Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 261246

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
546 AM PDT Wed Sep 26 2018

.SYNOPSIS...26/247 AM.

Low clouds and fog will shrink and be limited to most coastal
areas over the next few days before expanding back into the
valleys this weekend. A brief but significant warming trend is
expected today and Thursday. A cooling trend is expected late this
week into this weekend as an area of low pressure moves into the


.SHORT TERM (TDY-FRI)...26/545 AM.

Latest fog product imagery indicated plenty of low clouds
affecting coastal areas from around San Luis Obispo to the Orange
County border. Variable high clouds were also appearing on the
satellite imagery and will continue to move around the upper
ridge affecting areas mainly north of Point Conception today. As
expected, the marine layer depth which was over 2200 ft last
evening, has begun to shrink down to around 1200 ft around LAX
this morning. The combination of an upper level ridge building in
from the southwest and weaker onshore flow for today has already
caused the morning stratus to quickly scour out of much of the
L.A. County Valleys this morning. Some random patchy low clouds
could develop across the San Gabriel Valley early this morning,
but would be short lived.

In fact, some light offshore winds have developed early this
morning across the mountains and deserts, extending into some of
the interior valleys. Offshore wind speeds will be relatively
light, but some isolated gusts between 25 and 30 mph in favored
mountain locations will occur today. This offshore component
combined with the upper level ridge of high pressure will result
in a faster burnoff of the low clouds and bring substantial
warming across interior sections. With the weaker offshore trends
over the region today, expect highs to range between 8 to 15
degrees warmer in some L.A. and Ventura County Valleys todays
while coastal areas rise 4 to 10 degrees. Otherwise expect 2 to 6
degrees of warmer for most all other locations today. Highs are
expected to reach into the 90s for many valley, lower mountain
elevations, and the Antelope Valley, with a few degrees of warming
on Thursday as the ridge strengthens in the short term ahead of
an approaching upper level trough. Highs should remain the same
for most areas N of Point Conception and a couple degrees warmer
overall for areas to the south. Low clouds will push in once
again, but should remain to coastal areas, and possibly just sneak
into the San Gabriel and San Fernando Valleys, but not that
confident for that to occur tonight into Thu morning.

A cooling trend will begin Friday, but not quite as much as first
expected. A shortwave ridge will remain in place ahead of the
approaching trough but weaken some. However, onshore gradients
will increase which will bring widespread cooler conditions to
the forecast area. The seabreeze will kick in earlier across the
coast then spread inland into the valleys. Expect 4 to 8 degrees
of cooling for the most part. The marine layer will deepen and low
clouds will make a return to the coastal valleys once again by
Friday night into Sat morning.

Tropical Storm Rosa will likely be upgraded to a hurricane today
and then become a major hurricane by noon Thursday. There will be
the potential that remnants from Rosa could bring some moisture to
the region by early next week. However, it is a complex weather

.LONG TERM (SAT-TUE)...26/332 AM.

Both the GFS and ECMWF are in decent agreement through this
weekend as an upper trough will persist over the forecast area
allowing decent onshore flow, night through morning low clouds and
fog for coast and coastal valleys. Highs should be around normal
this weekend.

As mentioned in the previous long term forecast discussion,
confidence really drops off by Monday as a complex upper level
pattern sets up across the west coast and models are coming up
with quite different solutions. Tropical Storm Rosa is expected to
be upgraded to a hurricane over the next 24 hours. There are
several factors that do come into play next week. With an upper
trough situated off the California Coast, the potential for
remnants of Rosa could get pulled into to Southern California by
Monday and bring a continuous subtropical fetch of moisture across
the forecast area for a few days before the upper trough pushes
through the area. The European Model is trending in that
direction. While the GFS is much quicker pushing through a trough
keeping the bulk of the moisture to the east of the area into SE
Cal and Arizona. Some of the GFS ensembles do show a slight
chance for some rain potential across the SE portion of the
forecast area Mon-Tue. However, solutions continue to vary quite a
bit run to run and there`s a very large spread in the ensembles.
In respect to the models, the spread between the models remains
quite high.

If models trend more to the GFS, there could be some gusty
offshore winds early next week with the placement of what is left
of Rosa, with limited moisture to tap into. Again, will have to
see how the models finally trend.

The best chance for any precipitation here will probably be from
the upper trough approaching from the west but there are a number
of complicating factors with that too, including how much moisture
will be available, timing, and strength. And model solutions will
likely change several times between now and next week. In
following the earlier forecast, have kept small precip chances
in the forecast for next Monday and Tuesday, and added Wednesday
as well. As we get closer to this weekend models, should get a
better handle on the situation, but that being said, models have a
bias of not being very reliable with handling subtropical fetches
from the south. It should be a very interesting week ahead
pinning down the details. Stay tuned.



At 1115Z at KLAX, the marine layer was near 1200 feet deep. The
top of the inversion was around 2800 feet with a temperature of
25 degrees Celsius.

Good confidence in cstl/vly TAFs through 14Z then moderate
confidence 14Z-19Z due to uncertainty in clearing times which
could be off by as much as 2 hours. Good confidence again after
20Z. Good confidence that Vly sites will be clear tonight.

KLAX...Moderate confidence in clearing time with near equal chcs
between 17Z and 19Z. Good confidence in VFR conds aft 19Z. Good
confidence in no east wind component greater than 5 kt. Good
confidence of more low clouds tonight although with more
uncertainty about arrival time also vsbys will likely be lower
than today.

KBUR...Moderate confidence in clearing time with near equal chcs
between 15Z and 17Z. Good confidence in VFR conds aft 17Z. Good
confidence no cigs tonight.


.MARINE...26/158 AM.

For the outer waters, moderate to high confidence in wind and sea
conditions remaining below Small Craft Advisory levels (SCA)
through Friday. There is a 30 percent chance of Small Craft
Advisory winds for the outer waters near Point Conception
this evening, and again Friday afternoon and evening.

No SCA issues expected for the inner waters through at least
Friday, and likely through the weekend.

Areas of dense fog with visibility one nautical mile or less will
likely affect portions of the coastal waters N of Point
Conception, including the nearshore waters north of Point Sal,
this morning.

A large south swell event is becoming more likely starting over
the weekend.





No significant hazards expected.



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