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Eastern North Pacific Tropical Weather Discussion

AXPZ20 KNHC 290238

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
0405 UTC Thu Sep 29 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 0000 UTC surface analysis and satellite imagery through
0200 UTC.


Tropical Storm Ulika is centered near 15.8N 138.9W 1001 mb at
0300 UTC moving N or 360 DEG at 6 kt. Maximum sustained winds
have diminished to 45 kt with gusts to 55 kt. Water vapor imagery
shows a broad upper trough just to the west and northwest of Ulika
with its associated upper level southwest to west winds impinging
on the cyclone. As a result, low-level cloud lines are becoming
exposed in the western semicircle, with the system now under 
a shear regime. Scattered strong convection is seen within 45 nm
of the center in the north quadrant. Scattered moderate to
isolated strong convection is within 90 nm of the center in the
northeast quadrant. The upper trough will continue to weaken
Ulika during the next 48 hours as it turns northward, then
eventually decouples from the middle level circulation, leaving
the low level remnant circulation to turn westward within the 
next 48 hours or shortly afterwards 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 22.1N 115.1W 1007 mb
at 0300 UTC moving N or 360 DEG at 5 kt, or about 304 nm west-
southwest of the southern tip of the Baja California Peninsula.
Maximum sustained winds are 30 kt with gusts to 40 kt. Strong
southwesterly upper level winds continue to create south to
southwest over Roslyn, with the low level center completely
exposed, and convection well removed from the center of the
system, with convection of the scattered moderate type intensity
within 30 nm of 23N115W. Roslyn is forecast to continue to weaken
further to a post-tropical remnant low near 22.8N 115.4W by early
on Thursday as it turns to the north and then to the northwest. It
will dissipate by Friday evening well offshore of the Baja
California peninsula. Refer to the latest NHC forecast/advisory
under AWIPS/WMO headers WTPZ23 KNHC/TCMEP3, or visit the NHC
website at for additional details.


The monsoon trough axis extends from Colombia near 10N74W to
10N85W to 10N94w to 13N100W...where it fractures. It then resumes
from near 13N108W to 09N110W to 14N131W. Scattered moderate to
isolated strong convection is noted to the southwest of the 
first trough segment within 30 nm of a line from 09N102W to



A sub-tropical ridge axis extends in a northwest to southeast
orientation across the offshore zones west of the Baja California
peninsula to near 24N126W. 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 west of 115W tonight,
as weakening Roslyn begins to track to the northwest late tonight,
then west-northwest on Friday. 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 northwest winds, and 5 to 8 ft seas west of the
Baja California peninsula on Sun, and fresh to strong northwest
winds developing Sun night. A surface trough was defined on Ascat 
data from this afternoon, and visible satellite imagery shows that
this feature is part of a broad area of organizing low pressure 
west of central Mexico. Deep convection within this area has
diminished during the evening, however scattered moderate to
isolated strong convection is observed within 60 nm of the trough.
Strong upper-level west to southwest winds are blowing off the
cold tops of the convection, with the residual upper moisture
reaching beyond 250 nm.

Global models depict the trough to continue moving westward over
the next 48 hours. Any development of this trough is expected to
be slow during this time period as upper level winds will be only
marginally favorable. A fetch of fresh to strong east to southeast
winds with seas of 6-8 ft between the trough and the coast of
Mexico is expected to diminish Thursday morning as the broad area
of low pressure and trough pull further away from Mexico.

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


Light to gentle southerly winds are expected n of the monsoon
trough, while gentle to locally moderate south to southwest flow
is expected s of the monsoon trough through the upcoming weekend.
Combined seas of 4 to 6 ft primarily in long-period southwest
swell is expected this week.


As tropical storm Ulika moves west of 140W Thursday evening 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.