000 AXPZ20 KNHC 201559 TWDEP Tropical Weather Discussion NWS National Hurricane Center Miami FL 1311 UTC Tue Dec 20 2016 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 FEATURES... Gulf of Tehuantepec: A storm warning is in effect for the Gulf of Tehuantepec through late this morning. Cooler, drier air behind a stationary front over the western Gulf of Mexico has been moving across the Isthmus of Tehuantepec into the Gulf of Tehuantepec. Unfortunately, no recent wind observations have been available in the Gulf. There was, however, an altimeter pass showing peak wave heights of 14-16 ft. Seas likely to reach as high as 22 ft this morning given the strength and extent of the storm and gale force winds. Winds are forecast to diminish below storm force late this morning, with gale force winds persisting through Friday night as a relatively tight pressure gradient remains in place. Northerly flow will then pulse to strong to near gale force this weekend, with the strongest winds expected at night with the assistance of the nocturnal drainage flow. ...INTERTROPICAL CONVERGENCE ZONE/MONSOON TROUGH... The monsoon trough extends across the Gulf of Panama to 09N84W to 08N90W to 05N95W to 09N110W. The Intertropical Convergence Zone axis extends from 09N110W to 11N118W, then resumes at 10N124W to beyond 07N140W. Scattered moderate isolated strong convection is within 90 nm S of ITCZ axis W of 124W. ...DISCUSSION... OFFSHORE WATERS WITHIN 250 NM OF MEXICO... Gulf of California: the most recent scatterometer pass indicated NW winds of 20 to 30 kt in the northern Gulf, where seas are likely to 8 ft. Northerly winds of 20 to 25 kt are spilling southward across the remainder of the Gulf. These winds are the result of a strong pressure gradient between high pressure over the U.S. Rocky Mountain states and a trough of low pressure just inland across northwest mainland Mexico. Winds will diminish to 20 kt in the northern Gulf later today, with fresh to strong winds gradually shifting southward, spreading out of the entrance of the Gulf today through Thursday morning. Elsewhere, a surface trough induced by a cutoff low aloft extends into the offshore waters west of Baja California. Fresh to strong easterly winds were recently sampled by a scatterometer pass across the western part of forecast zone PMZ015, with deep convection also impacting this area. The trough is forecast to linger with a slow northwest drift through mid-week. Low pressure of 1012 mb is forecast to form along the trough near 23N118W by tonight. The low will move toward the N to a position near 29N119W by Wednesday night, then move eastward while dissipating by the end of the week. Seas of 8-10 ft in association with this trough/low pres are also expected to affect the offshore waters late today through Wednesday. NW swell will continue to impact the offshore waters W of Baja. Seas associated with the trough/low are forecast to merge with additional pulses of NW swell arriving Wednesday night through Friday. By Thursday morning, expect seas of 8-10 ft N of 20N w of 115W. A cold front will approach Baja California norte by the end of the week, moving across the area late Saturday. A brief surge of 20 to 25 kt winds both ahead of and behind the front is possible, with sea heights of 10 to 16 ft propagating across the NE forecast waters...including forecast zones PMZ009, PMZ011, and part of PMZ013 during the upcoming weekend. OFFSHORE WATERS WITHIN 250 NM OF CENTRAL AMERICA, COLOMBIA, AND WITHIN 750 NM OF ECUADOR... Gulf of Papagayo: Fresh to strong northeast winds are expected to pulse in and downstream of the Gulf of Papagayo to near 90W during the overnight and early morning hours through Saturday morning, with the assistance of the nocturnal drainage flow. Locally, these winds will produce up to 8 to 9 ft seas. The fresh northeast swells will merge with the Gulf of Tehuantepec gap wind event to provide a large area of 8 to 12 ft seas downstream of the gap regions. Gulf of Panama: Fresh northerly winds and seas to near 6 ft are noted across this Gulf to about 06N80W. These marine conditions will persist tonight, then mainly moderate northerly winds are expected on subsequent nights. Elsewhere across the region, winds will remain 20 kt or less, with seas below 8 ft through Saturday night. REMAINDER OF THE AREA... A 1033 mb high centered north of the area near 33N136W extends a ridge across the waters north of the ITCZ W of 120W. This ridge combined with the aforementioned trough continues to support a broad area of fresh to strong trades from roughly 08N to 26N west of 120W. Based on altimeter data, seas of 8 to 12 ft in mixed wind waves and NW swell, cover the majority of the waters north of the ITCZ west of 120W. The high center is forecast to move westward over the next 24 hours while slightly weakening. As a result, the aerial coverage of fresh to strong winds will decrease by Wednesday and remain limited through Saturday. A cold front will drop south into the northern waters Friday night with a brief surge of 20 to 25 kt northerly winds possible behind it north of 28N. Large northwest swells, building seas up to 16 ft along 30N, will arrive behind the front on Saturday. $$ GR