Area Forecast Discussion
Issued by NWS Los Angeles, CA
FXUS66 KLOX 291118
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
418 AM PDT THU SEP 29 2016
After one more rather warm day across the region today, a cooling trend
will begin Friday and continue through Monday as an upper trough
moves slowly into the West Coast. Night through morning low clouds
will increase west of the mountains through weekend. Warmer
weather is expected Tuesday and Wednesday as the trough moves east
and an upper high begins to build into the area from the south.
Low clouds and areas of dense fog have pushed into the Central
Coast early this morning. The marine layer was very shallow, so
there was rather widespread dense fog in that area. Have issued a
Dense Fog Advisory for the Central Coast and have included the
Santa Ynez Valley since low clouds and dense fog should push into
at least western portions of the valley before daybreak. Elsewhere,
skies were clear and should remain so today.
The weak upper low that brought showers and thunderstorms to the
mountains of L.A. and VTU Counties, the Antelope Valley, and
portions of the L.A. County valleys has lifted northeastward into
Nevada and weakens. A very weak upper low to the west of Baja
will drift into Baja later today. Just about all of the moisture
and instability from this feature should remain south and east of
the forecast area today. There may be a few buildups in the
mountains today, but do not expect any showers. Heights are
actually a bit higher across the region than they were on Wed.
This could actually lead to a degree or two of warming today,
although increasing onshore flow in the afternoon will probably
offset this in the valleys, and may more than offset it across the
coastal plain. So, will go with little change in max temps in the
valleys, mountains and deserts, and slight cooling across the
coastal plain. There should be more significant cooling, however,
on the Central Coast as the marine influence has returned more
A large upper low a few hundred miles west of Vancouver with a
trough extending southward through the Eastern Pacific will drop
southward later today through Friday. It will then move into the
Pac NW Friday night and Sat and the southern end of the
associated trough will push across CA. The WRF shows plenty of
moisture in the boundary layer on the Central Coast and Santa Ynez
Valley tonight, but little in the way of moisture s of Pt
Conception. This would suggest a very similar low cloud pattern
tonight/Fri morning. However, it would not be too much of a
surprise if there were some stratus across coastal sections of
L.A. and VTU Counties as well. With lowering heights, some
increase in onshore flow, and some cooling at 950 mb, expect
a few degrees of cooling in most areas on Fri, although temps
should still approach or exceed 90 degrees in the warmer valley
As the trough axis moves onshore late Fri and Fri night, expect
the marine layer to deepen, and expect more extensive night
through morning low clouds in coastal areas Fri night/Sat morning,
with clouds possibly spreading into the valleys. Across SBA
County, increasing N-S gradients will likely bring some gusty
winds to the Santa Ynez Mountains and to the south coast of
SBA County through and below passes and canyons Fri night, and
skies will likely remain clear there. More significant cooling is
expected across the region on Sat, with max temps down to near or
possibly slightly below normal levels.
Another upper low will drop south southeastward through the Pacific
to a position off the Oregon coast early Sunday morning, with a
trough extending southward off the CA coast, The marine layer
should continue to deepen, with night through morning low clouds
and fog in most coastal and valley areas. Decent N-S gradients
should bring another round of gusty winds to southern SBA County,
and will likely keep skies clear there. Max temps should be down a
few more degrees in most areas on Sunday, with temps below normal
in most areas. The exception may be across coastal sections of SLO
and SBA County due to the north-south gradients.
The upper low will swing across northern CA Sunday and into
Nevada Sunday night. There will probably be a fair amount of night
through morning low clouds in coastal and valley areas Sun night,
although cooling at 850 mb should weaken the inversion, so the
stratus may more patchy in nature, especially n of Pt Conception.
Max temps could be down a bit more on Monday, especially in the
mtns and deserts, and highs could stay in the upper 60s in the
Antelope Valley for the first time there since the end of May. The
upper trough will move eastward Tue and Wed and an upper high will
expand northward into the region. At the surface, there should be
some weak offshore flow. This should bring warmer weather to the
region each day, and skies should remain mostly clear.
At 1030Z, the marine inversion at KLAX was surface-based. The top
of the inversion was 1700 feet with a temperature of 26 degrees
Overall, moderate confidence in 12Z TAFs. This morning, LIFR/VLIFR
conditions persist across the Central Coast with VFR conditions
elsewhere. Expect this stratus/fog to dissipate by mid-morning
with VFR conditions for all sites this afternoon. For tonight,
will go with a persistence forecast although confidence in
LA/Ventura coast remaining clear is relatively low.
KLAX...moderate confidence in 12Z TAF. Moderate to high confidence
in VFR conditions through to this evening. For tonight, low
confidence in VFR forecast (with a 50% chance of IFR conditions
KBUR...high confidence in 12Z TAF. VFR conditions are expected to
continue through the period.
For the Outer Waters, high confidence in current forecast.
Northwest winds will be on the increase through the period. There
is a chance of winds increasing to Small Craft Advisory (SCA)
levels tonight and SCA level winds likely Friday through Monday.
There will also be a 30% chance of Gale Force winds on Saturday.
For the Inner Waters, moderate confidence in current forecast. For
the waters north of Point Sal, northwest winds will be on the
increase through the period with SCA level winds likely each
afternoon and evening from Friday through Monday. For the waters
south of Point Conception, moderate to high confidence in winds
remaining below SCA levels through Friday. However for Saturday
through Monday, there is a good chance of SCA level winds
developing across western sections each afternoon and evening.
Locally dense fog is possible across much of the coastal waters
CA...Dense Fog Advisory in effect until 9 AM PDT this morning For
zones 34>36. (See LAXNPWLOX).