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Eastern North Pacific Tropical Weather Discussion

AXPZ20 KNHC 291633

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
1605 UTC Tue Sep 29 2020

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
1540 UTC. 


Tropical Depression Eighteen-E is centered near 13.3N 108.5W at 
1500 UTC moving W at 9 kt. Estimated minimum central pressure is
1005 mb. Maximum sustained wind speed is 30 kt with gusts to 40 
kt. Scattered moderate to strong convection is noted from 10N to
20N between 103W and 116W. The system is forecast to steadily 
intensify as it moves on a westward to west-northwestward track 
during the next several days. The current forecast has the 
depression becoming a tropical storm this evening just outside of
the offshore waters near 13.8N 110.4W, then continuing WNW, and 
reaching hurricane intensity Wednesday evening. Please read the 
latest NHC Public Advisory at and Forecast/ 
Advisory at for 
more details. 

Gulf of Tehuantepec gale warning: A strong gale force gap wind 
event is expected across the Gulf of Tehuantepec late today 
through Fri night as cold front moves across the Gulf of Mexico. 
The pressure gradient between high pressure building behind the 
front and the eastern Pacific monsoon trough will tighten over 
the area. This will increase winds funneling through the Chivela
Pass into the Gulf of Tehuantepec. Winds will rapidly increase 
to gale force by late this afternoon, reaching 30-40 kt by 
tonight. At that time, seas will build to 15-18 ft. Swell 
generated from this event are forecast to reach 100W on Wed. 
Please read the latest NWS High Seas Forecast issued by the 
National Hurricane Center at website for more details.

Of note, the 20-year climatology of the Tehuantepec gap wind
events indicates that on average 16 gale-force events and 5 
storm force-events occur in the Gulf of Tehuantepec each cold 
season. Most gale-force events occurred during November and 
December, while most storm-force winds occurred in January. 
Based on latest model trends and the present evolving synoptic 
scale setup, this forecast event most likely will supersede the 
previously recorded earliest start date of October 3rd.


The monsoon trough extends from 10N74W to 10N84W to T.D.
EIGHTEEN-E NEAR 13.3N108.5W to 09N140W. Scattered moderate 
isolated strong convection is noted from 03N to 11N between 78W 
and 96W and within 120 nm S of trough between 127W and 140W. 


Please see above for more on the Gulf of Tehuantepec gale force
gap wind event.

Elsewhere, a weak ridge west of Baja California will maintain 
gentle to moderate NW to N winds across the region for the next 
few days. Gentle to moderate northerly winds will prevail across
the northern Gulf of California through tonight while mainly 
light to gentle winds are expected elsewhere across the Gulf most
of the forecast period. Elsewhere, light to gentle winds are 
generally expected across the waters from Las Tres Marias Islands
to Puerto Angel for the next several days. T.D. Eighteen-E is 
expected to become a large hurricane. The size of the system will
generate a large area of seas greater than 8 ft spreading from
swell out and away the cyclone center. This south swell will 
spread across the offshore waters west of the Baja California 
peninsula Thu into the weekend. 


Moderate southerly flow prevails S of the monsoon trough, with 
gentle winds north of the monsoon trough. Combined seas are 
currently 5 to 7 ft in a mix of wind waves and southerly swell. 
Long period SW swell is moving over the waters between Ecuador 
and the Galapagos Islands, with seas to 8 ft. These seas will 
subside below 8 ft tonight. Seas generated by the gap wind event 
in the Tehuantepec region will propagate across the offshore 
waters of Guatemala and El Salvador beginning on Wed morning.


Please see Special Features section above for newly developed

Outside of T.D. Eighteen-E, a weak pressure gradient prevails
with light to gentle winds prevailing N of the monsoon trough,
and gentle to moderate winds S of the monsoon trough. The cross
equatorial swell across the waters S of 15N has started to 
subside, with seas in the 6-8 ft range. These seas will continue 
to subside trough the middle of the week. New NW swell will 
propagate into the NW waters the middle of the week. Seas will 
build to 9 ft over the far NW waters by Wed night. T.D. 
Eighteen-E is expected to become a large hurricane. The size of 
the system will generate a large area of seas greater than 8 ft 
spreading well away from the cyclone center. By Saturday, seas 
greater than 8 ft will cover much of the waters N of 10N and W of