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Eastern North Pacific Tropical Weather Discussion

AXPZ20 KNHC 101059 AAA

Tropical Weather Discussion...Updated
NWS National Hurricane Center Miami FL
1059 UTC Mon Dec 10 2018

Updated ITCZ/Monsoon Trough section for convection

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 0600 UTC surface analysis and satellite imagery through
1030 UTC.


Gulf of Tehuantepec Gale Warning: High pressure is ridging 
southward along the eastern slopes of the Sierra Madre Oriental 
mountains of Mexico. The large pressure difference between the 
high pressure and lower pressure south of Mexico will maintain 
strong gale force winds over the Gulf until Tue night. Seas in 
the gale warning area are forecast to build to rather large 
values, in the 14 to 20 ft range, by late tonight. The strong 
gale force winds will continue through Tue, then diminish to 
minimal gale force Tue night. Winds will then rapidly decrease 
and become light by Wed evening as Strong high pressure over the 
Gulf of Mexico weakens and shifts E. Swell from this event is 
forecast to propagate south and southwest well away from the 
Gulf of Tehuantepec region, with the leading edge of the 8 ft 
seas expected to reach near 09N110W by late Tue night or Wed. 
This area of 8 ft seas will merge with a larger area of 8 ft 
seas approaching from the northwest by late Tue night. Please 
refer to the Eastern Pacific High Seas Forecast under AWIPS/WMO 
headers HSFEP2/FZPN03 KNHC for more specific marine details.


The monsoon trough axis extends from western Colombia near 04N75W
and continues to a 1010 mb low near 04N78W, to 04N85W and to 
05N93W, where latest scatterometer data indicated that the ITCZ 
begins and continues to 09N09W. It resumes at 09N115 to 08130W
and to beyond 07N140W. Numerous moderate to strong convection is 
seen within 150 nm north of the ITCZ between 131W and 135W.
Similar convection is within 90 nm either side of the ITCZ 
between 125W and 131W, and within 60 nm south of the ITCZ 
between 131W and 137W.



A 1026 mb high pressure system is located west of the southern
California/northern Baja California Norte border near 32N124W,
with a ridge extending south to near 24N124W and southeast from 
there to the Revillagigedo Islands. The surface analysis depicts
very strong high pressure of 1044 mb to be centered over the 
Great Basin as well. The pressure gradient between these high 
pressure systems and enhanced troughing across the Gulf of 
California is supporting fresh to strong north winds across 
central and southern sections of the Gulf of California. Seas 
with these winds presently have a maximum of around 6 ft. Beyond 
48 hours, model guidance suggests that the pressure gradient 
will further tighten leading to an increase in the coverage of 
these winds over almost all of the Gulf of California within 60 
nm of the Baja Peninsula on Thu and Thu night.

Latest satellite-derived sea height data reveals seas of 5 to 7
ft west of the Baja California Peninsula, however, wave model
guidance indicates that a new set of long-period northwest swell
producing much larger seas, in the range of 8 to 10 ft, will
reach the waters off the northern Baja California coast and near 
Guadalupe Island beginning tonight, then spread east to cover 
most of the waters off the entire Baja California peninsula and 
the Revillagigedo Islands through Tue night, before subsiding 
below 8 ft through Wed evening.


Gulf of Papagayo...Winds will pulse to strong speeds during the 
overnight and early morning hours over the Gulf of Papagayo 
tonight and Tue night, then to mainly fresh speeds Wed night 
as high pressure over the Gulf of Mexico builds southeastward 
toward the NW Caribbean Sea and Central America.

Elsewhere, gentle to moderate winds will persist on either side 
of a weakening monsoon trough and seas in the range of 4 to 6 ft.


A cold front is analyzed along a position from near 32N129W to 
26N140W at 06Z this morning. The front has slowed down, which 
has allowed long-period northwest swell generated by winds 
behind the front to propagate out ahead of the front. This will 
bring seas in excess of 8 ft to most of the area west of 120W 
through early Tue. This will reinforce the decaying northwest
well lingering south of 20N which are mixed with shorter period 
trade wind swell. A tightening pressure gradient resulting from 
strong high pressure that will build east-southeast behind the 
front will support mainly fresh northeast trade winds from 10N 
to 20N west of about 125W through Wed, with fresh to strong winds
west of 133W.

Wave model guidance suggests that a rather large area of 8 ft 
seas associated with mixed swell will stretch from about 03N to 
13N between 94W and 110W as a result of long-period northwest
swell mixing with shorter period northeast swell that propagate 
downstream from the Gulf of Tehuantepec and to a smaller extent 
those induced by the Gulf of Papagayo strong nocturnal pulsing 
northeast winds. This area of swell will begin to merge with an 
even larger area of seas associated with northwest swell which 
will cover almost all of the forecast area west of around 92W by 

A surface trough along a position from near 14N111W to 09N112W at
06Z is moving quickly westward. The trough is positioned to the 
east of a very well pronounced upper-level trough as seen in 
water vapor. The satellite imagery shows scattered moderate 
convection displaced to the east of the surface trough within 60 
nm of a line from 13N105W to 13N109W and to 13N111W. The upper-
level trough will amplify southward during the next couple of 
days, while at the same time the surface trough gradually loses 
its identity as it continues in a westward motion.