Tropical Weather Discussion

axpz20 knhc 151605

Tropical weather discussion
NWS National Hurricane Center Miami FL
1605 UTC Sat Dec 15 2018

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
1545 UTC.

...Special features...

Gulf of Tehuantepec gale warning: the tight pressure gradient
over southeastern Mexico resulting from the combination of strong
high pressure over northern Mexico and lower pressure south of 
Mexico is bringing strong gale force northerly winds over the 
Gulf of Tehuantepec. The observation from ship with call sign 
"vrjt8" indicated strong northeast gale force winds along with 
seas of 16 ft earlier this morning at 12z when it was located 
near 14n96w. First visible satellite images clearly reveal an 
arc cloud line located about 450 nm SSW of the Gulf while 
propagating away from it. This feature Marks the leading edge of 
the strong northerly surge, meaning that N to NE 30 to 45 kt
gale force winds are already funneling through The Isthmus of 
Tehuantepec and across the Gulf. This strong gap wind event will 
continue through Sun night, before diminishing on Mon as the 
strong high pressure over Mexico shifts eastward and weakens. 
Seas of 12 to 18 ft with this event will subside to 9 to 13 ft 
Mon and to less than 8 ft on Tue as the northerly winds diminish 
to fresh speeds. Please refer to the latest High Seas Forecast 
issued under AWIPS/WMO headers hsfep2/fzpn03 for more specific 
marine related details. 

...Intertropical convergence zone/monsoon trough...

The monsoon trough axis extends from northwestern Colombia to 
06n77w to 05n93w, where latest scatterometer data indicated that
it transitions to the ITCZ and continues to 05n108w to 08n126w 
and to beyond 07n140w. Scattered moderate convection is noted 
within 60 nm south of the ITCZ between 126w and 129w.


Offshore waters within 250 nm of Mexico... 

The weak ridge over the area has diminished ahead of a weakening 
cold front that extends from near 32n130w to 25n140w. The lighter 
pressure gradient is maintaining light to gentle breezes across 
the area. Northwest swell with seas to 9 ft persist in the 
offshore waters of Baja California through the revillagigedo 
islands. The front will dissipate this afternoon before reaching
the area, but associated northwest swell will reinforce the 
existing swell through sun. 

A second cold front will approach the region early next week. 
This will bring a new set of northwest swell, with seas in excess
of 12 ft reaching the northern coast of Baja California norte 
and Guadalupe island by early Mon. Swell of 12 to 15 ft will 
cover the baja offshore waters to the revillagigedo islands into 
mid week. Swell of this size and period will present hazardous 
seas for mariners, cause very powerful and dangerous surf along 
the Pacific reefs and beaches, and likely produce areas of modest
coastal flooding and beach erosion.

Over the Gulf of California, strong high pressure north of region
have been supporting moderate to fresh breezes across the basin.
These winds will diminish through this afternoon as the high 
pressure weakens and shifts eastward. 

A mid to upper-level disturbance approaching the area from the 
west may bring a few showers to the central and southern 
portions of the Baja California peninsula and Gulf of California 
through sun.

Offshore waters within 250 nm of Central America, Colombia, and 
within 750 nm of Ecuador... 

Gulf of papagayo...fresh to strong gap winds will pulse starting
tonight and persist into early next week as high pressure builds
north of the area. The Gap winds will diminish late Tue night.
Seas with these winds are expected to be 8 to 9 ft.Fresh winds 
will also pulse across the Gulf of Panama and Gulf of fonseca 
tonight and sun. 

Elsewhere, gentle to moderate winds will persist on either side 
of the monsoon trough through early next week, as seas remain in 
the 4 to 6 ft range.

Remainder of the area... 

A mid to upper-level trough noted on water vapor imagery to be 
north of about 18n and between 115w and 134w is supporting 
scattered moderate convection within 60 nm of a line from 
13.5n120w to 13n126w. The upper feature has also been supporting 
a surface trough, presently analyzed from near 14n130w to 
09n139w. Scatterometer data from overnight last night indicated 
fresh to strong winds on the northern end of the surface from 17n
to 19n between 121w and 128w. Concurrent altimeter data from 
early this morning showed seas to be as high as 12 ft in this 
area, due to the local winds and a component of longer period NW 
swell. NW swell in excess of 8 ft covers most of the area north 
of 06n and west of 115w. 

Farther north, a weakening cold front extending from near 32n130w
to 25n140w will dissipate this afternoon as it moves eastward.
Models highly suggest that a second and stronger cold front 
will approach the northwest section of the area tonight, and 
move eastward across the waters north of 20n through early next 
week. The main impact will be a new round of NW swell, with 
combined seas in excess of of 12 ft to as high as 20 ft 
propagating across most of the region north of 12n and west of 
120w sun through late Mon. 

During the early to middle part of next week, the leading edge 
of this next round of northwest swell will mix with shorter 
period northeast and east swell emerging from The Gap wind areas,
and possibly even southerly swell, to create an area of mixed 
seas south of 12n between 95w and 110w.



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