000 AXPZ20 KNHC 170844 TWDEP Tropical Weather Discussion NWS National Hurricane Center Miami FL 845 UTC Mon Dec 17 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 0900 UTC. ...SPECIAL FEATURES... Gulf of Tehuantepec Gale Warning: The tight pressure gradient over southeastern Mexico resulting from the combination of high pressure over northeast Mexico and lower pressure south of the region continues to produce gale force northerly winds to around 40 kt across the Gulf of Tehuantepec and downstream to near 14N95W, where seas are near 14 ft. Gap winds will diminish just below gale force late today as the strong high pressure over Mexico shifts eastward and weakens. Seas with this event will subside to 9 to 13 ft today and to less than 8 ft on Tue as the northerly winds diminish to 20 kt or less. Looking ahead, another gale event is expected by late Thu as a strong cold front moves into southern Mexico. Please refer to the latest high seas forecast issued under AWIPS/WMO headers HSFEP2/FZPN03 for more specific marine related details. ...INTERTROPICAL CONVERGENCE ZONE/MONSOON TROUGH... No significant monsoon trough was observed. The ITCZ extends from 05N90W to 08N125W to 06N140W. Scattered moderate convection was noted from 06N to 08N between 123W and 128W. ...DISCUSSION... OFFSHORE WATERS WITHIN 250 NM OF MEXICO... At the surface, a weak ridge will prevail over the area through Mon morning to maintain light to gentle breezes across the area waters. Northwest swell with seas 8 to 10 ft will dominate the offshore waters of Baja California through early morning before briefly subsiding to 6 to 8 ft through around noon today. A weak cold front will move eastward into the area by this afternoon, then will become diffuse. The front will be followed by moderate high pressure and freshening northerly winds across the offshore waters. This pattern will prevail through early Wed before the ridge begins to weaken. Also accompanying the front will be a new pulse of large NW swell, with the leading edge of 12 ft seas or greater reaching the northern coast of Baja California Norte and Guadalupe Island by late morning. The long period NW swell will bring heights of 12 to 16 ft across most of the waters off Baja California and through the Revillagigedo Islands. Swell of this size and period will produce hazardous seas for mariners, and create very large and powerful surf along the Pacific reefs and beaches. This high surf will also produce coastal flooding and beach erosion in many exposed locations from southern California to western Mexico. The swell will decay below 12 ft through Wed, ahead of a smaller reinforcing group of swell bringing seas again to 12 ft to Baja California Norte Thu. Elsewhere seas in excess of 8 ft will persist in Mexican open waters west of 105W through the week. Across the Gulf of California, strong high pressure north of region is supporting moderate to fresh breezes across the south half of the basin this morning, with little change expected through Tue. Strong high pressure will build north of the area by Wed, and will support fresh to strong winds across the northern and central Gulf, while seas build to 5-8 ft. OFFSHORE WATERS WITHIN 250 NM OF CENTRAL AMERICA, COLOMBIA, AND WITHIN 750 NM OF ECUADOR... A scatterometer pass from 0000 UTC indicate fresh to strong gap winds across the Gulf of Papagayo and Gulf of Fonseca. A tight pressure gradient between a wedge of high pressure through Central America and relatively lower pressure farther south will support s persistent fresh to strong gap winds across Papagayo into mid week. The same scatterometer pass also indicated smaller areas of fresh to strong offshore winds along the coast of western El Salvador and eastern Guatemala, assisted in part by low pressure over the northwest Caribbean of the coast of Honduras. The low is expected to weaken through today, allowing the cross-isthmus flow will diminish. Farther south, fresh to strong trade winds across the southwest Caribbean are supporting fresh gap winds into the Gulf of Panama, possibly pulsing to strong winds off the Azuero Peninsula Elsewhere, gentle to moderate winds will persist on either side of the monsoon trough through early next week, as seas remain in the 4 to 6 ft range. REMAINDER OF THE AREA... An upper trough from 32N130W to 10N140W is becoming a little more amplified and is supporting a persistent surface trough along roughly 131W from 08N to 16N. The combination of lower level trade wind convergence and divergence aloft on the east side of the upper trough is supporting scattered convection along the intertropical convergence zone between 120W and 130W. A scatterometer satellite pass from 0520 UTC showed strong trade winds along the ITCZ in this area as well. Farther north, a weakening cold front analyzed from 31N125W to 27N140W is expected to become diffuse as it moves eastward toward Baja California through tonight. A new round of long period NW swell is outrunning the front eastward, with the leading edge of 12 ft seas reaching from 31N122W to 16N140W. An altimeter satellite pass from 03 UTC showed seas to 22 ft near 30N133W. The NW swell will continue to move eastward, with 12 ft seas dominating most of the area west of 120W and north of 05N by late Tue. A pair of altimeter pass from around 0330 UTC and 0430 UTC showed confused seas 8 to 10 ft in an area probably from 03N to 14N between 92W and 113W, primarily in a mix of NW swell with shorter period NE and E swell from ongoing Mexican and Central American gap wind events. This will gradually diminish to 8 to 9 ft west of 100W by mid week in predominantly NW swell. $$ Christensen