AXPZ20 KNHC 172135

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
2137 UTC Tue Oct 17 2017

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 1800 UTC surface analysis and satellite imagery through 2100 UTC.


Gulf of Tehuantepec Gale Warning: Gale force winds are present across the Tehuantepec area as clearly depicted by recent ASCAT scatterometer passes. These gale force winds are supported by a tight pressure gradient between high pressure across the eastern slopes of the Sierra Madre Mountains in Mexico, and across the western Gulf of Mexico in the wake of a stationary front which extends from N to S over the central Bay of Campeche. Seas are in the range of 10 to 15 ft across the area of gale force winds with seas of 8 ft or greater spilling out to areas downwind of the gale. Global model guidance indicates that the high pressure will weaken through Thursday, with the corresponding tight gradient slackening. This will allow for the gale force winds to eventually diminish to below gale force just after sunrise on Thursday, however, northerly winds of 20 to 30 kt will continue through Friday along with seas of 8 to 10 ft. Looking ahead, another gale force wind event is possible early next week.


A tropical wave extends from 04N81W to 09N82W continuing northward offshore of the Caribbean coast of Nicaragua in the SW Caribbean Sea, moving W at 10 to 15 kt. Scattered moderate and isolated strong convection is from 04N to 06N E of 79W to the coast of Colombia. Scattered moderate convection is from 05N to 08N between 79W and 82W. Additional deep convection is in the SW Caribbean westward across Costa Rica and SE Nicaragua.


The monsoon trough axis extends from 09N75W to 08N97W to 10N114W. The intertropical convergence zone axis extends from 10N114W to 08N124W to 12N137W. Scattered moderate and isolated strong convection is from 07N to 10N between 91W and 95W, from 07N to 10N between 105W and 109W, and also within area bounded by 12N123W to 12N114W to 07N114W to 07N127W to 12N123W.



Please see the Special Features section above for information about the ongoing Gulf of Tehuantepec gale event.

A surface ridge extends across the waters off Baja California, with the pressure gradient between it and troughing along the eastern part of Baja California bringing gentle to moderate NW to N winds and seas of 5 to 7 ft. The ridge will remain in place during the next few days with little change in winds and seas over the offshore waters aside from the Gulf of Tehuantepec region as described above. By late Friday afternoon into Friday evening, a weakening cold front and accompanying set of large NW swell will propagate through the waters W of Baja California building seas to the 8 to 15 ft range. Also on Friday evening, NW winds will increase to fresh over the waters adjacent to Baja California Norte.

Mainly gentle to moderate NW to N winds are expected over most of the Gulf of California through early Wednesday, then winds become light and variable through Friday. Seas will be mainly 2 ft or less, except for higher seas of 3 to 5 ft at the entrance to the gulf in SW swell subsiding to 3 to 4 ft Wednesday through Thursday night, and then to 2 to 3 ft on Friday. Seas will build back to 3 to 5 ft near the entrance during the upcoming weekend.


Moderate to fresh offshore winds will develop across the Papagayo region each night through Thursday night, with seas of 5 to 7 ft. Light and variable winds and seas of 4 to 6 ft in SW swell will prevail N of the monsoon trough, while gentle to moderate SW to W winds and seas of 5 to 7 ft in SW swell will prevail S of the trough.

Swells originating from the gale force wind event in the Gulf of Tehuantepec will reach the far western section of the offshore waters of Guatemala and El Salvador, building seas to 8 to 10 ft there today through Wednesday, subsiding to 6 to 8 ft Wednesday evening and to 5 to 6 ft Thursday. Long period SW swell is forecast to reach the area between Ecuador and the Galapagos Islands from Thursday evening into Friday, building seas to 6 to 8 ft on Friday.


Low pressure of 1014 mb near 24N127W continues to rapidly weaken under very strong W to SW upper winds. A surface trough is just to the W of the low extending from near 30N130W to 22N129W. Latest satellite imagery shows the low level center exposed and elongated with no deep convection present. Winds are 20 kt or less with the low, however, some residual seas of 8 to 9 ft in mixed swell persist within 180 nm in the NE semicircle of the low as sampled by recent altimeter passes. This low will dissipate into a trough late tonight with associated seas subsiding to less than 8 ft by then.

A 1010 mb low pressure area is over the W central waters near 15.5N139W. Associated winds are moderate to fresh with some seas to 8 ft mainly NW swell within 180 nm in the NE semicircle of the low. This feature will move W of 140W through tonight with associated conditions E of 140W subsiding to less than 8 ft.

Elsewhere, gentle to moderate winds are noted across the northern forecast waters under the influence of a weak ridge. Seas continue to subside across the S central waters, with an area of 8 to 9 ft seas in mixed swell within an area bounded by 15N112W to 00N110W to 00N140W to 05N140W to 15N112W. The mixed swell is forecast to gradually decay through the end of the week, however a large batch of SW swell is forecast by Wave model guidance to propagate through the far southern waters beginning on Thursday with resultant combined seas building back to 8 to 9 ft in the SW corner and far S central waters through Friday before decaying during the upcoming weekend.

Looking ahead, a cold front is forecast by the global models to reach the far NW corner of the forecast region by Thursday night. A significant swell event will follow the front, with seas building to 10 to 17 ft over the NW waters Thursday night into early Friday before spreading across all the northern waters W of 115W, and across the west-central waters by late Friday night. Seas of 8 ft or greater will reach all the way to 110W by early Sunday then will begin to gradually decay while reaching to 100W thereafter.

$$ Lewitsky