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Eastern North Pacific Tropical Weather Discussion

AXPZ20 KNHC 281617

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
1605 UTC Wed Sep 28 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 1200 UTC surface analysis and satellite imagery through
1500 UTC.


Tropical Storm Ulika is centered near 14.5N 138.7W 995 mb at
1500 UTC moving NNE or 030 DEG at 6 kt. The maximum sustained
wind speed has diminished to 60 kt with gusts to 75 kt.
convection has gradually decreased in the past several hours,
with scattered moderate to strong occurring within 60 nm of
thecenter, and scattered moderate elsewhere within 120 nm across
the SE semicircle. Ulika has already begun to weaken earlier
than previously forecast, as an upper level trough to the NW is
imposing W to SW upper level wind shear, and also advecting dry
and stable middle level air into the W and SW portions of the
core circulation. This upper trough will continue to weaken
Ulika during the next few days as it turns northward, then
eventually decouples from the middle level circulation, leaving
the low level remnant circulation to turn westward within 3 days
as it remains embedded in the trade wind flow. Refer to the
latest NHC forecast/ advisory under AWIPS/WMO headers
WTPZ24KNHC/TCMEP4, or visit the NHC website at for additional details. After the center
moves w of 140W, refer to the latest CPHC forecast/advisory
under AWIPS/WMO headers WTPA22 PHFO/TCMCP2, or visit the CPHC
website at for additional details.   

Tropical Depression Roslyn is centered near 21.0N 115.2W 1007 mb
at 1500 UTC moving NE or 035 DEG at 9 kt. This is about 315 nm
wsw of the southern tip of the Baja California Peninsula.
Maximum sustained wind speed has diminished to 30 kt with gusts
to 40 kt. Strong southwesterly upper level winds continue
toproduce wind shear across Roslyn, with the low level center
completely exposed, and convection occurring around the
periphery of the circulation. Currently, scattered moderate to
strong convection is occurring from 45 to 180 nm across the N
quadrant, while scattered light to moderate convection is
elsewhere from 45 to 150 nm across the N semicircle. Roslyn is
forecast to continue to weaken further during the next few
daysas it turns N and then NW and remains well offshore of Baja
California. The remnant circulation is expected to gradually
dissipate in about 3 days. Refer to the latest NHC
forecast/advisory under AWIPS/WMO headers WTPZ23 KNHC/TCMEP3, or
visit the NHC website at for additional


The monsoon trough extends from Colombia near 10N74W to 10N93.5W
to 11.5N107W to 07.5N119W...where it breaks...then resumes from
near 13N117W to 14N135W. Isolated moderate convection is noted
from 05N to 08N between 85W and 99W. Scattered moderate to
strong convection is noted from 06N to 09.5N between 111W and
118W. Scattered moderate to strong convection is also observed
within 240 nm of the Pacific coastline from 90W to the Mexican
near 23N106W.



A sub-tropical ridge axis extends from nw to se across the
offshore zones w of the Baja Peninsula to near 24N124W.
Light to gentle n-ne winds are observed w of the Baja Peninsula.
Fresh to strong e to se winds,and combined seas of 7 to 11 ft
will expand n across the far offshore waters, generally from 11N
to 24N w of 115W today and tonight, as the weakening Roslyn
continues northward, then nnw later tonight. The subtropical
ridge will rebuild from 29N120W to 22N106W on Fri with gentle n
flow returning, except becoming moderate along the Baja coast
during the overnight hours. The pressure gradient will tighten
this weekend, with moderate to fresh nw winds, and 5 to 8 ft
seas, w of Baja on Sun, and fresh to strong nw winds developing
Sun night.

Light to gentle nw winds are expected across the entire Gulf of
California late today, with little change expected through Thu.
Light and variable winds expected across the far northern
gulf waters beginning on Thu night.

Fresh drainage flow is expected across the Gulf of Tehuantepec
tonight, increasing to fresh to strong flow on Thu, and then
pulsing overnight through the upcoming weekend. 


Light to gentle southerly winds are expected n of the monsoon
trough, while gentle to locally moderate s to sw flow is
expected s of the monsoon trough through the upcoming weekend.
Combined seas of 3 to 6 ft primarily in long-period sw swell is
expected this week.


As tropical cyclone Ulika moves w of 140W and Roslyn dissipates,
the pressure gradient will tighten around the subtropical ridge,
with fresh anticyclonic winds expected between the monsoon
trough/ITCZ and the ridge by Mon, with combined seas of 6 to 7
ft. The gradient will then relax some by the middle of next week.