000 AXPZ20 KNHC 081003 TWDEP Tropical Weather Discussion NWS National Hurricane Center Miami FL 1005 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 0600 UTC surface analysis and satellite imagery through 0945 UTC. ...SPECIAL FEATURE... Gulf of Tehuantepec: 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 in the late afternoon or early evening hours. 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 08N74W to the Gulf of Panama near 09N79W to 09N88W and to 09N97W. The ITCZ extends from 07N101W to 06N114W to 08N127W to 10N133W and to beyond 07N140W. Numerous moderate and scattered strong convection is within 120 nm north and 60 nm south of the ITCZ west of 135W. ...DISCUSSION... OFFSHORE WATERS WITHIN 250 NM OF MEXICO... 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 main marine issue for mariners will be extensive coverage of long period northwest swell currently propagating through these waters, with resultant seas in the 7 to 10 ft range. The northwest will gradually subside through Sun night. With the strong high pressure building in next week, yet another large set of long-period northwest swell will propagate through these same waters with seas in the 8 to 10 ft range. 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 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 08N to 10N between 95W and 99W, but will shrink in coverage through this afternoon and the seas will lower to less than 8 ft by this evening. Elsewhere, the monsoon trough will gradually lose longitudinal coverage through next week as it will generally exist roughly from 07N to 09N over the next few days, with gentle to moderate winds on either side of the trough and seas in the range of 4 to 6 ft. REMAINDER OF THE AREA... The earlier cold front has become stationary near 30N140W. A cold front is within about 400 nm to its northwest moving quickly eastward. This cold front is expected to reach from near 32N138W to 28N140W this evening, accompanied by fresh winds and a new set of large long-period northwest swell, with seas of 8 to 11 ft. This cold front will weaken as it moves eastward toward Baja California Norte through early next week. The northwest swell is forecast by Wave model guidance to sweep through just about the entire area, with the leading edge of 8 ft or greater seas reaching to the offshore waters of Baja California by late on Tue. Combined waves from trade winds and swell will support a large area of 8 to 10 ft seas south of 17N and west of 105W by Sun night. This area will retreat to the W of 120W by Tue afternoon, but an area of strong trade winds will develop between 10N and 25N by Tue night in response to the resultant gradient between strong high pressure that will build eastward over the northern and central waters with comparatively lower pressure in the sub-tropical and tropical regions of the area. A surface trough was introduced to the 06Z analysis along a position from near 13N97W to 07N91W based on Ascat data from late last night and further supported by model guidance. Fresh to locally strong northeast to east winds were noted within 90 nm west and 60 nm east of the trough from 08N to 11N. Satellite imagery shows scattered showers and isolated thunderstorms within 60 nm of the trough from 07N to 09N. The trough is forecast to continue westward through Sun night and weaken. $$ Aguirre