NWS Los Angeles, CA wrote its last area forecast discussion before the first event at Jan 18, 10:19 PM PST, 4 hours 34 minutes earlier.
Area forecast discussion, Jan 18, 10:19 PM PST, 4 hours 34 minutes before
Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
1019 PM PST Mon Jan 18 2021
.SYNOPSIS...18/646 PM.
Very strong northeast winds will affect much of Southwest
California late tonight through Wednesday morning, especially
across Los Angeles and Ventura Counties. Damaging wind gusts will
be possible at times during the period. It will remain dry except
for isolated showers over the eastern San Gabriels on Tuesday.
There will be a widespread chance of light rain Friday through
the weekend.
&&
.SHORT TERM (MON-THU)...18/913 PM.
The latest water vapor imagery shows broad troughing over the
region with a retrograding trough located over the Great Basin.
This trough will dig southwest to just west of the Channel Islands
by late Tuesday morning, then dig south towards Baja California.
A cold air mass can seen settling into the Great Basin on infrared
imagery currently, where high pressure will set up at the surface
through Tuesday morning as a precursor to the offshore flow
pattern expected across the region.
A difficult forecast is in store as the upper-level trough
circulation will be cut-off from the mean flow pattern aloft, and
depending how far offshore the trough moves, more moisture could
be entrained into the trough. The latest model solution keep the
best moisture to the south and east of the area, but wrap-around
moisture could reach the interior slopes of the San Gabriels. A
few stray showers cannot be ruled out pushing into the foothills.
PoPs have been nudged up slightly and cloud coverage have been
increased as there looks to be a decent amount of mid and high
level cloudiness streaming over a mostly dry air mass across the
coastal and valley areas. While the less confidence in the sky
coverage and rain chances, there is more confidence in the winds
with the system. Strong surface high pressure settling up over the
Great Basin will be the driving surface feature and the trough
aloft will provide wind and thermal support at the lower and
middle levels for strong Santa Ana and moderate Santa Lucia wind
events. The current headlines look agreeable and only a high wind
warning was added for the Santa Cruz and Anacapa Islands as
damaging wind gusts are expected for the interior portions of the
Channel Islands.
A stratus deck in place across the Los Angeles County coast,
producing low clouds and patchy dense fog, is surging through
Cahuenga Pass and will likely make into the lower portions of the
San Fernando and San Gabriel Valleys late this evening. The
developing offshore flow could skim over the top the wedged in
marine layer and delay the onset of gusty northeast winds until
after sunrise on Tuesday.
Another update will be issued shortly.
***From Previous Discussion***
Overall, 12Z models in very good synoptic agreement through the
short term period. At upper levels, strong low will retrograde
over the area Tuesday then southwest and south of the area on
Wednesday and Thursday. Near the surface, moderate to strong
offshore flow will continue.
Forecast-wise, main issue continues to be a strong Santa Ana wind
event tonight through Wednesday morning. 12Z models do not
indicate any major changes to previous forecast thinking with
respect to the strength of the event. Offshore gradients will
increase tonight/Tuesday then weaken slightly Tuesday
night/Wednesday then weaken more noticeably on Thursday. On
Tuesday morning, the LAX- DAG is forecast to range between -7.0
and -9.2 MB and between -6.5 and -7.5 MB on Wednesday morning.
Just for reference, the 97th percentile LAX-DAG gradient is -6.8
MB. So, these forecast gradients are very noteworthy.
Additionally, models continue to indicate very impressive upper
level support (northeast winds 45-55 knots) with strongest winds
Tuesday and Tuesday night. With strong offshore gradients and
impressive upper level support, high confidence in a strong Santa
Ana wind event. Will continue with current HIGH WIND WARNINGs from
early Tuesday morning through Wednesday morning. Also, will issue
WIND ADVISORIES leading into the current slate of warnings as
well as issue WIND ADVISORIES for the Central Coast, Antelope
Valley and interior sections of San Luis Obispo and Santa Barbara
counties. Please refer to LAXNPWLOX for more detailed info about
the plethora of wind products in effect. For Wednesday night and
Thursday, the offshore winds will be much weaker, but still likely
at advisory levels for many areas, especially Ventura and LA
counties.
Otherwise, secondary issue continue to be the possibilty of some
light showers as moisture wraps around the upper low. Models
indicate the potential for some very light, scattered showers
across the San Gabriel Mountains as well as the southern Inner
Coastal Waters Tuesday and Tuesday evening. Will keep mention of
slight chance showers for these areas with the afternoon forecast.
Precipitation totals, if any, will be very light.
As for temperatures, Tuesday will be cooler for most areas as
offshore winds bring in some significant cold air advection. The
only exception will be coastal areas where the downsloping effect
of the winds will bring some warming. On Wednesday, all areas will
warm up with continued offshore flow and more limited cold air
advection. On Thursday, expect some cooling most areas as weak
onshore flow returns Thursday afternoon.
.LONG TERM (FRI-MON)...18/202 PM.
For the extended, 12Z models have the same general synoptic idea
through the period. At upper levels, low develops along the
Northern CA coast on Friday then drop southward Saturday and
Sunday with another trough moving across the area on Monday. Near
the surface, weak onshore flow is forecast to prevail.
Forecast-wise, this will be a noticeable pattern change for the
area. With the upper low dropping south, it will be able to
pick up some moisture. So, there is the potential for some
precipitation across the area Friday afternoon through Sunday
morning. At this time, there could be some decent rainfall totals
with this system. Preliminary QPF amounts range from 0.10 to 0.50
inches across San Luis Obispo, Santa Barbara and Ventura counties
with 0.50 to 1.00 inch possible across LA county (with highest
amounts over the foothills). For Monday, models differ a bit with
the timing of the upper level trough, but still indicate the
chance for some additional light precipitation.
As for temperatures, expect much cooler conditions through the
extended with all areas well below seasonal normals.
&&
.AVIATION...19/0619Z.
At 04Z at KLAX, the marine layer was 1200 feet deep. The top of
the inversion was around 2000 feet with a temperature near 21 C.
Low to moderate confidence in TAFs. Through 12Z IFR cigs are
possible at KOXR, KSMO, KLAX, KLGB and KBUR. Cigs could rapidly
dissipate anytime after 08Z. Low confidence in wind forecast
after 12Z with very strong NE to E winds possible at all
terminals.
There will be areas of turbc and LLWS with a 20 percent chc of SVR
turbc anywhere but esp near and over hier trrn.
KLAX...Low confidence in cig forecast through 12Z with rapid
clearing possible at any time aft 08Z. Low confidence in wind
forecast with a chc of a 12kt east wind component 12Z-04Z and a
slight chc of an 18 kt east wind component 21Z-04Z. LGT ocnl MDT
turbc on over the field and to the east aft 18Z.
KBUR....Low confidence in cig forecast through 12Z with rapid
clearing possible at any time. Low confidence in wind forecast
with 35 kt gust possible aft 21Z. lgt ocnl mdt turbc and LLWS
likely aft 16Z.
&&
.MARINE...18/746 PM.
High confidence that Small Craft Advisory (SCA) level hazardous
seas over 10 feet will continue through Tuesday afternoon across
the waters along the Central Coast southward to San Nicolas
Island. North winds will increase overnight, with moderate
confidence in forecast of Gales over PZZ670 late tonight through
Tuesday morning. Otherwise, NE winds at SCA levels are expected
over the outer waters Tuesday through Wednesday morning over
PZZ670-673. PZZ679 is likely to see NE Gales Tuesday and Tuesday
night, with winds diminishing some Wednesday morning. Gusty NW
winds at SCA levels are then possible over the outer waters
Thursday night through Saturday.
Offshore winds are also expected nearshore north of Point Sal, at
SCA levels Tuesday through Tuesday night, with seas diminishing
below 10 feet Tuesday evening. There will be another chance of SCA
NW winds Thursday night into Friday.
High confidence in strong northeast winds and choppy short-period
seas across all the inner waters inside the Southern California
Bight and out to all the Channel Islands, San Nicolas Island, and
Catalina Island late tonight through Wednesday morning. The
strongest winds are expected Tuesday morning through afternoon. A
Gale Warning is in effect from late tonight through Wednesday
morning with winds likely peaking between 30 and 45 kt with local
gusts as high as 50 kt possible. Choppy wind- generated waves of
5 to 8 feet are also expected. East facing harbors, such as Avalon
Harbor and other east facing harbors across the Channel Islands
will be affected with strong winds and steep seas. These
conditions are hazardous to boaters, and there could be damage to
boats that are not anchored properly. Mariners should avoid these
areas, or remain in safe harbor until conditions improve.
&&
.BEACHES...18/759 PM.
A long period west to northwest swell continue to affect the
coastal waters through Tuesday, resulting in high surf and
dangerous rip currents on all west and northwest facing beaches.
Surf will peak through Tuesday morning, then slowly subside
through Tuesday evening. There is a chance that surf will subside
more slowly than expected, and Surf Advisories may need to be
extended past Tuesday night.
For the Central Coast, large surf will likely subside to 12 to 16
feet on Tuesday along favored northwest-facing beaches, then
subsiding through Tuesday evening.
For the Los Angeles and Ventura County beaches, large breaking
waves of 6 to 8 feet will be likely along exposed west-facing
beaches, with some higher sets to 9 feet for the Ventura County
beaches. The west- facing beaches of the Santa Barbara County
South Coast will likely see surf of 5 to 7 feet. The surf will
subside below 7 feet in all areas Tuesday evening.
&&
.FIRE WEATHER...18/400 PM.
Gusty offshore winds and low humidities will continue to bring elevated to brief
critical fire weather conditions through early evening for Los Angeles and Ventura
counties as well as the coastal foothills of San Luis Obispo County. Wind gusts
will continue to range between 30 and 50 mph across these areas, with humidities
in the 12 to 25 percent range.
A very strong and damaging northeast to east wind event is expected to impact a
large portion of Southwest California late tonight through Wednesday morning.
Red Flag Warnings are in effect for many portions of Southwest California later this
evening through Wednesday morning. The strongest and most widespread winds are
expected to be Tuesday through Tuesday night when widespread damaging wind
gusts of 60 to 80 mph will be likely in the mountains, foothills, and some valley
locations in Los Angeles and Ventura counties. Isolated gusts as high as 90 mph will
be possible in the mountains of Los Angeles county. Even coastal areas (including
portions of the LA Basin) will likely see locally damaging wind gusts of 50 to
65 mph. Portions of San Luis Obispo and Santa Barbara counties (especially the
mountains,Central Coast, and Santa Ynez Valley) could see strong northeast to
east winds Tuesday into Tuesday night, with wind gusts of 30 to 50 mph at lower
elevations and 50 to 70 mph at higher elevations. Humidity levels tonight through
Tuesday are expected to range between 12 and 25 percent across coastal and
valley areas, and 15 to 30 percent across the mountains, with humidity levels
gradually rising on Tuesday night.While humidity levels will be above 15 percent
for some areas, the strength of this wind event and the current state of the very
dry fuels warranted this Red Flag Warning issuance. Significant wind impacts will
be likely with this event, including the threat of widespread downed trees and
powerlines, as well as power outages. Winds are then expected to diminish
Thursday through next weekend, with a chance of precipitation Friday and
Saturday, and additional precipitation chances next week.
&&
.LOX WATCHES/WARNINGS/ADVISORIES...
CA...High Surf Advisory in effect until 10 PM PST Tuesday for
zones 34-35-39>41-87. (See LAXCFWLOX).
Wind Advisory in effect from 4 AM Tuesday to 4 AM PST
Wednesday for zones 34>38. (See LAXNPWLOX).
Wind Advisory in effect until 4 AM PST Tuesday for zones
40-41-44-45-51-52-88-547. (See LAXNPWLOX).
High Wind Warning in effect from 4 AM Tuesday to 4 AM PST
Wednesday for zones 40-41-44-45-51-52-88-547. (See LAXNPWLOX).
Wind Advisory in effect until 1 AM PST Tuesday for zones
46-53-54. (See LAXNPWLOX).
High Wind Warning in effect until 4 AM PST Wednesday for
zones 46-53-54. (See LAXNPWLOX).
Wind Advisory in effect from 7 AM Tuesday to 4 AM PST
Wednesday for zone 59. (See LAXNPWLOX).
Red Flag Warning in effect until 10 AM PST Wednesday for
zones 234>236-251-252. (See LAXRFWLOX).
Red Flag Warning in effect until 10 PM PST Tuesday for zones
240-241-244>246-253-254-288-547-548. (See LAXRFWLOX).
High Wind Warning in effect from 7 AM Tuesday to 4 AM PST
Wednesday for zone 550. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect until 3 AM PST Wednesday for
zones 645-673. (See LAXMWWLOX).
Gale Warning in effect from 3 AM Tuesday to 10 AM PST
Wednesday for zones 650-655-676. (See LAXMWWLOX).
Gale Warning in effect until 9 AM PST Tuesday for zone 670. (See LAXMWWLOX).
Small Craft Advisory in effect until 3 AM PST Tuesday for
zone 676. (See LAXMWWLOX).
&&
.HAZARD POTENTIAL OUTLOOK (THU-MON).
Light to moderate precipitation is possible Friday through the
weekend.
&&
PUBLIC...Hall/Thompson
AVIATION...Rorke
MARINE...Smith
BEACHES...Smith
FIRE...Gomberg
SYNOPSIS...Smith
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