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Eastern North Pacific Tropical Weather Discussion

AXPZ20 KNHC 252157

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
2157 UTC Sun Sep 25 2016

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.


Newly formed Tropical Depression Eighteen-E is near 16.1N 119.6W
at 25/2100Z or about 680 nm southwest of the southern tip of the
Baja California peninsula, moving north at 5 kt. Estimated minimum
central pressure is 1005 mb. Maximum sustained wind speed is 30 kt
with gusts 40 kt. Numerous moderate showers and thunderstorms are
active within 210 nm of the southeast semicircle of the center of
T.D. Eighteen-E. The depression is forecast to intensify to
tropical storm strength tonight and move within 180 nm to the west
of Clarion Island Monday night before weakening. See latest NHC
forecast/advisory under AWIPS/WMO headers WTPZ23 KNHC/TCMEP3 or
visit the NHC website at for more details .


A tropical wave is located to the north of 08N along 96W/97W
drifting west. This likely enhanced a large area of late night and
early morning showers and thunderstorms in the Gulf of Tehuantepec
that have since diminished. 

Another area of showers and thunderstorms has persisted over
Panama and Costa Rica and their adjacent waters over the past
couple of days, propagating slowly westward. Specifically,
scattered moderate to isolated strong convection is noted north of
04N between 80W and 87W off western Panama and southwest Costa
Rica. This may be evidence of another tropical wave moving from
the southwest Caribbean into the eastern Pacific. Unlike the
previous tropical wave, this tropical wave would be fairly low
latitude and is probably better defined in the lower to mid levels
of the atmosphere than at the surface. The main impact will be
persistent scattered to numerous showers and thunderstorms over
the next couple of days mainly east of 80W.


The monsoon trough extends from 10N86W to 11N97W to 08N107W to
09N113W. It resumes at 13N123W to 1011 mb low pressure near
11N139W to beyond 11N140W. Scattered moderate convection is within
60 nm south of the monsoon trough axis between 92W and 100W and
within 120 nm south of axis between 135W and 140W. 



The pressure gradient between relatively low pressure over
southern Arizona and northwest Sonora compared to strong high
pressure farther north over the Great Basin is contributing to the
potential for fresh eastern winds across the far northern Gulf of
California from late tonight through Monday, locally reaching 25
kt at times. Wave height development will be limited by fetch
however, and winds will diminish by late Monday. Elsewhere winds
and seas will remain fairly limited across the region through mid
week, except in areas beyond 200 nm off Baja California Sur that
will be impacted by increased winds and seas by mid week on the
periphery of the by then weakening T.D. Eighteen-E, tracking
mainly just outside of the offshore waters zones.

Farther south, a cold front is forecast to move across the Gulf
of Mexico waters early in the week. High pressure behind the
front will likely induce the first Gulf of Tehuantepec gap wind
event of the season with fairly weak pulse Tuesday night into
Wednesday morning followed by a strong pulse Wednesday night into
Thursday morning. At this time, marine guidance suggests
northerly winds of 20-25 kt and building seas to 8 ft by early


Mainly light and variable winds are noted north of the monsoon
trough while gentle to locally moderate south to southwest flow
is expected south of monsoon trough the middle of the week. Combined
seas of 3-6 ft, primarily in long-period southwest swell, are
expected through the middle of the week.


An area of low pressure located near 12N138W or about 1100 nm
east-southeast of the Big Island of Hawaii continues to produce
disorganized showers and thunderstorms. Satellite imagery
indicates scattered moderate convection within 120 nm in the
southeast quadrant of the center of the low pressure. Any
development of this disturbance should be slow to occur during the
next couple of days. After that time, environmental conditions
could become more conducive for development. This system is
expected to move west-northwestward during the next couple of
days, and then turn northward as it approaches the Central Pacific
basin. The pressure gradient between this low and the subtropical
ridge to the north is resulting in an area of fresh to strong
northeast from 13N-22N W of 130W. An earlier altimeter pass
provided observations of combined seas of 8 to 12 ft within the
area of the trade winds. This marine conditions will diminish in
areal extent, covering the waters from 13N to 17N between 135W
and 140W by this evening. Seas of 8 to 9 ft will persist from 12N
to 16N west of 138W through early Monday morning, just north of
the low pressure.