000 AXPZ20 KNHC 200839 TWDEP Tropical Weather Discussion NWS National Hurricane Center Miami FL 938 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 0600 UTC surface analysis and satellite imagery through 0900 UTC. ...SPECIAL FEATURES... Gulf of Tehuantepec: A storm warning is now in effect, induced by a very tight pressure gradient across the region due to strong 1036 mb high pressure over northern Mexico and lower pressures along the southeast Mexican coast. This gradient is combined with cooler, drier air funneling through the Chivela Pass behind a front that has stalled in the southwest Gulf of Mexico. A recent scatterometer pass only clipped the far southwest portion of the area showing gale force winds, while a ship with call-sign C6BZ9 traversing the offshore Gulf of Tehuantepec waters recently reported 42 knot elevated winds. Seas are building to up to 18 to 23 ft with the storm 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. ...INTERTROPICAL CONVERGENCE ZONE/MONSOON TROUGH... The monsoon trough axis extends from 04N77W to 06N100W to 10N111W. The intertropical convergence zone axis extends from 10N111W to 12N120W to 07N140W. Scattered moderate convection is from 08N to 12N between 112W and 115W, within 90 nm either side of the axis between 120W and 126W, and also within 90 nm south of the axis west of 132W. ...DISCUSSION... OFFSHORE WATERS WITHIN 250 NM OF MEXICO... Gulf of California: Gale force winds have diminished in the past few hours, with a 0430 UTC ASCAT-B scatterometer pass indicating 20 to 30 kt winds in the northern Gulf, where seas are also likely 8 to 10 ft. Northerly 20 to 25 kt winds are spilling southward to 25N in the central 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 in the northern gulf by late this morning, with fresh to strong winds gradually shifting southward, spreading out of the entrance of the gulf by tonight through Wednesday night. Elsewhere, a deep layered surface trough induced by a cutoff low aloft extends into the offshore waters west of Baja California. Fresh to strong winds were recently sampled by a scatterometer pass southwest of the southern tip of the peninsula, 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 28N119W by late Wednesday night, dissipating as it moves northeast toward southern California by the end of the week. A set of northwest swell of 8 to 10 ft will arrive Wednesday night through Friday. 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 large northwest swells of 10 to 16 ft arriving behind the front by Sunday morning. 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 from the Gulf of Papagayo 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 gaps. Gulf of Panama: Fresh northerly winds will pulse through the morning, with moderate northerly pulsing winds then expected on subsequent nights. Elsewhere across the region, winds will remain 20 kt or less, and seas will remain below 8 ft through Saturday night. REMAINDER OF THE AREA... A 1033 mb high centered north of the area near 34N136W extends a ridge to the southeast across the waters north of the ITCZ. This ridging combined with elongated surface troughing embedded in the ITCZ continues to support an area of fresh to strong trades from roughly 08N to 26N west of 120W. Northwest swells of 8 to 10 ft cover the majority of the waters north of the ITCZ west of 120W. The high center is forecast to move southeast 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 will arrive behind the front, up to 17 to 20 ft along 30N by Saturday. $$ LEWITSKY