000 AXPZ20 KNHC 081548 TWDEP Tropical Weather Discussion NWS National Hurricane Center Miami FL 1249 UTC Sat Dec 8 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 1200 UTC surface analysis and satellite imagery through 1500 UTC. ...SPECIAL FEATURE... Gulf of Tehuantepec Gale Warning: In the wake of a Gulf of Mexico cold front, strong high pressure will begin to ridge southward along the eastern slope of the Sierra Madre Oriental mountains of Mexico beginning tonight. The pressure difference between the high pressure and lower pressure to the south of Mexico will induce a tight pressure gradient over southeastern Mexico on Sunday. This will result in a rather quick onset of strong to near gale force winds northerly winds to surge southward through the Isthmus of Tehuantepec and out across the Gulf of Tehuantepec near daybreak Sun. These northerly winds will increase to strong gale force beginning Sun afternoon. Seas are forecast to build to the range of 10 to 16 ft with these gale force winds by late Sun night. The strong gale winds will continue through Tue, before diminishing to minimal gale force Tue night and diminish further to strong to near gale force early on Wed as the strong high pressure over eastern Mexico weakens as it shifts eastward. Swell from this event is forecast to propagate south and southwestward well away from the Gulf of Tehuantepec region, with the leading edge of the 8 ft seas expected to reach near 09N110W by late Tue night into Wed. Please refer to the Eastern Pacific High Seas Forecast under AWIPS/WMO headers HSFEP2/FZPN03 KNHC for more specific marine details. ...INTERTROPICAL CONVERGENCE ZONE/MONSOON TROUGH... The monsoon trough axis extends from northern Colombia near 09N76W across Panama to 08N93W. The ITCZ extends from 08N100W to 08N140W. Numerous moderate and scattered strong convection is within 90 nm of the ITCZ west of 130W. ...DISCUSSION... OFFSHORE WATERS WITHIN 250 NM OF MEXICO... Recent altimeter passes confirm seas of 8 to 10 ft across much of the offshore waters of Baja California. This is due to persistent long period NW swell. This is expected to subside below 8 ft through early Sun. Another group of NW swell will reach the northern coastal waters of Baja California Norte by late Mon, as well as Guadalupe Island. Swell in excess of 8 ft will cover most the offshore waters of Baja California and the Revillagigedo Islands through late Tue, before subsiding below 8 ft through mid week. The pressure gradient between a 1026 mb high analyzed near 32N126W moving northeastward and comparatively lower pressure over much of the Baja California Peninsula will maintain mainly moderate to occasionally fresh north to northeast winds over the offshore waters through Mon night. Winds begin to increase late Mon night into early Tue over the NE part of the area as strong high pressure builds east-southeastward across these waters. The combination of strong high pressure that will build over the Great Basin with troughing along and near the Gulf of California is expected to produce a narrow swath of northwest to north winds across the Gulf of California within about 60 nm of the E coast of Baja California Sur from Sun afternoon afternoon to late Sun night. OFFSHORE WATERS WITHIN 250 NM OF CENTRAL AMERICA, COLOMBIA, AND WITHIN 750 NM OF ECUADOR... Gulf of Papagayo...Nocturnal fresh to locally strong northeast winds and seas of 6 to 8 ft will diminish to fresh winds this afternoon. These winds will resume to pulse during mostly overnight and early morning hours over the Gulf of Papagayo Sun night through Wed night as high pressure that will shift eastward over the Gulf of Mexico builds southward toward the northwest Caribbean Sea and Central America. An area of moderate to fresh east winds well west and southwest of the Gulf has seas in the 6 to 8 ft range from mixed swell. This area is defined from 08N to 10N between 95W and 99W, but will shrink in coverage through this afternoon and seas will lower to less than 8 ft by this evening. Elsewhere, gentle to moderate winds will persist on either side of a weakening monsoon trough and seas in the range of 4 to 6 ft. REMAINDER OF THE AREA... A cold front is dissipating near 30N140W, ahead of a stronger reinforcing front approaching the area from the west. Long period NW swell will continue propagating SE to encompass almost all of the forecast waters west of 110W through the afternoon, ahead of a reinforcing group of swell approaching from the northwest associated with the second front. Much of the older swell will decay north of 20N through early Sun. Combined seas from trade winds and swell will support a large area of 8 to 10 ft seas south of 20N by Sun night, eventually mixing with the longer period NW reinforcing swell moving across the basin west of 120W through Tue. The combination of low level trade wind convergence and divergence aloft on the east side of an upper trough farther west near Hawaii is supporting scattered convection along the ITCZ west of 130W. This will persist at least through Sun. Farther east, a surface trough persist near 100W from 06N to 12N. Earlier scatterometer data indicated winds to 25 kt on the northern portion of the trough. Satellite imagery shows scattered showers and isolated thunderstorms persisting within 60 nm of the trough from 07N to 09N. The trough is forecast to continue westward through Sun night and weaken. Meanwhile a large area of mix swell will emerge from 03N to 13N east of 110W consisting of the NW swell and shorter period NE swell downstream of the Gulf of Tehuantepec. $$ Christensen