INITIAL POSITION IS RESULTING IN SLIGHT SHIFTS IN THE FORECAST, AND NUMERICAL GUIDANCE IS LIKELY HAVING SIMILAR ISSUES. THE SYSTEM IS EXPECTED TO TRACK WEST-NORTHWESTWARD TO WESTWARD ALONG THE SOUTHERN PERIPHERY OF THE SUBTROPICAL RIDGE THROUGH THE FORECAST PERIOD. ADDITIONAL CONSOLIDATION AND INTENSIFICATION IS EXPECTED AS THE DEEP UPPER LOW IN THE WEST SEA CONTINUES TO FILL AND DRIFT POLEWARD, ENHANCING POLEWARD OUTFLOW. GRADUAL INTENSIFICATION IS FORECAST DUE TO THE BROAD SIZE, REACHING A PEAK INTENSITY OF AROUND 85 KNOTS/CAT 2 AS THE SYSTEM APPROACHES TAIWAN. AFTER 96H, INTERACTION WITH TAIWAN WILL WEAKEN THE SYSTEM SLIGHTLY. THE DEGREE OF WEAKENING IS HIGHLY DEPENDENT ON WHETHER THE ACTUAL TRACK TAKES IN-FA OVER LAND, OR KEEPS IT OVER WATER.

09W(IN-FA).SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS AREA OF VIGOROUS DEEP CONVECTION WRAPPING INTO A RAGGED CENTRAL CORE. TRACKING OF THE CENTER HAS BEEN DIFFICULT IN THE OVERNIGHT HOURS, HOWEVER AN 181708UTC AMSR2 IMAGE ARRIVED LATE IN THE CYCLE TO ASSIST. THE AMSR LOW LEVEL CIRCULATION CENTER (LLCC) APPEARS BROAD AND ILL-DEFINED, WITH DEEP CONVECTION DISPLACED TO THE NORTHEAST. THE INITIAL INTENSITY WAS ROUNDED UP FROM THE KNES T3.0 (45 KNOTS) TO 50 KNOTS BASED ON THE AMSR WINDSPEED DATA PRODUCT. THE LARGE SIZE OF 09W COMBINED WITH REMNANTS OF DRY AIR ARE INHIBITING THE SYSTEM FROM INTENSIFYING AT A FASTER RATE, DESPITE AN OTHERWISE FAVORABLE ENVIRONMENT.

09W(IN-FA).MODEL DISCUSSION: CONFIDENCE IN THE TRACK FORECAST THROUGH 72H IS MEDIUM GIVEN REASONABLE AGREEMENT AMONG THE MULTI-MODEL CONSENSUS. BEYOND 72H THE SPREAD GREATLY INCREASES, WITH ECMF A NOTABLE OUTLIER ON A MORE NORTH-WESTWARD TRACK. DUE TO THE THE PRESENCE OF 10W AND EXPECTED FUTURE DEVELOPMENT IN THE WAKE OF 09W, THERE IS POTENTIAL FOR DIRECT CYCLONE INTERACTION, A KNOWN SOURCE OF INCREASED NUMERICAL MODEL ERROR. THERE IS LOW CONFIDENCE IN THE JTWC INTENSITY FORECAST WITH A LARGE SPREAD IN INTENSITY GUIDANCE.

TS 10W. WARNING 2 ISSUED AT 18/21UTC.THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: TS 10W IS FORECAST TO CONTINUE STEADILY TRACKING WEST-NORTHWESTWARD TO WESTWARD THROUGH 72H UNDER THE STEERING INFLUENCE OF THE WEAK SUBTROPICAL RIDGE TO THE NORTH. AFTER 72H, THE SYSTEM WILL SLOW UNDER COMPETING STEERING INFLUENCES AND TURN GRADUALLY BACK TO THE SOUTHEAST UNDER THE STEERING INFLUENCE OF THE LOW-LEVEL WESTERLY FLOW THAT IS EXPECTED TO BUILD IN TO THE SOUTH THROUGH THE COMING DAYS. IN GENERAL, THE NUMERICAL MODEL GUIDANCE SUPPORTS THE FORECAST TRACK, HOWEVER, THERE IS A LARGE DEGREE OF SPREAD DUE TO THE COMPLEX, EVOLVING SYNOPTIC STEERING ENVIRONMENT. TS 10W SHOULD INTENSIFY TO A PEAK OF 55 KNOTS AT 36H JUST PRIOR TO LANDFALL WITH STEADY WEAKENING AS THE SYSTEM TRACKS OVER LAND. TD 10W WILL LIKELY RE-INTENSIFY NEAR 120H OR SHORTLY AFTER REEMERGING BACK OVER WATER. THE INTENSITY IS HIGHLY DEPENDENT ON THE DEGREE THAT THE ACTUAL TRACK REMAINS OVERS LAND, AND COULD BE HIGHER IF TS 10W REMAINS NEAR THE COAST.

TS 10W. SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A COMPACT AREA OF DISORGANIZED PERSISTENT CONVECTION. THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON EXTRAPOLATION FROM A 181413UTC ASCAT-C PASS WITH GOOD COVERAGE OF THE LOW LEVEL CIRCULATION CENTER (LLCC), AS WELL AS RADAR FIXES. DESPITE BEING GENERALLY CHARACTERIZED BY 30 KNOT BARBS IN THE EARLIER ASCAT, THE INTENSITY WAS SET AT 35 KNOTS WHICH IS SLIGHTLY HIGHER THAN AGENCY FIXES, BASED ON IMPROVING BANDING STRUCTURES IN THE RADAR LOOP, AND ASSUMING SLIGHT INTENSIFICATION HAS OCCURRED SINCE THE ASCAT OVERPASS TIME. THE COMPACT NATURE OF 10W ALSO SUGGESTS THAT DVORAK ESTIMATES MAY BE SLIGHTLY UNDER-REPRESENTATIVE.

TS 10W. MODEL DISCUSSION: THERE IS LIMITED MODEL GUIDANCE AVAILABLE AT THIS TIME, WITH A LARGE SPREAD AMONG THESE SOLUTIONS GIVEN THE COMPLEX SYNOPTIC STEERING ENVIRONMENT MADE EVEN MORE UNCERTAIN DUE TO THE PRESENCE OF 09W AND A POTENTIAL FUTURE AREA OF DEVELOPMENT IN ITS WAKE. THIS UNCERTAINTY IS EVIDENT IN THE LARGE CIRCULAR CONE OF POTENTIAL 34 KNOT WINDS. ALTHOUGH RAPID INTENSIFICATION IS NOT EXPECTED GIVEN THE SHORT TIME TO LANDFALL AND CURRENT POOR ORGANIZATION, IT REMAINS A POSSIBILITY GIVEN OTHERWISE FAVORABLE FACTORS.

TS 09W(IN-FA) AND TS 10W.