000 AXPZ20 KNHC 091003 TWDEP Tropical Weather Discussion NWS National Hurricane Center Miami FL 1005 UTC Sun Dec 9 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 Gale Warning: High pressure is pressing southward along the eastern slopes of the Sierra Madre Oriental mountains of Mexico. The large pressure difference between the high and lower pressure south of Mexico will result in an abrupt onset of strong to near gale force north to northeast winds, which will surge southward across the Isthmus of Tehuantepec and out over the Gulf of Tehuantepec. These winds have recently begun to affect the northern tier of the Gulf. These north winds will quickly increase to strong gale force beginning this afternoon. Seas in the gale area are forecast to build to between 10 and 17 ft by late Sun night. The strong gale force winds will continue through Tue, then diminish to minimal gale force Tue night. Winds will then quickly decrease and become light by Wed evening as the strong high over the Gulf weakens and shifts eastward. Swell from this event is forecast to propagate south and southwest 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 or 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 08N76W to across southern Panama and Costa Rica and to 08N90W, where latest scatterometer data indicates that ITCZ begins and continues to 08N102W where it briefly ends to the east of a surface trough that extends from 14N103W to 07N104W. The ITCZ resumes from 08N106W to 07N120W to 06N130W and to beyond 07N140W. Scattered moderate to isolated strong convection is seen within 180 nm north of the ITCZ west of 133W. Scattered moderate convection is within 120 nm south of the ITCZ between 114W and 117W, also within 90 nm south of the ITCZ between 125W and 127W and within 60 nm north of the ITCZ between 127W and 129W. ...DISCUSSION... OFFSHORE WATERS WITHIN 250 NM OF MEXICO... Recent satellite-derived wind data confirmed long period northwest swell are maintaining seas of 7 to 9 ft spread across much of the offshore waters of Baja California. Seas are expected to subside a couple of feet by tonight as the NW swell decay. Another round of NW swell will reach the northern coastal waters of Baja California Norte by Mon evening, as well as Guadalupe Island. Seas exceeding 8 ft will encompass most the offshore waters of Baja California and the Revillagigedo Islands through Tue evening, before subsiding below 8 ft through mid week. High pressure of 1027 mb is centered near 31N125W. This high will be overtaken with a larger high pressure system that will build eastward over the northern and central waters Mon. The pressure gradient between this area of high pressure and lower pressure in the vicinity of the Baja Peninsula will primarily maintain 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 northeast part of the area as the extensive area of high pressure builds east- southeastward across those waters. The combination of strong high pressure building over the Great Basin and troughing along and near the Gulf of California will soon result in a narrow swath of fresh to locally strong northwest to north winds across the Gulf of California within about 30 nm of the E coast of Baja California from 25N to 27N through late tonight. OFFSHORE WATERS WITHIN 250 NM OF CENTRAL AMERICA, COLOMBIA, AND WITHIN 750 NM OF ECUADOR... Gulf of Papagayo...Winds and seas have diminished for the time being. However, Winds will once again pulse to strong intensity during the overnight and early morning hours over the Gulf of Papagayo beginning tonight and through Wed night as high pressure shifting eastward over the Gulf of Mexico builds southward toward the NW Caribbean Sea and Central America. 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 extends from near 32N135W to 27N140W. Yet another batch of long-period NW swell is following in behind this front, with seas in the 8 to 11 ft range except for large seas of 10 to 13 ft in the far northwest corner of the area. Swell, primarily northwest in direction, already covers a good chunk of the forecast waters within the area bounded by 18N140W to 21N131W to 19N108W to 08N102W to 03N103W to 03.4S108W to 03.4S120W to the equator at 120W and at 136W, then to 07N140W and to 18N140W. This swell will continue to decay through Mon, while the swell associated with the cold front will propagate through the waters covering the area northwest of a line from near 25N113W to 14N115W and to the equator near 126W by late on Tue. This swell will produce seas in the range of 8 to 11 ft by then, with the exception of seas in the 10 to 12 ft range in the far western portion of the area where the northwest swell will combine with windwave swell produce by strong northeast winds. A surface trough was again nicely depicted by an Ascat pass last night. The inferred position of this trough is from 14N103W to 07N104W. The Ascat pass revealed east to southeast winds within about 60 nm either side of the trough from 10N to 12N. Seas with these winds are in the range of 6 to 8 ft. Satellite imagery shows scattered showers and thunderstorms within 120 nm west of the trough from 10N to 14N, and within 60 nm east of the trough from 07N to 14N. The trough is forecast to continue to move westward and weaken through tonight. Wave modeL guidance suggests that a rather large area of 8 ft seas associated with mixed swell will exist Wed morning from about 03N to 13N between 94W and 110W as a result of long period northwest swell mixing with shorter period northeast swell that propagate downstream from the Gulf of Tehuantepec and to a smaller extent those induced by the Gulf of Papagayo strong pulsing northeast winds. $$ Aguirre