NWS Los Angeles, CA wrote its last area forecast discussion before the first event at Dec 4, 4:51 AM PST, 1 hour 37 minutes earlier.
Area forecast discussion, Dec 4, 4:51 AM PST, 1 hour 37 minutes before
Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
451 AM PST Mon Dec 4 2017
.SYNOPSIS...
Strong offshore winds will develop across most of Ventura and Los
Angeles Counties today and continue through Friday. Winds will be
strongest tonight through Tuesday, and again late Wednesday night
into Thursday, with damaging wind gusts expected in some areas.
Afternoon temperatures will be near normal through Tuesday, except
below normal in the mountains and deserts. Warming is expected
Wednesday, with above normal temperatures in most areas Thursday
through the weekend.
&&
.SHORT TERM...(TDY-WED)
***Strongest and longest duration Santa Ana wind event so far this
season will bring extreme fire danger to Southwest California
this week***
Patchy low clouds have developed across coastal sections of L.A.
County and portions of the San Gabriel Valley early this morning.
Some low clouds were also banking up against the northern slopes
of the VTU County mountains and the interior slopes of the Sierra
Madre mountains in Santa Barbara County. Otherwise, skies were
mostly clear across the region. Surface pressure gradients were
trending sharply offshore, and were now almost 4 mb offshore
between KLAX and KDAG. Any low clouds west of the mountains should
dissipate by mid morning, while clouds on the interior and
northern mountain slopes could linger a bit longer.
A cold upper trough across the Great Basin will dig southeastward
today. At the surface, strong high pressure will expand from
Oregon southeastward into Nevada. Decent subsidence has spread
across the region this morning, and winds aloft have increased.
Gusty north to northeast winds were already affecting the mountains,
and will increase in strength early this morning, with gusts to
60 to 65 mph expected. High Wind Warning are already in effect for
the mtns of L.A. and VTU Counties including the Santa Monica Range.
During the early morning hours, strong winds are expected to
spread into the valleys of L.A. and VTU Counties, coastal sections
of VTU County, and coastal sections of L.A. County below passes
and canyons from Malibu to the Hollywood Hills. Wind gusts to 40
to 50 mph are expected in these areas. Advisory level winds will
continue in most areas through this evening. Warming due to
offshore flow will be offset by the coolness of the air mass, so
expect max temps today to be similar to those on Sunday across the
coastal plain, a bit lower in the valleys, and significantly
lower in the mtns and deserts. Immediate beach areas could have a
degree or two of warming today.
Another piece of shortwave energy will drop southward through CA
later today and early tonight. Behind this system, strong
subsidence will overspread the region tonight, and north to
northeast winds aloft will increase. In addition, there will be a
fair amount of cold air advection, and offshore gradients will
continue to sharpen. The WRF continues to show 10 mb offshore
gradients between KLAX and KDAG Tue morning. Expect winds to
increase across L.A. and VTU Counties tonight, with damaging wind
gusts expected across much of VTU County, the mountains of L.A.
County, and the Santa Clarita and San Fernando Valleys of L.A.
County. Winds will likely gust to between 70 and 80 mph in the
wind prone locations in the mountains, and to between 60 and 65
mph in the VTU County coastal and valley areas, and the Santa
Clarita and San Fernando Valleys. High Wind Warnings are in
effect for these areas from late this evening through Tue afternoon,
though there is a chance that warning level winds in coastal and
valley areas could hold off until after midnight. Advisory level
winds will continue across coastal sections of L.A. County from
Malibu to Hollywood, and in the San Gabriel Valley, especially in
the Hills. Winds will begin to diminish by late Tue afternoon,
dropping below warning levels in coastal and valley areas, but
probably continuing at low end warning levels in the mountains.
In wind sheltered locations, it will become quite cold tonight. A
Hard Freeze Watch is in effect for the interior valleys of SLO
County tonight and early Tue, while a Freeze Watch is in effect
for the Santa Ynez and Cuyama Valleys. There could be some temps
between 33 and 35 degrees on the Central Coast and in the Ojai
Valley tonight. On Tue, there should be a couple of degrees of
warming west of the mountains, especially on the coastal plain,
with little change or some cooling in the mountains and the
Antelope Valley.
The EC and GFS both show an upper high just off the Pac NW Tue
night and Wed. Gradients will remain strongly offshore, but upper
support will weaken some. Wind gusts could still reach 60 mph in
the mtns Tue night and Wed, while advisory level winds are
expected in most coastal and valley areas of L.A. and VTU
Counties. Wind advisories will likely be needed in most of these
areas once the High Wind Warning expires late Tue afternoon.
Tue night will be another very cold night in wind sheltered
locations, especially across SBA and SLO Counties and in the Ojai
Valley. Freezes are likely across much of interior SLO and SBA
Counties, and hard freezes are possible. Frosts or freezes
are possible in the Ojai Valley, and perhaps on the Central
Coast. There should be a several degrees of warming in most areas
west of the mountains Wed, with a few degrees of warming in the
mtns and deserts.
.LONG TERM...(THU-SUN)
The GFS and EC are in good agreement showing a highly amplified
upper ridge along the West Coast Wed night and Thu. Low level
offshore surface pressure gradients are forecast to increase
once again Wed night and early Thu. This should keep warning level
winds in the mountains of L.A. and VTU Counties Wed night into
Thu. Winds may increase once again in coastal and valley areas
of VTU and L.A. Counties late Wed night into Thu, but will
probably stay in the high end of advisory levels. However,
warning level gusts can not be ruled out in these areas.
There will likely be another round of frost and freezes in wind
sheltered areas Wed night across SBA and SLO Counties, and in the
Ojai Valley. Max temps on Thu should be similar to those Wed in
most areas, because rising heights/thicknesses should be offset
by some cold air advection.
A large upper high will linger across the West Coast through the
weekend. Surface pressure gradients will weaken slightly Fri, but
remain moderately offshore, then change little on Sat. Expect
gusty winds to continue across much of VTU and L.A. Counties
through Sat, probably reaching advisory levels in many areas.
There may be a bit of warming on Fri and Sat, with temps several
degrees above normal in most areas. On Sunday, gradients weaken a
bit more, but there still should be locally gusty winds across
portions of L.A. and VTU Counties. Max temps on Sunday should be
about the same as those on Sat in most areas.
&&
.AVIATION...04/1215Z...
At 0530Z, the marine layer was 1700 feet deep at KLAX with a weak
inversion. The top of the weak inversion was around 3100 feet
with a temperate near 11 degrees celsius.
Generally hi confidence in the 12Z TAFs. VFR conditions are
generally expected at the airfields thru Tuesday morning. There
is a 50 percent chance of mvfr cigs across the LA County coast
through 16z including KLAX and KLGB prior to the onset of the
offshore flow condition. Gusty N to NE winds are forecast to
develop this morning at KOXR, KCMA, KBUR, KLGB and KVNY, with
further strengthening expected tonight. There is a chance of LLWS
with moderate UDDFS across airfields in LA and Ventura counties
today. By tonight, LLWS and strong UDDFS will be likely across
these same airfields.
KLAX...Moderate confidence in the 12Z TAF with MVFR conditions
through 16z, then VFR conditions generally expected after that.
NE winds 5 to 10 kt are expected today, before switching to an
afternoon sea breeze by early afternoon. There is a 20 percent
chance of LLWS and moderate UDDFS vcnty KLAX today, increasing to
70 percent chance of LLWS and strong UDDFS by tonight.
KBUR...Generally hi confidence in the 12Z TAF with VFR conditions
expected thru this evening. Gusty N-NE winds expected today, with
further strengthening tonight. There is a 30 percent chance of
LLWS and moderate UDDFS vcnty KBUR today, increasing to 70
percent chance of LLWS and strong UDDFS by tonight through Tue
morning.
&&
.MARINE...04/300 AM.
The large northwest swell has finally subsided to around or just
under 10 feet. A mix of gusty northwest to north winds and seas
just below 10 feet will continue across the outer waters through
today. Small Craft Advisories (SCA) will continue over the
northern portion of the outer waters thru this evening, while
gusty NE winds will move into PZZ676 today through Tuesday night.
Across the inner waters and Southern California Bight, northeast
winds are expected to increase to SCA levels this morning through
this evening with SCA level gusts expected from Ventura to around
Malibu then around San Pedro Channel through this evening, with
winds possibly increasing to Gale Force late this evening through
late Tuesday morning. A Gale Watch has been issued, advertising
gusts to 35 knots from Ventura to Santa Monica. There is a 40%
chance of SCA level gusts over portions of the Central Coast
waters during this time.
Northeast winds 25 to 30 knots will likely affect the San Pedro
Channel out to Catalina Island during this time with short-period
hazardous seas developing. The wind-driven seas could cause
boating hazards for east-facing harbors and inlets, such as Avalon
Harbor and the offshore islands.
The offshore winds will begin to diminish in speed by Tuesday
afternoon, but SCA conditions will likely continue during the
night to morning hours south of Point Conception through at least Thursday
morning.
&&
.FIRE WEATHER...04/400 AM...
Confidence is high for an extended period of moderate to strong
Santa Ana winds for Southwest California through at least Thursday.
This will likely be the strongest and longest duration Santa Ana
wind event so far this season. During this event, northeast winds
gusting between 40 and 55 mph will be common across coastal/valley
areas of LA/Ventura counties and between 50 and 65 mph for the
mountains. However, during the peak of the event (late tonight
through Tuesday), damaging wind gusts of 50 to 65 mph are expected
across coastal/valley areas, with mountain areas potentially
seeing gusts of 70 to 80 mph! Humidities are expected to drop to 8
to 15 percent today, then widespread single digit humidities
Tuesday through Thursday. Humidity recoveries at night will be
very poor across the foothills, mountains, and wind prone areas,
adding to the fire weather threat.
The combination of gusty Santa Ana winds, very low humidities,
and extremely dry fuels will lead to the longest duration of
critical fire weather conditions we have seen so far this season,
with Red Flag Warnings in effect for much of Los Angeles county
and all of Ventura county. Late tonight through Tuesday will be
a very critical time period as strong damaging winds will coincide
with single digit humidities. During this period, there will be a
strong offshore pressure gradient (LAX-Daggett projected to be
around -10 mb), ample upper level wind support, and strong
cold air advection. If fire ignitions occur this week, there will
be the potential for very rapid fire spread, long range spotting,
and extreme fire behavior. Long range computer models are showing
the potential for the Santa Ana winds persisting into Friday or
Saturday, which may require the extension of the Red Flag Warning.
Everyone needs to exercise extreme caution when handling any
potential fire ignition sources...such as campfires, cigarettes,
welding and brush clearing equipment.
&&
.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Frost Advisory in effect until 8 AM PST this morning for zone
36. (See LAXNPWLOX).
Freeze Watch in effect from late tonight through Tuesday
morning for zones 36-38. (See LAXNPWLOX).
Hard Freeze Watch in effect from late tonight through Tuesday
morning for zone 37. (See LAXNPWLOX).
Freeze Warning in effect until 8 AM PST this morning for zone
38. (See LAXNPWLOX).
Wind Advisory in effect until 10 PM PST this evening for
zones 40-44-45-88-547. (See LAXNPWLOX).
High Wind Warning in effect from 10 PM this evening to 3 PM
PST Tuesday for zones 40-44-45-88-547. (See LAXNPWLOX).
Wind Advisory in effect until 3 PM PST Tuesday for zones
41-87. (See LAXNPWLOX).
High Wind Warning in effect until 4 PM PST Thursday for zones
46-53-54. (See LAXNPWLOX).
Red Flag Warning in effect until 6 AM PST early this morning
for zones 239-252. (See LAXRFWLOX).
Red Flag Warning in effect until 6 PM PST Thursday for zones
240-241-244>246-253-254-288-547-548. (See LAXRFWLOX).
PZ...Small Craft Advisory in effect until 10 PM PST this evening
for zones 650-655. (See LAXMWWLOX).
Gale Watch in effect from this evening through Tuesday
morning for zones 650-655. (See LAXMWWLOX).
Small Craft Advisory in effect until 9 PM PST this evening
for zone 670. (See LAXMWWLOX).
Small Craft Advisory in effect until 3 AM PST Wednesday for
zone 676. (See LAXMWWLOX).
&&
.HAZARD POTENTIAL OUTLOOK...(WED-SUN)
Strong Santa Ana winds are expected through at least Thursday or
Friday across Los Angeles and Ventura Counties, along with
critical fire conditions. Additionally, there may be widespread
frost and freeze conditions across interior sections of San Luis
Obispo and Santa Barbara Counties, the Ojai Valley, and possibly
the Central Coast.
&&
PUBLIC...DB
AVIATION...Kaplan
MARINE...Kaplan
FIRE...Gomberg/DB
SYNOPSIS...DB
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