Eastern North Pacific Tropical Weather Discussion (Text)

AXPZ20 KNHC 171602

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
1605 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 1200 UTC surface analysis and satellite imagery through
1530 UTC.


Gulf of Tehuantepec Gale Warning: October marks the beginning of
the Tehuantepec wind events. The first gale-force event of a 
season typically occurs in mid-October. Currently, gale force 
winds of 30 to 40 kt are present across the Tehuantepec area 
as inferred from the 12 UTC isobaric analysis, model data and 
an ASCAT pass from last night. The analysis suggests that 
the 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 cold front over the 
central Bay of Campeche with lower pressures along the coast 
of Mexico and S of Mexico supports these gale force gap winds.
Seas have build to the range of 10-15 ft with these winds.
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 20-30 kt will continue 
through Friday along with seas of 8-10 ft.


The monsoon trough axis extends from along the coast of NW
Colombia to 09N79W to 09N88W to 08N100W to 09N112W where
scatterometer data indicates the ITCZ then begins and continues
to 09N124W to 13N138W. Scattered moderate to strong convection 
is within 180 nm N of the axis between 113W-118W, and within 120
nm N of the axis between 118W-121. Scattered moderate isolated 
strong convection is S of the axis between 80W-83W. Scattered 
moderate convection is within 30 nm of the axis between 92W-94W, 
and between 105W-108W.



Please see 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-N winds and seas of 
5-6 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 
11-14 ft range. Also on Friday evening, NW winds will increase 
to 20-25 kt over the waters adjacent to and within 180 nm W 
of Baja California Norte.

Mainly gentle to moderate NW-N winds are expected over most of 
the Gulf of California through early Wednesday, then winds 
become light and variable through Friday, with seas of 2 ft or
less except for higher seas of 4-5 ft at the entrance to the 
gulf in SW swell subsiding to 3-4 ft Wednesday through Thursday 
night and to 2-3 ft on Friday.


Moderate to fresh offshore winds will develop across the Papagayo
region each night through Thursday night, with seas of 6-7 ft. 
Light and variable winds and seas of 4-6 ft in SW swell will 
prevail N of the monsoon trough, while gentle to moderate SW to W
winds and seas of 5-7 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 
8-10 ft there today through Wednesday and subsiding to 6-8 ft 
Wednesday evening and to 5-6 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 7 to 
8 ft on Friday.


Low pressure of 1013 mb near 23N127W continues to rapidly weaken
under very strong W to SW upper winds. Latest satellite imagery 
shows that the only remaining convective activity with the low is
well removed 180-240 nm to the NE of the low, and is quickly 
dissipating. Previous strong winds with this low have diminished 
to moderate to fresh winds within 150 nm NE of the low, with seas
of 8-10 ft in mixed swell based on latest available altimeter 
data. The low will degenerate into a trough near 128W from
21N-28N late tonight into Wednesday.

A 1010 mb low pressure embedded in the ITCZ region near 15N138W 
contains winds of 20-25 kt with seas of 8 to 9 ft within 60 nm of 
it in the NE semicircle. Seas are 8-9 ft in mixed swell elsewhere
from 14N-19N W of 134W. Model guidance indicates that this low 
will weaken to a trough just to the W of 140W by early on 
Wednesday. Seas to 8 ft in mixed swell are expected by that time 
from 15N-20N W of 134W. This mix swell will then decay late 
Wednesday night allowing for these seas to subside to 5-7 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 south-central waters, and
currently seas of 8-9 ft are noted S of 20N between 115W-128W,
also S of 15N between 105W-115W and S of 15N W of 128W. The
culprit mixed SE and SW 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 of 8-9 ft.

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 10-17 ft over the NW waters by early Friday before
spreading across all the northern waters W of 115W, and across 
the west-central waters by late Friday night.


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Page last modified: Tuesday, 17-Oct-2017 16:02:51 UTC