Area Forecast Discussion
Issued by NWS Los Angeles, CA

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815
FXUS66 KLOX 230434
AFDLOX

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
934 PM PDT Tue Oct 22 2019

.SYNOPSIS...22/841 PM.

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 increased fire weather conditions. Temperatures will
start cooling down Saturday.

&&

.SHORT TERM (TNT-FRI)...22/933 PM.

Offshore flow remains in place across the region beneath an
upper-level ridge of high pressure near 35N and 130W. Some cooling
will develop on Wednesday as offshore pressure gradients are
weaker and 950 mb temperatures cool slightly. While NBM/GFS/ECMWF
solutions are taking a cooler approach for Wednesday, with today`s
high temperatures, 950 mb temperatures cooling by a degree, and
offshore flow remaining in place, it is hard to argue against the
latest NAM-WRF solution in keeping the thermal trough off the
coast. With almost a ten degree difference of bias corrected NAM
MOS to the previously mentioned models above, the forecast
confidence in temperatures for Wednesday is moderate at best. It
would not be surprising to see daytime high temperatures closer
to today`s levels. For now, the forecast nudges away from the
short-term guidance, more weighted to the latest NAM MOS
guidance.

Reenforcement will arrive in the offshore flow pattern on
Wednesday night and into Thursday. NAM-WRF solutions continue to
fall inline with the ECMWF solutions in suggesting a strong
event, while the GFS solutions remain on the lighter side point
toward a weak to moderate event. The forecast remains consistent
at this point favoring the NAM-WRF and ECMWF solutions.

***From Previous Discussion***

Offshore flow expected to get another shot in the arm Wednesday
night and Thursday as the next trough swings south out of Canada
and into the northern Rockies and Great Basin. The GFS (including
most of its ensemble members) continues to indicate a light
offshore event at best while the ECMWF and NAM both are suggesting
a strong Santa Ana, possibly the strongest of the season so far
LAX-DAG gradients in excess of -8mb and 50-60kt of wind between
the surface and 850mb. The absence of cold air advection is
concerning and this isn`t a slam dunk event yet, but the ECMWF did
a better job with the last event and it`s been consistent the
last few days with this one, even getting stronger. Because of
this, confidence is high enough now to go with a high wind watch
for the LA/Ventura valleys and mountains, including the Santa
Monica`s but not the San Gabriel Valley. Could make a case for a
watch for the interior portion of the Ventura Coast in the
Camarillo area but it`s rare to get warning level gusts especially
without the cold air advection. But certainly gusts to 50 are a
reasonable expectation and same with the Malibu coast.

This will be another hot event as the eastern Pacific Ridge
expands east following the trough passage with 500mb heights
approaching 590dm. After a little cool down tomorrow look for
highs to jump again Thu and Fri with 90s again commonplace across
coast/valleys. Would not be surprised if heat advisories are again
needed. Could even make a case for an excessive heat warning for
some of the coastal areas which will be 20+ degrees above normal.
Overnight lows will be very warm as well, especially in windy
areas. Case in point, the low this morning in Morro Bay was 73
degrees thanks to a steady northeast wind. Could be looking at
more of the same later this week.

.LONG TERM (SAT-TUE)...22/203 PM.

The ridge will start to break down over the weekend as another
trough approaches from the north. Gradients will also be trending
strongly onshore so look for significant cooling as early as
Saturday and continuing Sunday. May even start to see some marine
layer clouds returning to southern coastal areas by Sunday with
temps back to near normal in most areas.

Forecast confidence drops after Sunday as there are big
differences in handling of the next trough early next week. The
deterministic 12z ECMWF run went back to a sharply cutting upper
trough with almost 50dm of height change from the previous run
over California. The ensemble mean is weaker but still much colder
than the GFS ensemble mean. Still seems too unlikely for rain in
our area to include in the official forecast but not a zero
chance. More likely we`ll see another bump up in offshore winds
early next week but with cooler temps than what we`ve seen.
Current forecast leans towards the GFS next week given the better
consistency with it but definitely low confidence.

&&

.AVIATION...23/0037Z.

At 00Z, there was no marine layer at KLAX.

High confidence in the current forecast. VFR conditions are
expected at all terminals. There is a slight chance to chance of
moderate wind shear and turbulence at terminals south of Point
Conception through 04Z.

KLAX...VFR conditions are expected through the period at this
time. There is a 30 percent chance of moderate wind shear and
turbulence through 03Z. There is a 10 percent chance of northerly
cross winds greater than 10 knots between 04Z and 08Z. Any east
winds should be less than 7 knots.

KBUR...VFR conditions are expected through the period at this
time. There is a 20 percent chance of moderate wind shear and
turbulence through 03Z.

&&

.MARINE...22/859 PM.

Wind and seas will likely remain below Small Craft Advisory
through Wednesday. There is a 40 percent chance of Small Craft
Advisory (SCA) level winds near Point Conception on Wednesday
afternoon and evening.

After Wednesday night, there is 70 percent chance of SCA level
winds and a 30 percent chance of gales across the southern
California bight from Ventura to Santa Monica. There is a 40
percent chance of SCA level winds in the San Pedro Channel and out
to Catalina Island. The strongest winds will occur in the morning
but could hang through mid afternoon, or possibly redevelop
Thursday evening. Otherwise, conditions will remain below SCA
levels through the forecast period.

&&

.FIRE WEATHER...22/150 PM.

Through Wednesday 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.

Wednesday night 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.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...High Wind Watch in effect from late Wednesday night 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...NONE.

&&

.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.

&&

$$

PUBLIC...Hall/MW
AVIATION...Hall
MARINE...Hall
FIRE...Kittell
SYNOPSIS...Phillips/Delerme

weather.gov/losangeles



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