000 AXPZ20 KNHC 171602 TWDEP Tropical Weather Discussion NWS National Hurricane Center Miami FL 1605 UTC Tue Oct 17 2017 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 1200 UTC surface analysis and satellite imagery through 1530 UTC. ...SPECIAL FEATURES... Gulf of Tehuantepec Gale Warning: October marks the beginning of the Tehuantepec wind events. The first gale-force event of a season typically occurs in mid-October. Currently, gale force winds of 30 to 40 kt are present across the Tehuantepec area as inferred from the 12 UTC isobaric analysis, model data and an ASCAT pass from last night. The analysis suggests that the pressure gradient between high pressure across the eastern slopes of the Sierra Madre Mountains in Mexico, and across the western Gulf of Mexico in the wake of a cold front over the central Bay of Campeche with lower pressures along the coast of Mexico and S of Mexico supports these gale force gap winds. Seas have build to the range of 10-15 ft with these winds. Global model guidance indicates that the high pressure will weaken through Thursday, with the corresponding tight gradient slackening. This will allow for the gale force winds to eventually diminish to below gale force just after sunrise on Thursday, however, northerly winds of 20-30 kt will continue through Friday along with seas of 8-10 ft. INTERTROPICAL CONVERGENCE ZONE/MONSOON TROUGH... The monsoon trough axis extends from along the coast of NW Colombia to 09N79W to 09N88W to 08N100W to 09N112W where scatterometer data indicates the ITCZ then begins and continues to 09N124W to 13N138W. Scattered moderate to strong convection is within 180 nm N of the axis between 113W-118W, and within 120 nm N of the axis between 118W-121. Scattered moderate isolated strong convection is S of the axis between 80W-83W. Scattered moderate convection is within 30 nm of the axis between 92W-94W, and between 105W-108W. ...DISCUSSION... OFFSHORE WATERS WITHIN 250 NM OF MEXICO... Please see Special Features section above for information about the ongoing Gulf of Tehuantepec gale event. A surface ridge extends across the waters off Baja California, with the pressure gradient between it and troughing along the eastern part of Baja California bringing gentle to moderate NW-N winds and seas of 5-6 ft. The ridge will remain in place during the next few days with little change in winds and seas over the offshore waters aside from the Gulf of Tehuantepec region as described above. By late Friday afternoon into Friday evening a weakening cold front and accompanying set of large NW swell will propagate through the waters W of Baja California building seas to the 11-14 ft range. Also on Friday evening, NW winds will increase to 20-25 kt over the waters adjacent to and within 180 nm W of Baja California Norte. Mainly gentle to moderate NW-N winds are expected over most of the Gulf of California through early Wednesday, then winds become light and variable through Friday, with seas of 2 ft or less except for higher seas of 4-5 ft at the entrance to the gulf in SW swell subsiding to 3-4 ft Wednesday through Thursday night and to 2-3 ft on Friday. OFFSHORE WATERS WITHIN 250 NM OF CENTRAL AMERICA, COLOMBIA, AND WITHIN 750 NM OF ECUADOR... Moderate to fresh offshore winds will develop across the Papagayo region each night through Thursday night, with seas of 6-7 ft. Light and variable winds and seas of 4-6 ft in SW swell will prevail N of the monsoon trough, while gentle to moderate SW to W winds and seas of 5-7 ft in SW swell will prevail S of the trough. Swells originating from the gale force wind event in the Gulf of Tehuantepec will reach the far western section of the offshore waters of Guatemala and El Salvador, building seas to 8-10 ft there today through Wednesday and subsiding to 6-8 ft Wednesday evening and to 5-6 ft Thursday. Long period SW swell is forecast to reach the area between Ecuador and the Galapagos Islands from Thursday evening into Friday, building seas to 7 to 8 ft on Friday. REMAINDER OF THE AREA... Low pressure of 1013 mb near 23N127W continues to rapidly weaken under very strong W to SW upper winds. Latest satellite imagery shows that the only remaining convective activity with the low is well removed 180-240 nm to the NE of the low, and is quickly dissipating. Previous strong winds with this low have diminished to moderate to fresh winds within 150 nm NE of the low, with seas of 8-10 ft in mixed swell based on latest available altimeter data. The low will degenerate into a trough near 128W from 21N-28N late tonight into Wednesday. A 1010 mb low pressure embedded in the ITCZ region near 15N138W contains winds of 20-25 kt with seas of 8 to 9 ft within 60 nm of it in the NE semicircle. Seas are 8-9 ft in mixed swell elsewhere from 14N-19N W of 134W. Model guidance indicates that this low will weaken to a trough just to the W of 140W by early on Wednesday. Seas to 8 ft in mixed swell are expected by that time from 15N-20N W of 134W. This mix swell will then decay late Wednesday night allowing for these seas to subside to 5-7 ft. Elsewhere, gentle to moderate winds are noted across the northern forecast waters under the influence of a weak ridge. Seas continue to subside across the south-central waters, and currently seas of 8-9 ft are noted S of 20N between 115W-128W, also S of 15N between 105W-115W and S of 15N W of 128W. The culprit mixed SE and SW swell is forecast to gradually decay through the end of the week, however a large batch of SW swell is forecast by Wave model guidance to propagate through the far southern waters beginning on Thursday with resultant combined seas of 8-9 ft. Looking ahead...a cold front is forecast by the global models to reach the far NW corner of the forecast region by Thursday night. A significant swell event will follow the front, with seas building to 10-17 ft over the NW waters by early Friday before spreading across all the northern waters W of 115W, and across the west-central waters by late Friday night. $$ Aguirre