Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 09/28/20

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard CA

 836 PM PDT Sun Sep 27 2020

.SYNOPSIS...27/820 PM.

Gusty offshore flow will help to drive temperatures warmer this
week, with triple digits in the valleys and near 90 degrees along
the coast Tuesday through at least Thursday. Temperatures will
begin to cool by Friday and will drop close to normal in some
locations by next Sunday.

&&

.SHORT TERM (SUN-WED)...27/835 PM.

Upper level ridge of high pressure building over the west
coast will continue to be the main weather feature bringing
another extended heat wave to the region this week. Some light
offshore winds developed today across the interior, especially in
the mountains, which brought some warming and drying to these
areas. This evening, pressure gradients have become weakly
onshore, with LAX-Daggett pressure gradient at +1.1 mb as of 8 pm.
This weak onshore flow in combination with an eddy circulations
off the coast is resulting in low clouds and fog returning to
coastal areas this evening. The 00z Vandenberg sounding showing a
strong marine inversion, currently around 1200 feet deep, while
ACARS date over the LA Basin showing a depth around 1500 feet. The
marine layer depth is expected to shrink some overnight due to
rising heights and pressure gradients trending offshore. This
could result in some patchy dense fog, especially along the
Central Coast.

Latest 00z NAM model still showing pressure gradients trending
offshore quickly late tonight, then peaking on Monday morning
when the LAX-Daggett pressure gradient is forecasted to range
between -3 and -4 mb. The peak of this weak to locally moderate
Santa Ana wind event is expected to occur on Monday morning
through the early afternoon hours. During this time, wind gusts
of 30 to 45 mph are expected across the typical wind prone
mountains and valleys, with the strongest winds focused across
the Los Angeles county mountains. Portions of the Bobcat Fire
(especially interior sections) are expected to see northeast
winds gusting 25 to 35 mph on Monday with humidities lowering
to between 10 and 15 percent.

While the offshore winds will begin to ramp up overnight
across the mountains, it will likely be delayed until after
sunrise on Monday across the lower elevations due to the
persistent marine layer influence. Areas that do see the gusty
Santa Ana winds on Monday will see significant warning and
drying especially in the valleys where temperatures could
locally soar to around 100 degrees. The gusty Santa Ana
winds, low humidities, and very warm temperatures will bring
elevated to brief critical fire weather conditions for all areas
away from the coast, with critical Red Flag Warning conditions
expected in the LA/Ventura county mountains and Santa Clarita
Valley.

*** From previous discussion ***

So to summarize will go with heat advisories for the LA/Ventura
valleys Tue-Thu with triple digit temps expected, especially the
western SF Valley and in the foothills. Will hold off issuing for
the coast and the mountains for now and see how gradients are
shaping up Monday and how the marine layer is responding. There`s
still a chance that inland coastal temps in LA/Ventura Counties
could jump up into the 90s by Tuesday but there`s almost an equal
chance the marine layer will still be around. Current forecast
highs reflect the higher numbers but there`s a big spread in the
possible outcomes.

As far as winds go advisories were previously issued for the
LA/Ventura Mountains and Santa Clarita Valley. Warm Springs
already gusting to 45 mph today though that site tends to be an
outlier. It`s possible low end advisories will be needed for the
Ventura valleys Monday but holding off on that for similar
reasons. The upper support for gusty winds drops off dramatically
Tuesday so even with a moderate offshore gradient winds will be
lighter and more localized.

Tuesday night/Wednesday look pretty similar with the primary
exception being an increase in offshore flow along the Central
Coast resulting in a boost in temps. This could also lead to the
development of some warm Sundowners Tuesday night across srn SB
County. If hot temps develop Tuesday across LA/Ventura County
coastal areas those will likely continue into Wednesday, though
there is a slight onshore trend in gradients by afternoon.

.LONG TERM (THU-SUN)...27/140 PM.

Temps are going to remain above normal through next weekend but
expect to start seeing some cooling as early as Thursday as
onshore flow starts to return earlier in the day. Those trends
will continue into the weekend but a pretty strong ridge will
remain in place until at least Saturday. By Sunday we could be
looking at temps getting back closer to normal but still a tad
above. Might start seeing some marine layer clouds returning to
the coast by next weekend but a little early to say with any
confidence.

&&

.AVIATION...28/0059Z.

At 0030z at KLAX... the inversion was around 1300 ft. The top of
the inversion was around 2000 ft with a temperature of 26 degrees
Celsius.

Overall... Moderate to high confidence in current TAFs.  Low
clouds and fog will likely be widespread in all coastal areas
tonight. Marine layer depth expected to decrease overnight due
to building upper level ridge of high pressure and offshore
pressure trends near the surface. Conditions will be VLIFR/LIFR
on the Central Coast and IFR/LIFR south of Point Conception.
There is a 20 percent chance of LIFR/IFR conditions at KBUR
and KVNY on Monday morning.

KLAX... Moderate confidence in current TAF. IFR conditions likely
tonight into mid morning Monday at KLAX, with +/- 2 hours
arrival and burnoff times from current TAF. There is a 20 percent
chance of LIFR conditions late tonight or early Monday morning.
There is a 40 percent chance of afternoon sea breeze cigs on
Monday afternoon.

 KBUR... Moderate confidence in current TAF.  Developing
offshore flow will likely keep low clouds from reaching KBUR
early Monday morning, however there is a 20 percent chance
of LIFR/IFR cigs.

&&

.MARINE...27/759 PM.

Across the outer waters... Winds will be elevated and gusty today
and there is a sixty percent chance of winds reaching Small Craft
Advisory (SCA) level Wednesday through Friday during the
afternoon and evening hours. Otherwise, conditions will remain
below SCA level.

Across the nearshore waters of the Central Coast... Conditions
will remain below SCA level through the forecast period.

Across the inner waters south of Point Conception... There is a
thirty percent chance of winds reaching SCA level across the
western portion of the Santa Barbara Channel Wednesday through
Friday during the afternoon and evening hours. Otherwise and
elsewhere, conditions will remain below SCA level through the
forecast period.

Patchy fog, locally dense with visibility under one mile, will
continue through Monday. The lowest visibility and most widely
spread conditions will occur in the overnight and morning hours.

&&

.FIRE WEATHER...27/602 PM.

Surface high pressure building over the Great Basin will bring
weak Santa Ana winds to the area tonight and Monday. Northeasterly
wind gusts between 30 and 45 MPH will develop across the mountains
of Ventura and Los Angeles counties late tonight and Monday morning.
At the same time, poor humidity recovery tonight will lead to very
low humidity values (7-15%) on Monday. With high temperatures rising
into the 90s, critical fire weather conditions are expected in these
areas Monday morning through Monday afternoon. Therefore, the
FIRE WEATHER WATCHES have been upgraded to RED FLAG WARNINGS.

Elsewhere across the area, elevated fire weather conditions are
expected late tonight and Monday due to the hot and dry condition
as well as weak offshore winds. A lingering marine layer influence
will moderate conditions along the coastal plain.

For Tuesday through Friday, weak offshore flow is expected during the
night and morning hours with weak onshore flow in the afternoon and
evening hours. Very hot and dry conditions will persist through the
period. Relative humidity away from the coast will drop into the
teens and single digits with limited overnight recovery. So,
elevated fire weather conditions will continue across all areas.
Additionally, the hot and dry conditions will be favorable for
plume dominated fire behavior on existing and new fires.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Heat Advisory in effect from 10 AM Tuesday to 8 PM PDT
      Thursday for zones 44>46-88-547. (See LAXNPWLOX).
     Wind Advisory in effect from 3 AM to 3 PM PDT Monday for
      zones 53-54-88. (See LAXNPWLOX).
     Red Flag Warning in effect from 5 AM to 5 PM PDT Monday for
      zones 253-254-288. (See LAXRFWLOX).
PZ...NONE.

&&

.HAZARD POTENTIAL OUTLOOK (WED-SUN).

Hot temperatures expected through at least Thursday with near
critical fire weather conditions.

&&

$$

PUBLIC...Gomberg/MW
AVIATION...Gomberg
MARINE...Kj
FIRE...RAT
SYNOPSIS...Kj

weather.gov/losangeles