Scientific Forecaster Discussion
fxus66 klox 231150
Area forecast discussion
National Weather Service Los Angeles/Oxnard California
450 am PDT Wed Oct 23 2019
Hot and dry conditions will continue across most of the region
for the rest of the week, accompanied by weak to locally moderate
Santa Ana winds over portions of Ventura and Los Angeles
Wednesday. Santa Ana winds will strengthen Thursday into Friday
leading to critical fire weather conditions in many areas.
Temperatures will start cooling down Saturday.
Short term (tdy-fri)...23/355 am.
Offshore gradients continued across the region this morning,
with gradients between klax and kdag slightly weaker than at this
time on Tue, and gradients between ksba/kbfl and klax/kbfl nearly
the same as they were 24 hour ago. Some gusty winds continued
across the mountains and valleys of l.A. And vtu counties this
morning, and these may become a bit more widespread around
sunrise, but winds will remain well below advisory levels today.
Skies were clear across the region this morning, and clouds should
not be a factor in the forecast through Friday. The upper high
that has been nosing into the region will shift southwestward
today as a large and broad trough drops into The Rockies. Heights
and thicknesses will drop a bit across the region today, as will
temps at 950 mb. Profiler data already shows very slight cooling
in the lowest 1500 feet during the past 24 hours, but only about 1
degree c or so. Pressure gradients are forecast to become neutral
this afternoon, which should allow for earlier and stronger
seabreezes today. With all this in mind, expect a few degrees of
cooling today, especially across the coastal plain. However, it
will still be hot, with highs likely rising above 90 degrees in
most locations in the valleys and across interior sections of the
The trough will sharpen as it digs into the southern rockies and
into The Four Corners area tonight and Thursday. The upper high in
the eastern Pacific will amplify just off the West Coast. This
will set up a northerly flow pattern aloft across California.
At the surface, low level gradients will turn sharply offshore
tonight and Thu morning. The ec and WRF continue to show stronger
offshore gradients than does that GFS, but the ec has been very
consistent, so have placed a bit more Faith in its solution.
Along with the increasing offshore gradients, northeast winds at
850 mb increasing to about 55 knots across portions of l.A. And
vtu counties Thu morning, and there is a broad area of fairly
strong subsidence. The only ingredient that is not completely
ideal is that there is a lack of good cold advection, and for this
reason, have refrained from upgrading the high wind watches to
warnings at this point. In the end, all the other factors may
end making up for the lack of cold advection.
Gradients will remain strongly offshore Thu night and Fri, but
upper support should weaken. It appears that the most likely time
period for High Wind Warning level winds would be Thu into Thu
evening for areas under the high wind watch, but gusty, at least
advisory level winds will linger through late Fri morning or early
Have issued wind advisories for coastal sections of vtu County
and for coastal sections of l.A. County (for areas mainly from
Malibu to the Hollywood hills) for very late tonight through early
Fri afternoon, with the strongest winds expected Thu. Winds could
actually get close to High Wind Warning levels across portions of
the Ventura County coastal plain on Thu.
There will likely be some cooling across the mountains, deserts
and possibly the interior valleys on Thu, while there may be some
warming near the immediate coast. There should be several degrees
of warming in most areas on Fri which should be quite a hot day.
Highs should rise well into the 90s across interior sections of
the coastal plain and in the valleys. Red flag warnings are in
effect from late tonight through Friday evening for most of l.A.
And vtu counties for the moderate to strong Santa Ana winds and
very low relative humidities.
Long term (sat-tue)...23/447 am.
An upper trough will drop into the northern rockies on Sat,
forcing the upper high over the region to the south and west into
the Pacific. Offshore gradients will weaken quite a bit Sat
morning, but there may still be some locally gusty NE winds,
although well below advisory levels. Gradients may actually turn
onshore during the afternoon. There should be several degrees of
cooling, though temps should still be well above normal, and highs
in the hottest valley locations may still reach 90 degrees.
Beyond Sat, there are major differences in the models, and forecast
confidence becomes much lower. The GFS is much more progressive,
showing the trough dropping through The Rockies on Sunday, and
moving into the Midwest on Monday and Tue, while an upper high
builds into the West Coast. This would bring some additional
cooling on Sunday, followed by some warming early next week. The
00z operational run of the ec, on the other hand, shows the upper
trough digging sharply to the west across Nevada on Sunday,
evolving into a closed low over central California by early Mon. It is
then forecast to drift into Arizona on Tue. This is a much colder
solution, and even would threaten at least eastern sections of the
forecast area with some showers on Mon. With this feature, the ec
has not be consistent. Therefore, have kept the current forecast
as is, leaning towards the GFS next week given its better
consistency. However, confidence is low.
At 09z, there was no marine layer at klax.
High confidence in ceiling and visibility unlimited 12z tafs. There will be a 30% chance for low level wind shear
and uddf across airfields across l.A. And Ventura County coast
and valley locations from 10z Thu through at least Fri morning.
Klax...VFR conditions are expected through the 30-hour 12z period.
There is a 30% chance for easterly wind component to exceed 8
knots through 16z this morning.
Kbur...VFR conditions are expected through the period at this
time. There is a 20 percent chance of moderate wind shear and
turbulence starting 10z Thu.
Outer waters...winds will remain below Small Craft Advisory
thresholds through at least this weekend. There is a 40% chance
that Small Craft Advisory level gusts could occur across the western portion of the
northern outer waters of zones (pzz670-673) this afternoon
through early evening. The exception would be the northern
portion of the southern waters (pzz676) where the potential for
easterly gale force gusts could occur across the northern portion
Thursday morning through mid afternoon hours.
Inner waters north of Point Sal...not anticipating any winds reaching
Small Craft Advisory levels through this weekend.
Inner waters S of Point Conception...winds will remain below Wind
Advisory levels through late Wednesday evening. A moderately
strong Santa Ana wind event will develop pre dawn on Thursday.
Northeast winds could strengthen quickly to gale force winds as
early as 8 am between Ventura and Malibu and out to The Channel
islands. A gale watch has been issued early Thursday morning
through the mid afternoon hours. Gusts to 35 knots will be
possible in areas mentioned. Otherwise expect moderate to strong
Small Craft Advisory level winds across the San Pedro Channel to Catalina Island.
Mariners should be prepared for choppy seas across east facing
harbors including Avalon Harbor Thursday. There is a 60% chance
that Small Craft Advisory level gusts will continue through Friday morning for the
Fire weather...23/449 am.
Through this afternoon, elevated to brief critical fire weather
conditions will persist with very warm and dry air and fuels.
Breezy offshore winds with gusts between 15 and 30 mph will
continue over the wind prone areas.
Tonight through Friday, a moderate to strong Santa Ana wind event
will bring widespread critical conditions to Los Angeles and
Ventura counties, and elevated to brief critical conditions for
Santa Barbara and San Luis Obispo counties. The previous Fire
Weather Watch has been converted to a red flag warning for most of
Los Angeles and Ventura counties. There is still some uncertainty
in the strength of the winds, but peak gusts between 45 and 65
mph is the most likely outcome. Humidities will lower to 2 to 9
percent and temperatures will remain well above normal with highs
generally in the 90s. This has all the makings for a dangerous
fire weather scenario, similar to or worse than the recent October
10-11 event that produced the saddleridge fire.
The fuels and vegetation are critically dry. The expected weather
will create an environment ripe for large and dangerous fire
growth, especially Thursday and Friday. We urge everyone to be
extremely cautious with any potential fire ignition sources. Fires
have started from things like cigarettes, Camp fires, welding or
brush clearing equipment, and dragging towing chains.
California...Wind Advisory in effect from 5 am Thursday to 3 PM PDT Friday
for zones 40-41. (See laxnpwlox).
High wind watch in effect from late tonight through Friday
morning for zones 44>46-53-54-88-547. (See laxnpwlox).
Red flag warning in effect from 1 am Thursday to 10 PM PDT
Friday for zones 240-241-244>246-253-254-288-547-548. (See laxrfwlox).
Pz...gale watch in effect from late tonight through Thursday
afternoon for zones 650-655-676. (See laxmwwlox).
Hazard potential outlook (fri-tue).
Hot and dry conditions along with strong Santa Ana winds are
expected Friday, especially la/Ventura counties. Fire weather
conditions will be critical to extreme.