342 AXPZ20 KNHC 270917 TWDEP Tropical Weather Discussion NWS National Hurricane Center Miami FL 1005 UTC Thu Sep 27 2018 Tropical Weather Discussion for the eastern Pacific Ocean from the Equator to 32N, east of 140W. The following information is based on satellite imagery, weather observations, radar, and meteorological analysis. Based on 0600 UTC surface analysis and satellite imagery through 0915 UTC. ...SPECIAL FEATURE... Hurricane Rosa centered near 17.1N 114.1W at 27/0900 UTC or 550 nm WSW of Puerto Vallarta Mexico moving W at 10 kt. Estimated minimum central pressure is 981 mb. Maximum sustained wind speed is 75 kt with gusts to 90 kt. Currently numerous strong convection is observed in bands within 270 nm southeast and 240 nm northwest semicircles of center. Environmental conditions are favorable for Rosa to gradually intensify. Swells generated by Rosa will affect portions of the coast of southwestern Mexico and the southern Baja California Peninsula late this week and over the weekend. These swells are likely to cause life- threatening surf and rip current conditions. Please consult with products from your local weather office. See latest NHC forecast/advisory under AWIPS/WMO headers MIATCPEP5/WTPZ35 KNHC for additional details. ...TROPICAL LOW... A surface low is analyzed along the monsoon trough at 10.5N93W and is estimated at 1009 mb. Scattered moderate isolated strong convection is currently observed within 120 nm north of the estimated center. This low is forecast to move to near 11N96.5W on Fri night, and near 12N100W on Sat night, with at least strong cyclonic winds developing within 240 nm over the north semicircle. There will be much uncertainty until this system becomes more defined. At this time the low is expected to track northwest just beyond 240 nm seaward through the upcoming weekend, with an envelop of at least strong cyclonic winds, and seas building to 12 ft, shifting west across the offshore waters generally south of 16N west of 97W through Sun night, with seas of 8 ft or greater extending further outward across the waters from 11N to 20N west of 98W. There is a possibility that the low will recurve north early next week with the associated conditions then shifting back across the offshore waters beyond 200 nm, roughly from 16N to 21N west of 109W, during the middle of next week. This cyclone currently has a low chance of becoming a tropical cyclone through 48 hours, and a medium chance thereafter. Refer to the East Pacific Tropical Weather Outlook under WMO/AWIPS headers ABPZ20 KMIA/MIATWOEP for additional details. ...INTERTROPICAL CONVERGENCE ZONE/MONSOON TROUGH... The monsoon trough extends W off the Caribbean coast of Colombia at 10N76W to the Pacific coast of Costa Rica at 09N85W then through the surface low at 10.5N93W to 10N101W where it loses identity. The monsoon trough resumes west of Rosa at 13N116W and continues southwest to 11N129W where scatterometer winds indicate a transition to an ITCZ which continues west to 11N138W. Scattered moderate to strong convection is observed north of 12n between 81W 100w. Scattered moderate isolated strong convection is noted within 150 nm either side of a line from 07N77W to 10N88W to 09N105W to 14N109W, and within 120 nm either side of a line from 13N116W to 11N129W. ...DISCUSSION... OFFSHORE WATERS WITHIN 250 NM OF MEXICO... See the Special Features paragraph above for information on Tropical Cyclone Rosa. A west to east ridge along 24N will gradually retract west of the area ahead of Tropical Cyclone Rosa. Gentle to moderate northwest flow is forecast west of the Baja California through Fri, then becoming northerly early this upcoming weekend, with marine conditions deterioating across the southern waters as early as Sat night with these conditions shifting northeast across the waters west of Baja Norte on Mon night and Tue. Gulf of California: Light to gentle mostly southerly winds expected across the gulf waters through Sun night. The southerly flow will increase to near gale conditions to the north of 27N thereafter with tropical storm conditions possible north of 29N on Tue. Gulf of Tehuantepec: Fresh northerly flow forecast through late this morning. Expect seas to build to 8 ft on Fri as a tropical low previously described passes to the southwest. OFFSHORE WATERS WITHIN 250 NM OF CENTRAL AMERICA, COLOMBIA, AND WITHIN 750 NM OF ECUADOR... Gulf of Papagayo: Moderate to locally fresh east nocturnal drainage flow expected across and downstream of the Gulf of Papagayo through Sat night. See Tropical Low paragraph above concerning a tropical low developing south of Guatemala which will affect the far offshore Gautemala waters through late this afternoon. Gentle east winds are then forecast north of the monsoon trough which is meandering between 09N and 12N. Moderate southwest flow is forecast south of the monsoon trough. REMAINDER OF THE AREA... See the Special Features paragraph above for information on Tropical Cyclone Rosa. A broad ridge continues across the subtropics accompanied by moderate anticyclonic flow south of 23N west of 120W. Long period swell from Rosa will spread west to along 130W over the upcoming weekend. A series of weak cold fronts, or troughs, will sweep east across the far NW waters through the upcoming weekend accompanied by a moderate west to northwest wind shift. $$ Nelson