Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 032354 AAA

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
454 PM PDT Fri Apr 3 2020

updated aviation discussion

.SYNOPSIS...03/1204 PM.

Onshore flow increases tonight as a Catalina Eddy brings low
clouds back over most of the south coasts and valleys. Saturday
will see increasing high clouds as a storm system approaches
southern California. Moderate to heavy rain will overspread the
area Sunday night through Monday with lingering rain and mountain
snow showers Monday night through Thursday.


.SHORT TERM (TDY-MON)...03/147 PM.

Quiet before the storm as the saying goes. Nice sunny conditions
today with mild temperatures and very little to complain about. A
few clouds have lingered over the LA Basin today following the
weak marine inversion from last night. Winds are a little gusty
through the highway 14 corridor into the Antelope Valley and along
the central coast...gusts to 40 mph near Poppy Park and 35 mph
near Gaviota. Expect an offshore pressure gradient to help winds
remain gusty west of Goleta on the south SBA coast this evening,
diminishing after midnight.

Saturday morning will be mostly cloudy with patchy fog across most
beaches and valleys from LA to Ventura County and will likely
push into Santa Barbara by sunrise. A rather strong eddy
circulation near Catalina Eddy will be responsible to this return
of low clouds. Morning clouds will knock a few degrees off the
afternoon highs from what we are experiencing today. Northern
areas will see increasing high clouds Saturday afternoon as a low
pressure trough develops north of California.

Sunday is our transition day as low pressure near the Oregon coast
merges with a much colder trough dropping down the British
Columbia coast. Also, a storm system over the central Pacific is
very active with thunderstorms today and this is where much of our
moisture will originate from for Sunday night and Monday. A
rather organized cold front will push southward from the Bay Area
Saturday night with light rain developing across San Luis Obispo
County early Sunday. As colder air aloft builds into the region,
the cold front will become a little steeper as lapse rates
increase and southerly flow increases. This will result in a
significant rain band with moderate orographic flow into our
mountain ranges by Sunday evening. Between 6pm-midnight the SBA
County foothills/mountains will be seeing moderate to heavy rain
which will spread southward across Ventura County overnight and
into LA County by Monday morning. Embedded isolated thunderstorms
will be possible along and near the front during this time period.

To complicate matters, both the NAM and GFS models are showing
another disturbance rotating into the base of the low pressure on
Monday. This slows the front down as the trough axis pivots more
north-south and temporarily aligns with the surface features. The
slower southward movement favors steady moderate to heavy rain
over the eastern LA County for a longer duration, therefore,
higher rain amounts in the foothills/mountains. In total it may
take a solid 24-hours to see the steady rain and front traverse
from our northern counties to the eastern LA County line.

There will be a break in the steady rain to the more convective
showery-type rainfall by afternoon (north) and evening (south) on
Monday. Models do indicate strong diffluent flow and another
potential round of showers and isolated thunderstorms increasing
Monday night into Tuesday morning as the upper low moves closer to
southwest CA.

Rainfall totals look good overall with 0.75-1.5" for coasts and
valleys, especially Ventura and LA, and 1.5-3.0" for foothills and
mountains through Monday night. This amount of rain could meet or
exceed our normal amount during a typical month of April, in just
one-day. Rain rates will mostly be in the 0.25-0.50"/hour range
with local rates over 0.75"/hr in heavier cells. This should not
be a significant threat for recent burn scars, but minor debris
will be possible. Snow levels will begin above 5500 feet Sunday
night-Monday, then fall closer to 4500 feet by Tuesday morning.
Think the main accumulations will remain above 6k feet, but some
local amounts will be possible near Tejon Pass at some point on
Tuesday. A winter storm watch may be needed in the day or so.

.LONG TERM (TUE-FRI)...03/139 PM.

Both Tuesday and Wednesday will remain cold and unstable with a
fairly good chance of showers developing each day after a little
heating, and some of those showers could be heavy and contain
small hail or ice pellets. Temperatures aloft range from -28 to
-31 degrees C so it doesn`t take much to get towering cumulus with
 ice and a lightning flash or two. The main impact will likely be
 low snow levels between 4-4.5k feet for the duration and travel
 problems over mountain passes, including the Grapevine at times.
 Confidence in the forecast goes down on Thursday due to the
 timing issues with the low pressure system. If it hangs around
 there could be additional showers, but the trend looks to be a
 decreasing threat of showers by Thursday afternoon/evening.

Friday would appear to be a drier day with high pressure building
into the west coast and the low finally shifting to the east.
Warmer and dry conditions expected into next weekend.



At 23Z at KLAX, there was no marine inversion.

Moderate to high confidence of VFR conditions everywhere through
this evening. Low-moderate confidence in timing, categories, and
coverage of stratus later tonight into Saturday morning to early
afternoon for the coast and vlys. Clouds could be solid in
coverage, or spotty with many breaks due to possibilty weak

Hi confidence in VFR conditions for KWJF and KPMD thru Sat
afternoon, with gusty afternoon and evening W winds thru the

KLAX...Hi confidence in VFR prevailing through about 08Z tonight.
Moderate confidence in presence of CIGS, low confidence in timing
with a projected start time between 06Z and 09Z and lingering
through about 21Z Sat followed by VFR cigs thru Sat evening.
Moderate confidence in CIGS at MVFR. East winds are likely, with a
40 percent chance of exceed 8 KT, best chance 10-16Z.

KBUR...Hi confidence in VFR prevailing through about 12Z late
tonight. Moderate confidence in presence of CIGS, low confidence
in timing with a projected start time between 10Z and 13Z and
lingering through about 20Z. Moderate confidence in CIGS at MVFR.
Southeast winds will be prevalent through the night into Sat.


.MARINE...03/150 PM.

High confidence in Gale Force winds through tonight off the
Central Coast down to San Nicolas Island, with Small Craft
Advisory (SCA) winds bleeding into the nearshore waters of the
Central Coast and the western half of the Santa Barbara Channel.
The rest of the waters may need a SCA for short period seas, but
more probable the seas will stay just under those thresholds. High
confidence NW winds will decrease quickly on Saturday, allowing
for enhanced SE winds from Orange to Santa Barbara Counties,
including the San Pedro Channel and Anacapa Passage.

A storm system is still on target to impact the area Sunday
through Wednesday. Thunderstorms will be possible any time, but
especially Tuesday and Wednesday. South wind gusts between 25 and
35 kt are also possible Sunday into early Monday anywhere as the
system moves down the coast.


PZ...Small Craft Advisory in effect until 3 AM PDT Saturday for
      zones 645-650. (See LAXMWWLOX).
     Gale Warning in effect until 11 PM PDT this evening for zone
      670. (See LAXMWWLOX).
     Gale Warning in effect until 4 AM PDT Saturday for zones
      673-676. (See LAXMWWLOX).



Rain, possibly heavy at times, and mountain snow above 5000 feet
are expected Monday. Numerous rain and mountain snow showers are
expected with possible thunderstorms Tuesday to Wednesday. Showers
will decrease on Thursday. Impacts will include minor urban
flooding and difficult mountain travel.



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