WTPA45 PHFO 190300 TCDCP5 TROPICAL STORM UNALA DISCUSSION NUMBER 1 NWS CENTRAL PACIFIC HURRICANE CENTER HONOLULU HI CP022013 500 PM HST SUN AUG 18 2013 GOES VISIBLE IMAGERY AT 2330Z AND A 0020Z ASCAT PASS DEMONSTRATED THAT THE SUSPECT AREA FORMERLY KNOWN AS INVEST 90C WAS ACTUALLY TROPICAL STORM UNALA. ASCAT SHOWED A SHARP TROUGH WITH A RAGGED 30 NM SWATH OF TROPICAL STORM FORCE WINDS ACROSS NORTHEAST AND NORTHWEST SECTIONS...BUT NO CLOSED CIRCULATION CENTER. HOWEVER...GOES VISIBLE IMAGERY AT 2330Z SHOWED A LOW LEVEL CIRCULATION CENTER...LLCC...POPPING OUT TO THE WEST SOUTHWEST OF DEEP CONVECTION. THE COMBINATION OF THE TWO DATA SETS COMPELLED US TO UPGRADE THIS SYSTEM IN SHORT ORDER. INITIAL STRENGTH IS SET AT 35 KT BASED MAINLY ON SCATTEROMETER DATA. FIXES FROM CPHC AND JTWC INDICATED 30 KT STRENGTH...WHILE SAB ALONE HAD THIS SYSTEM STRONGER AT 35 KT. THERE WERE A FEW STRONGER WIND BARBS WITHIN THE ASCAT PASS...BUT WITH THE EMERGENCE OF THE LLCC IT IS LIKELY THIS SYSTEM IS ACTUALLY PAST ITS PEAK...MAKING 35 KT SEEM REASONABLE. SHIPS SLOWLY STRENGTHENS UNALA THROUGH 120 HOURS...MAKING IT A 50 KT TROPICAL STORM AT THE END OF 5 DAYS. HOWEVER...INTERFERENCE FROM TROPICAL STORM PEWA OR ITS REMNANTS SHOULD HINDER DEVELOPMENT. SHIPS SEEMS TO PARAMETERIZE THIS BY HAVING UNALA PASS THROUGH RATHER BRUTAL SHEAR BEYOND 12 HOURS...BUT FAILS TO WEAKEN THE SYSTEM APPROPRIATELY. WE WILL KEEP UNALA AT MARGINAL TROPICAL STORM STRENGTH THROUGH 48 HOURS...THEN WEAKEN IT AFTERWARD AS A NOD TO THE FORECAST SHEAR. INITIAL MOTION IS TOWARD THE WEST AT 18 KT ALONG THE SOUTH SIDE OF A BROAD SUBTROPICAL RIDGE. THIS TRACK WILL GRADUALLY BECOME NORTHWESTERLY IN THE 36 TO 48 HOUR TIMEFRAME AS UNALA BEGINS TO GAIN LATITUDE ALONG THE WESTERN LIMB OF THE RIDGE. TRACK GUIDANCE IS TIGHTLY PACKED IN A SIMILAR MANNER...BUT WITH A QUICKER SWING TO THE NORTH. INITIAL MOTION AND THE PROXIMITY OF PEWA KEEPS THE FORECAST TRACK WITHIN THE LEFT HALF OF THE TRACK GUIDANCE ENVELOPE. FORECAST POSITIONS AND MAX WINDS INIT 19/0300Z 17.1N 178.3W 35 KT 40 MPH 12H 19/1200Z 17.2N 178.9E 35 KT 40 MPH 24H 20/0000Z 17.6N 175.3E 35 KT 40 MPH 36H 20/1200Z 18.4N 172.1E 35 KT 40 MPH 48H 21/0000Z 19.9N 169.0E 35 KT 40 MPH 72H 22/0000Z 24.0N 163.9E 30 KT 35 MPH 96H 23/0000Z 29.5N 160.6E 30 KT 35 MPH 120H 24/0000Z 34.9N 158.8E 30 KT 35 MPH...POST-TROP/REMNT LOW $$ FORECASTER POWELL PALMBAYWEATHER.info
Posted on: Mon, 19 Aug 2013 03:00:10 +0000
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