NWS Los Angeles, CA wrote its last area forecast discussion before the first event at Feb 24, 8:12 PM PST, 6 hours 41 minutes earlier.
Area forecast discussion, Feb 24, 8:12 PM PST, 6 hours 41 minutes before
Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
812 PM PST Wed Feb 24 2021
.SYNOPSIS...24/810 PM.
Moderate to strong Santa Ana winds will develop through Thursday
morning across the wind prone areas of Los Angeles and Ventura
Counties as offshore flow establishes. Another round of gusty
winds will return late Saturday into Sunday. Dry conditions with
temperatures above seasonal averages are expected through early
next week.
&&
.SHORT TERM (WED-SAT)...24/805 PM.
The latest satellite imagery show an upper-level trough over
Central Utah this evening and a blocking ridge of high pressure
aloft centered near 36N and 145W. The trough will brush the area
tonight as it continues to move southeastward. Weak onshore flow
in place this evening is attempting to cling to the area, but
surface high pressure building into the Great Basin will win out
by late tonight or early Thursday morning. Some semblance of a
marine layer stratus deck remains over the Santa Monica Bay and
into the San Pedro Channel this evening. Dense fog is, at times,
affecting Western Los Angeles County beaches this evening. Low
clouds and fog are likely to stay wedged in through at least
midnight tonight before the offshore flow establishes and
strengthens. Dense fog will likely linger along the beaches in
Los Angeles County tonight, and possibly push farther eastward
into Downtown and East Los Angeles later this evening.
With the stratus wedging into the southern coastal areas this
evening, the start times of the wind advisories were delayed by
about 3 hours for the Los Angeles and Ventura County coast and
Catalina and Santa Barbara Islands. Confidence is good for
stratus to start scouring away around midnight as northeast winds
start to filter in. Otherwise, the timing of the wind advisories
and High Wind Warnings look agreeable and no additional updates
are planned to the forecast.
***From Previous Discussion***
Overall, 12Z models in good synoptic agreement through the short
term period. Inside slider will move into the Great Basin tonight
and Thursday to be followed closely by another inside slider on
Saturday. With this type of pattern, winds will continue to be the
main issue through the short term period.
For this afternoon through Thursday, forecast looks very much
on track. As the inside slider moves into the Great Basin this
evening, northerly flow will increase across the area. 12Z models
indicate SBA-SMX gradient peaking around -2.5 MB and SBA-BFL
gradient between -4.0 and -5.4 MB. So, with some decent upper
level northerly flow, will anticipate advisory-level north winds
across the Santa Ynez Range and I-5 Corridor. Late this evening,
the flow will turn to the northeast and remain northeast through
Thursday afternoon. LAX-DAG gradient forecast to peak between -7.8
MB and -9.0 MB Thursday morning. NAM has upper level winds 40-55
knots while the GFS forecasts winds around 35 knots. Given the
forecast surface gradient and upper level winds, moderate to
strong Santa Ana winds will develop late this evening through
early Thursday afternoon, before diminishing. Current slate of
HIGH WIND WARNINGs and WIND ADVISORIES look good for Santa
Barbara/Ventura/LA counties (see LAXNPWLOX for latest details).
For Thursday night/Friday, the offshore winds across Ventura and
LA counties will diminish, turning weakly onshore Friday
afternoon. However for San Luis Obispo and Santa Barbara counties,
continued northerly surface gradients and upper level winds
indicate potential for advisory-level Sundowner and Santa Lucia
winds with gusts up to 50 mph possible.
For Friday night/Saturday, the next inside slider will bring
a reinforcing shot of northerly winds to the area. Therefore,
advisory-level winds will likely continue with the strongest winds
across the mountains as well as the SBA South Coast.
Other than wind, the forecast is very benign. Skies should remain
mostly clear through the period as offshore flow keeps the stratus
at bay. As for temperatures, expect cold air advection to cool
most areas on Thursday except for the coastal plain of Ventura/LA
counties where downsloping will bump up temperatures a bit. For
Friday, all areas should be several degrees warmer with weakening
offshore wind and associated cold air advection. For Saturday, all
areas will cool a few degrees as the inside slider brings the
gusty northerly winds.
.LONG TERM (SUN-WED)...24/149 PM.
For the extended period, forecast confidence starts out high for
Sunday, but drop significantly for the first half of next week.
For Sunday, both the GFS and ECMWF indicate strong offshore
gradients and decent upper level support. So, another round of
Santa Ana winds is expected with advisory-level northeast winds
likely. With the offshore flow, temperatures west of the mountains
will be a few degrees above normal while interior sections will be
near or a couple degrees below normal.
For Monday through Wednesday, forecast confidence hits rock bottom
as GFS and ECMWF are all over the place. The GFS indicates dry
conditions Monday/Tuesday then bring a decent storm across the
area on Wednesday. Conversely, the ECMWF keeps the area dry
through Wednesday, dropping an inside slider across the Great
Basin on Tuesday which would bring additional offshore winds.
Respective ensembles reinforce the particular solutions of each
model. So, given the model uncertainty, will keep the forecast dry
for now and hope for some model clarity over the next few days.
&&
.AVIATION...25/0056Z.
At 23Z, the marine layer depth was around 900 ft deep at KLAX.
The top of the marine inversion was around 1300 ft deep with a
temperature of 15 degrees Celsius. There was another inversion up
to around 1800 feet.
VFR conditions are expected through the period, except for a
moderate chance of VLIFR to LIFR conditions at Los Angeles County
coastal terminals through 08Z. Periods of moderate to strong low-
level wind shear and turbulence are possible at terminals south
of Point Conception through 20Z.
KLAX...There is a 40 percent chance of VLIFR to LIFR conditions
at Los Angeles County coastal terminals through 08Z. East winds
greater than 7 knots are likely between 08Z and 16Z. Periods of
moderate to strong low-level wind shear and turbulence are
possible between 10Z and 16Z.
KBUR...VFR conditions are expected through the period. Periods of
strong wind shear and turbulence are likely between 06Z and 20Z.
&&
.MARINE...24/806 PM.
Across the outer waters, Small Craft Advisory (SCA) level winds
and seas across the northern portion along the Central Coast will
continue into the overnight hours, then will likely drop below
SCA level for the rest of Thursday. By Thursday night or early
Friday morning, SCA level hazardous seas will return, and SCA
level winds will also likely follow suit, lasting through the
weekend. There is a 30-40% chance of Gale force winds on Friday
and Saturday.
Across the southern outer waters from Point Conception to San
Nicolas Island, SCA level NW winds are likely to continue into
tonight. As a Santa Ana develops, winds will shift to NE later
tonight through early Thursday afternoon remaining at SCA level,
with a 30% chance of Gale force gusts across the eastern portion.
SCA level NW winds are expected to develop late Thursday night or
early Friday and continue through the weekend, along with
hazardous seas. There is a 20% chance of Gale Force winds Friday
night into Saturday.
Gale Force NE winds are expected across to develop late tonight
and persist through Thursday morning across eastern portions of
the SBA Channel, the inner waters from Pt. Mugu to Santa Monica,
and south of Palos Verdes out to Catalina Island. Winds will
diminish quickly by the afternoon. These gusty NE winds will
cause steep short period NE wind waves. These waves will create
particularly hazardous conditions in the vicinity of NE facing
harbors such as Avalon and Two Harbors.
Seas across the outer waters and inner waters north of Pt. Sal
will build Thursday night into the weekend. Moderate confidence
that seas will surpass 15 ft across the outer waters at the peak
Friday night and Saturday.
&&
.BEACHES...24/806 PM.
A relatively large long period west to northwest swell off the
coast of Central CA will subside through the evening, reducing
surf across the Central Coast to below advisory level. However,
this will be relatively short-lived as another large northwest
swell enters the waters Thursday night into early Friday. A new
High Surf Advisory will be likely for the Central Coast on Friday
through Sunday.
&&
.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory in effect until midnight PST tonight for zones
39-52. (See LAXNPWLOX).
Wind Advisory in effect from midnight tonight to 3 PM PST
Thursday for zones 40-41. (See LAXNPWLOX).
Wind Advisory in effect until 3 PM PST Thursday for zones
44-45-547. (See LAXNPWLOX).
High Wind Warning in effect until 3 PM PST Thursday for zones
46-53-54-88. (See LAXNPWLOX).
Wind Advisory in effect from 3 AM to noon PST Thursday for
zone 87. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect until 9 PM PST this evening
for zone 645. (See LAXMWWLOX).
Gale Warning in effect until noon PST Thursday for zones
650-655. (See LAXMWWLOX).
Small Craft Advisory in effect until 3 AM PST Thursday for
zones 670-673. (See LAXMWWLOX).
Small Craft Advisory in effect until 3 PM PST Thursday for
zone 676. (See LAXMWWLOX).
&&
.HAZARD POTENTIAL OUTLOOK (SAT-WED).
Gusty north to northeast winds are possible Saturday and Sunday.
&&
PUBLIC...Hall/Thompson
AVIATION...Hall
MARINE...Sweet/Stewart
BEACHES...Stewart
SYNOPSIS...Hall
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