NWS Los Angeles, CA wrote its last area forecast discussion before the first event at Jan 20, 9:21 AM PST, 32 minutes earlier.
Area forecast discussion, Jan 20, 9:21 AM PST, 32 minutes before
Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
921 AM PST Mon Jan 20 2025
.SYNOPSIS...20/413 AM.
Santa Ana winds will arrive this morning and clear out the low
clouds. A moderate to strong Santa Ana wind event will affect
Ventura and Los Angeles Counties today and Tuesday, with weaker
offshore flow Wednesday and then another bump up in winds
Thursday. Temperatures will be below normal today and Tuesday,
with a warming trend later in the week. Humidity levels will be
low to very low. Frost or freezing temperatures are likely in
wind sheltered areas each night today through Thursday. There is
a developing chance of rain next weekend.
&&
.SHORT TERM (TDY-WED)...20/920 AM.
***HIGH WIND WARNINGS ARE IN EFFECT THIS MORNING THROUGH EARLY
TUESDAY AFTERNOON FOR MOST OF VENTURA COUNTY AND LOS ANGELES
COUNTY DUE TO LIKELIHOOD WIDESPREAD DAMAGING WINDS***
***DANGEROUS FIRE WEATHER CONDITIONS EXPECTED FOR MUCH OF VENTURA
AND LOS ANGELES COUNTIES THIS MORNING THROUGH TUESDAY EVENING
POSSIBLY CONTINUING THROUGH THURSDAY***
***UPDATE***
A 2500 ft deep marine layer brought low clouds to all of the
csts/vlys early this morning. Just before dawn, however, the upper
level flow turned to the NE and some subsidence brought those
winds down to the sfc (The sfc grad in the W/E direction was
neutral and did not contribute) and quickly pushed the low clouds
out to sea.
The E/W offshore flow has rapidly increased over the last 4 hours
from 0 to 3.8 mb. The N/S offshore flow has also increased from
2.3 to 3.7 mb. Since the offshore push is nearly equal in both
directions this wind event will be a classic NE Santa Ana with the
axis of strongest winds located from the mtns NE of the Santa
Clarita Vly to the beach at the VTA/LA county line. At 850 mb
there are 40 to 50 kt winds out of the NE. Currently, these winds
aloft have combined with the offshore flow to produce 50 to 65 mph
winds in the mtns. There are the marine inversion is preventing
the gusts to reach the csts/vlys but it will soon mix out and the
gusty winds will overspread the csts/vlys.
Still looking for a major increase in winds late this afternoon or
early evening and upper support peaks near 60 kts and the
offshore push from the east leaps to around 9 mb.
Classic Santa Ana temperature pattern today with 4 to 8 locally 10
degrees of warming across the csts/vlys due to the offshore flow,
but 5 to 10 degrees of cooling inland as the NE winds sweep in
much cooler air from the interior. Most cst/vly max temps today
with be a few degrees either side of 65.
No update to the current fcst planned. Will be adding freeze
warnings and frost advisories to the package this afternoon for
the very cold wind sheltered inland areas.
***From Previous Discussion***
The latest in a seemingly endless series of Santa Ana wind events
was beginning to unfold across the region early this morning. A
strong vort max was dropping southward through Nevada into eastern
CA this morning, and this will help to carve a trough just to the
east of the region today. Upper level winds will become northerly
and increase sharply this morning.
Winds at 850 mb and 700 mb winds will increase to between 50 and
60 knots later today, then may become a bit stronger early
tonight. Temperatures at 850 mb will drop from 4 C to 0 C across
the Antelope Valley today, then down to -5 C tonight. This will
set up a very sharp thermal gradient over the San Gabriel
Mountains, with a 15 degree C temperature difference in 25 miles
or so by late tonight. Offshore pressure gradients will increase
during the day today (going against typical diurnal trends) into
tonight, peaking in the -8 to -10 mb range by early Tue morning.
Winds are expected to increase quickly in the mountains of L.A.
and VTU Counties before sunrise, then overspread the valleys
around daybreak, and the push across coastal plain later this
morning. Winds are expected to peak this afternoon or early
tonight, then remain strong overnight and early Tue, Winds will
slowly diminish Tue afternoon. Max gusts across many wind prone
coasts and valleys will average 50 to 70 mph. The highest gusts
will likely be focused near Santa Clarita Valley and near the
Highway 118/126 corridors, affecting cities including Simi Valley,
Moorpark, Porter Ranch, and Santa Paula.
In addition, with the strong upper level wind support extending
well up to 700 mb level, there is potential for locally
strong/damaging mountain wave wind activity across the foothills
of the San Gabriel and San Fernando Valleys, mainly focused near
the Highway 210/118 corridors from Porter Ranch to San Fernando,
eastward to La Crescenta/Altadena, extending to Azusa/Glendora
and Monrovia. The highest probability of any stronger mountain
wave wind activity would most likely be from 2 pm this
afternoon to 2 am Tuesday.
In the mountains, max gusts will likely average 60 to 80 mph,
with isolated gusts in the 80 to 100 mph range possible in the
most favored mountain locations (most likely in the San Gabriels
and Santa Susanas).
While offshore winds will likely peak in many areas late
this afternoon into tonight when there is the strongest upper
level wind support. However, strong offshore winds will likely
continue in many areas into Tuesday morning due to the strong
offshore pressure gradients and continued cold air advection.
While the offshore winds will likely diminish some by Tuesday
afternoon, there will likely continue to be a long duration of
critical Red Flag fire weather conditions across much of
LA/Ventura counties potentially persisting into Thursday due to
the long duration of offshore winds combining with widespread
single digit humidities. In fact, this will likely be the driest
air mass we have seen with all offshore wind events this season,
with many areas seeing humidities fall to between 2 and 5 percent
Tuesday through Thursday.
These strong offshore winds will bring hazardous winds and
dangerous wind waves to Catalina Island, Avalon Harbor as well
as the Channel Islands. For all of the details on the winds and
marine hazards, please see the LAXNPWLOX and LAXCFWLOX products.
Like most Santa Anas, there will be some cooling today across the
interior due to cool air advection while the coasts and vlys see
some warming due to the the compressional warming. Most areas are
expected to warm on Tuesday.
Gusty NE winds will likely continue Tue night into Wed, and Wind
Advisories may be needed for some areas. Rising heights and
warming at 850/950 mb should lead to warming in most areas Wed,
with max temps approaching 80 degrees in the warmest locations.
With clear skies and very low humidity values, it will become very
cold in wind sheltered areas tonight and especially Tue night.
Lows will drop to between 10 and 15 degrees in the Antelope
Valley, and into the lower to mid 20s in interior portions of the
SLO valleys and the Cuyama Valley. Freezing temperatures are
possible each night in wind-sheltered portions of coastal SLO and
SBA Counties. Freezes are possible in the colder valleys of L.A.
and VTU County, especially Tue night when there is less wind.
.LONG TERM (THU-SUN)...20/523 AM.
There will be another bump up in offshore gradients Wed night and
Thu. Though upper support will not be significant, gradients will
be steep, with widespread advisory level winds likely in most of
Ventura County, in the mountains of L.A. County including the
Santa Monicas, and in the northern and western L.A. County
Valleys. Winds could even approach High Wind Warning levels
in some areas.
Heights will rise Wed night and Thu as a strong upper level high
builds into CA. Max temps will be up a few more degrees Thu, with
highs likely rising into at least the lower 80s in the warmest
valley and interior coastal sections of L.A. and VTU Counties.
The upper high will break down and move east on Friday. There
will be enough offshore flow in the morning to keep skies free of
low clouds, but high clouds will increase during the day ahead on
an upper low. There will be a big switch to onshore flow in the
afternoon and this will lead to a big 4 to 8 degree cool down.
Things start to get interesting for the weekend. There is not
great mdl or ensemble agreement so exact confidence in low but the
general 48 hour confidence has grown quite a bit. There will be a
compact upper low off the coast and a cold pos tilt retrograding
trof moving out of the Great Basin and into the state. At some
point these two features will merge into one upper low. The exact
phasing, timing and location of this system is still very much up
in the air.
Looking at the entire 48 hour period...There is a 60 to 70 percent
chc of rain sometime during this period. About 40 percent of the
ensembles show rainfall amounts of over half an inch for most
coast and vlys. This could also be a significant mtn snow event
with snow levels at or even under 4000 ft. Once last detail is
that the most likely (40 percent) set up would allow for an
enhanced convective potential with difluent flow aloft and -25
degree C 500mb. Early estimates call for a 25 percent chc of a
TSTM during this period.
Please note the official forecast is diced into 6 hour blocks and
the chance of rain in each 6 hour period is much less and the chc
of a TSTM is below 15 percent in any one period which to low to
mention in the forecast.
This weekend system is evolving and hopefully will be refined over
the next few days.
&&
.AVIATION...20/1207Z.
At 07Z at KLAX, the marine layer was 2800 ft deep. The top of the
inversion was at 4500 ft with a temperature of 9C.
Overall, moderate confidence in 12Z TAF Package. Timing of
Cigs/vsby restrictions may be off +/- 2 hours. Flight cats could
be off one or two especially during worst conditions. After 18Z
Mon, VFR conds are expected for airfields. However, LLWS and
turbulence will then become problematic for airfields especially
across LA/VTA counties. Timing of wind shift changes may be off a
couple hours, and wind gusts could be off 10-15 kts.
KLAX...Moderate confidence in 12Z TAF. There is a 30% chance of
east wind component of 6-8 kts before 22Z Mon, with an increase to
70% + until 09Z Tue.
KBUR...Moderate confidence in 12Z TAF. Increasing northeast winds
by Monday late morning into afternoon. There is a 30% chance of
brief gusts of 40+ knots by late Monday afternoon. Lgt-mdt turbc
is possible to likely after 16Z.
&&
.MARINE...20/845 AM.
For the Outer Waters, Gale Force winds from the northeast are
expected from around noon today through at least Tuesday afternoon
for the entirety of the southern zone (PZZ676), and through at
least early morning hours Tuesday in the southern portion of the
central zone (PZZ673). For the central zone, there is a 30% chance
for Gales to linger into Tuesday afternoon. Small Craft Advisory
(SCA) level winds are then likely through Tuesday evening (60%
chance) and could then linger through Wednesday morning (30%
chance) around the Channel Islands. Late Wednesday into Thursday
afternoon, SCA winds are highly likely for similar areas (80-90%
chance), and there is a 50% chance for Gales. In the northern
Outer Waters, sub-advisory conditions are likely, except for a
30% chance for SCA level winds Tuesday afternoon through Wednesday
morning.
For the Inner Waters North of Pt Sal (PZZ645), there is
a 20% chance of Small Craft Advisory (SCA) level wind gusts this
evening thru Tuesday morning. Then there is a 40% chance for SCA
level NE winds Tuesday afternoon through Wednesday evening. Then,
sub-advisory conds are expected Thursday.
For the Inner Waters South of Point Conception, from Ventura
Harbor to Malibu and out past the Channel Islands, there is a
80-90% chance of GALE FORCE E/NE wind gusts through early Tuesday
afternoon. For the nearshore waters from Channel Islands Harbor to
Malibu and out to Santa Cruz Island, there is a 50-60% chance of
STORM FORCE wind gusts this afternoon into Tuesday morning. From
the Orange County shores through the San Pedro Channel, there is a
60-80% chance of GALE FORCE wind gusts Monday evening through
Tuesday morning, with a 80% chance of 25-35 kt winds impacting
Avalon Harbor. Steep, choppy seas of 5 to 8 feet will be possible
during this time frame. From Ventura to Malibu and out to the
Channel Islands Tuesday afternoon through Wednesday, SCA level
winds are likely to continue from Tuesday afternoon through late
night (60-70% chance), and a 40-50% chance continuing all the way
through Wednesday evening. Another round of Santa Ana Winds are on
tap for late Wednesday into Thursday, with an 90% chance for SCA
winds in similar areas, and a 50-60% chance a Gale Warning will be
needed.
&&
.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory remains in effect until 2 PM PST Tuesday for
zones 87-367-370-381>383-548. (See LAXNPWLOX).
High Surf Warning in effect until 2 PM PST Tuesday for zone
87. (See LAXCFWLOX).
Coastal Flood Advisory in effect until 2 PM PST Tuesday for
zone 87. (See LAXCFWLOX).
High Wind Warning remains in effect until 2 PM PST Tuesday
for zones 88-354-355-358-362-369-371>380-549-550. (See LAXNPWLOX).
Red Flag Warning in effect until 10 PM PST Tuesday for zones
288-354-355-358-362-369-371>380-548. (See LAXRFWLOX).
Fire Weather Watch in effect from Tuesday evening through
Thursday evening for zones 288-354-355-358-362-369>380. (See LAXRFWLOX).
Red Flag Warning in effect until 2 PM PST Tuesday for zones
367-370-381-382. (See LAXRFWLOX).
PZ...Gale Warning in effect until 2 PM PST Tuesday for zones
650-655-676. (See LAXMWWLOX).
Gale Warning in effect until 9 AM PST Tuesday for zone 673. (See LAXMWWLOX).
&&
PUBLIC...DB/Rorke/DB
AVIATION...Black
MARINE...Lewis/Black
SYNOPSIS...DB/RM/DMB
weather.gov/losangeles
Experimental Graphical Hazardous Weather Outlook at:
https://www.weather.gov/erh/ghwo?wfo=lox