WTPN21 PGTW 171330 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/TROPICAL CYCLONE FORMATION ALERT (INVEST 91W)// RMKS/ 1. FORMATION OF A SIGNIFICANT TROPICAL CYCLONE IS POSSIBLE WITHIN 175 NM EITHER SIDE OF A LINE FROM 27.3N 157.8E TO 33.5N 162.4E WITHIN THE NEXT 12 TO 24 HOURS. AVAILABLE DATA DOES NOT JUSTIFY ISSUANCE OF NUMBERED TROPICAL CYCLONE WARNINGS AT THIS TIME. WINDS IN THE AREA ARE ESTIMATED TO BE 18 TO 23 KNOTS. METSAT IMAGERY AT 171200Z INDICATES THAT A CIRCULATION CENTER IS LOCATED NEAR 27.6N 157.8E. THE SYSTEM IS MOVING NORTHWARD AT 06 KNOTS. 2. REMARKS: THE AREA OF CONVECTION (INVEST 91W) PREVIOUSLY LOCATED NEAR 27.1N 156.7E IS NOW LOCATED NEAR 27.6N 157.8E, APPROXIMATELY 700 NM NORTHWEST OF WAKE ISLAND. RECENT PHASE CLASSIFICATION WORKSHEETS INDICATE THE SYSTEM HAS TRANSITIONED FROM SUBTROPICAL TO TROPICAL SYSTEM AND THUS IS NO LONGER CLASSIFIED AS A SUBTROPICAL DEPRESSION. ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A LOW LEVEL CIRCULATION CENTER (LLCC) THAT HAS BECOME ALIGNED WITH STRONG CONVECTION THAT WAS PREVIOUSLY PUSHED OFF TO THE EAST. A 171106Z ASCAT METOP-B BULLSEYE CONFIRMS THE TROPICAL CONVERSION WITH A SYMMETRICAL LLCC, BUT THE WIND FIELD REMAINS LOPSIDED, WITH LIGHT WINDS IN THE WESTERN HALF OF THE SYSTEM, AND EMBEDDED AREAS OF 25-30 KNOT WINDS ALONG THE EASTERN PERIPHERY, UNDER THE FLARING CONVECTION, WHICH ARE NOT CONSIDERED TO BE REPRESENTATIVE OF THE OVERALL INTENSITY OF THE SYSTEM. ENVIRONMENTAL ANALYSIS REVEALS AN IMPROVEMENT IN CONDITIONS DUE TO A DECREASE IN VWS (DOWN TO 5-10 KNOTS), AN INCREASE IN POLEWARD OUTFLOW, BUT NOT MUCH A CHANGE IN THE SSTS DEPARTMENT (27-28C). GLOBAL MODELS ARE IN AGREEMENT THAT 91W HAS A BRIEF WINDOW OF 6-12 HOURS TO INTENSIFY BEFORE TROUGHING OUT AND GETTING PICKED UP BY AN APPROACHING LONGWAVE TROUGH AND TRANSITIONING ONCE AGAIN TO A SUBTROPICAL THEN EXTRATROPICAL LOW. MAXIMUM SUSTAINED SURFACE WINDS ARE ESTIMATED AT 18 TO 23 KNOTS.MINIMUM SEA LEVEL PRESSURE IS ESTIMATED TO BE NEAR 1005 MB. THE POTENTIAL FOR THE DEVELOPMENT OF A SIGNIFICANT TROPICAL CYCLONE WITHIN THE NEXT 24 HOURS IS HIGH. 3. THIS ALERT WILL BE REISSUED, UPGRADED TO WARNING OR CANCELLED BY 181330Z.// NNNN