Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 12/09/21
Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
726 PM PST Wed Dec 8 2021
.SYNOPSIS...08/108 PM.
Another storm system will move into the region later tonight
into Thursday with light rain and mountain snow. Precipitation
will taper off during the day with breezy conditions developing
through Friday. Cool daytime highs and chilly overnight lows will
continue through the weekend, then a significant storm system will
bring periods of heavy rain and high elevation snow Monday into
Tuesday.
&&
.SHORT TERM (WED-SAT)...08/713 PM.
***UPDATE***
Latest satellite and surface observations indicate partly to
mostly cloudy skies across the area. Current AMDAR soundings
indicate a deep moist layer around 4000 feet with a weak
inversion. As for winds, there are some west to northwest gusts
25-35 MPH across the mountains and deserts.
Forecast-wise for the immediate short term, focus is on rain.
Overnight, will expect some light, warm-frontal type, rain to
develop across the area, most likely after midnight. The main
front will sweep across the area late Tuesday morning and
afternoon per latest model data. Any rain overnight tonight will
not be problematic for the area. Additionally, will be expecting
some increase in southwesterly winds overnight as the system
approaches. At this time, the strongest southwesterly winds look
to develop across the LA Mountains and Antelope Valley, so the
current WIND ADVISORIES will remain in place.
Overall, current forecast has good handle on timing of rain, POPs
and winds. So, do not anticipate any significant updates this
evening.
***From Previous Discussion***
A weak low pressure system will slide down the California coast
tonight and generate light rain across southwest California into
Thursday. Rain amounts still mostly under a quarter inch but as
much as a half inch in the eastern San Gabriel foothills and
mountains. Overall a very low impact event with showers tapering
off from the northwest by mid to late morning across SLO/Santa
Barbara Counties and late morning to mid afternoon for LA/Ventura
Counties. A few inches of snow are likely at higher elevations,
mostly above 6000`. Lingering showers are possible Thursday
evening across the north facing slopes bordering Kern County as
moisture banks up from the San Joaquin Valley and rides up the
slopes. As the colder air behind the low moves in late Thursday
snow levels will lower rapidly to around 4500`, possibly
resulting in some light accumulations above pass level and maybe
even a dusting of snow down to I5.
Following the rain gusty west to northwest winds will kick up,
especially coastal areas, with gusts to around 30 mph. Then
overnight into Friday winds will shift to more of a northerly
direction and impact the mountains with some wind also getting
down into the LA Valleys and LA Basin, especially the west side
from Sepulveda Pass through west LA and South Bay areas. Advisory
level winds likely in the mountains and windy but not quite as
strong down at lower elevations. On Friday winds will shift to
northeast and weaken with speeds mostly topping out in the 15-25
mph range.
A quiet weather day Saturday after a chilly start. Expecting lows
in the Antelope Valley in the low to mid 20s, coastal valleys
down to the mid to upper 30s, and the LA Basin in the low to mid
40s.
.LONG TERM (SUN-WED)...08/141 PM.
One last quiet weather day Sunday before the next system arrives
Sunday night into Monday. Models and ensembles continue to be in
excellent agreement with this system with overall timing and
strength. Rainfall forecast is unchanged from yesterday, 1-3"
coast/valleys, 3-5" foothills and mountains. South facing slopes
from the Santa Ynez Range to the San Gabriels will see the
highest totals. Would not be surprised to see some final numbers
over 5 when it`s all said and done.
A couple adjustments from yesterday include the likely possibility
of the heaviest band of rain now coming through Monday afternoon
into evening instead of Tuesday morning. Both the 12z GEFS and
ECMWF ensembles are moving that in a little faster than earlier
runs. Peak precipitable water values will be in the 1-1.4" range
which is significant when combined with southerly flow of 30-50kt
going up the coastal slopes and really enhancing an already
dynamic system. One last change from yesterday is the added
convective potential with colder air coming in and CAPE values
rising to well over 1000 j/kg. As a result, Tuesday precip may
end up being a little more variable but with the potential for
some very heavy showers and storms. The one caveat being the
strong west to southwest flow aloft that would likely push storms
along pretty quickly.
If models stay the same it`s very likely that rainfall rates
could exceed burn area debris flow thresholds at times Monday
and/or Tuesday.
Snow levels will start out pretty high with this system Monday but
then fall quickly Tuesday as that colder air arrives. Resort
levels (7000` and above) should do very well with this event with
a couple feet of snow (or more) but accumulations below that will
depend on the timing of the colder air arrival and how much
precip is left by then. So for now a lower confidence forecast for
the snow below 7000`.
We should be drying out Wednesday but still quite cool with some
brisk and breezy northwest winds at times.
&&
.AVIATION...09/0029Z.
At 00Z at KLAX, there was no marine layer.
Low confidence in all TAFs. Low confidence in cig heights during
frontal passage. Timing of cat changes and rain may be off by +/-
2 hours. All TAF sites have a 40 percent chance of returning to
VFR conditions as early as after the rain passes.
KLAX...Low confidence in the 00Z TAF. Timing of cat changes and
rain may be off by +/- 2 hours. There is a 30 percent chance that
cigs may be BKN007-009 during frontal passage. There is a
40 percent chance of returning to VFR conditions as early as
after the rain passes. There is a 40 percent chance of east to
southeast winds around 7 knots from 10-16Z.
KBUR...Low confidence in the 00Z TAF. Timing of cat changes and
rain may be off by +/- 2 hours. There is a 40 percent chance of
cigs staying MVFR during the frontal passage and returning to VFR
conditions as early as after the rain passes.
&&
.MARINE...08/725 PM.
High confidence that NW winds will strengthen across the
northernmost waters later tonight and into the overnight hours
ahead of an incoming storm. Gusty winds will spread across the
waters from north to south early Thursday. High confidence in at
least Small Craft Advisory (SCA) level conditions over all the
waters by mid to late Thursday morning lasting through at least
Friday morning. There is higher confidence (60-70% chance) of
Gales across the outer waters from the Central Coast to San
Nicolas Island developing by Thursday afternoon, so a Gale Warning
has been issued for this portion of the waters. There is low
confidence (30% chance) for Gales over the inner waters south of
Point Conception. These gusty winds will create dangerous short-
period seas over all the waters. Seas over 10 feet are likely
across the outer waters, mainly on Thursday.
&&
.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory in effect until 3 AM PST Friday for zone 54. (See LAXNPWLOX).
Wind Advisory in effect from 1 AM to 9 PM PST Thursday for
zone 59. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect from 4 AM to 9 PM PST Thursday
for zone 645. (See LAXMWWLOX).
Small Craft Advisory in effect from 10 AM Thursday to 3 AM
PST Friday for zones 650-655. (See LAXMWWLOX).
Small Craft Advisory in effect from 1 AM to noon PST Thursday
for zones 670-673. (See LAXMWWLOX).
Gale Warning in effect from noon Thursday to 3 AM PST Friday
for zones 670-673-676. (See LAXMWWLOX).
Small Craft Advisory in effect from 7 AM to noon PST Thursday
for zone 676. (See LAXMWWLOX).
&&
.HAZARD POTENTIAL OUTLOOK (SAT-WED).
A strong storm is expected to bring periods of heavy rain Monday
into Tuesday with possible flooding impacts. At higher elevations
heavy snow accumulations are possible.
&&
$$
PUBLIC...MW
AVIATION...Lund
MARINE...Lund/Sweet
SYNOPSIS...MW
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