Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 270343
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
843 PM PDT Mon Jul 26 2021

.SYNOPSIS...26/830 PM.

The chance of showers and thunderstorms will continue winding down
through the evening. Some showers and isolated thunderstorms are
possible Tuesday afternoon over Los Angeles and Ventura County
Mountains as well as the Antelope Valley. Above normal
temperatures can be expected through the week.

&&

.SHORT TERM (MON-THU)...26/842 PM.

Easterly wave brought a significant influx of monsoonal moisture
with scattered rain showers to the region. The highest rainfall
totals were focused across LA county, where several stations
reported between 0.10 and 0.40 inches since late last night. Any
rainfall in July is quite rare for coastal/valley areas in
Southwest California. In fact, downtown Los Angeles reported 0.12
inches of rain, which is a record rainfall for this date (previous
record was 0.04 inches set in 2013).

With convective threat diminishing this evening, Flash Flood Watch
for LA/Ventura mountains and Antelope Valley was cancelled early.
While there was abundant moisture in the atmosphere today, with
precipitable water values reaching 2.0 inches, there was just too
much cloud coverage and subsidence behind the upper level wave
for any deeper convector to develop this afternoon. Current
doppler radar imagery this evening showing a band of showers
continuing across San Luis Obispo county, with a few light showers
elsewhere. This band of showers over San Luis Obispo county is
expected to shift north of our area after midnight. Otherwise
the moist lower levels of the atmosphere should allow low clouds
and fog to develop across many coastal areas tonight into Tuesday
morning.

For Tuesday, the overall air mass will be gradually drying with
precipitable water values dropping to between 1.0 and 1.4 inches
by the afternoon hours, compared to values of 2.0 inches observed
today. Despite the lower moisture content through the
atmosphere, the 850 mb dewpoints are still expected to range
between 10 and 12 degrees Celsius across interior portions of LA
county. In addition, the lack of cloud coverage on Tuesday will
help to better destabilize the atmosphere. 00Z NAM model continues
to show best convective threat on Tuesday across the LA county
mountains and Antelope Valley, due to highest moisture and best
instability. In fact, 850 mb dewpoint values are still in the +10
to +12 degree Celsius range for interior LA county, while lifted
index values climb to between -5 and -7 in the afternoon hours,
along with K-index values in the 40 to 44 range. Also of note, are
surface based CAPE values exceeding 1000 J/KG across the LA
county mountains and Antelope Valley on Tuesday afternoon. If
thunderstorms develop on Tuesday across the LA county mountains
and Antelope Valley, the main threats would be brief heavy
downpours and gusty winds, with the strongest wind gust potential
in the Antelope Valley, where model soundings showing more of an
inverted-v structure, along with DCAPE values approaching 1400
J/kg.

*** From previous discussion ***

Also, will need to watch the coastal and valley areas of LA
county for potential shower activity as the NAM indicates some
pretty instability during the day. However, at this time, think
the potential for any coastal/valley showers is below mentionable
levels. Otherwise, Tuesday should be a mostly clear day (after any
stratus dissipates) and noticeably warmer. In fact, many areas
away from the coast will be 5-10 degrees warmer than today.

For Tuesday night through Thursday, a very benign pattern is
anticipated. The marine inversion will remain quite shallow with
H5 heights remaining rather high. So with continued onshore flow,
stratus/fog should develop across the coastal plain and lower
coastal valleys during the night and morning hours. Other than the
stratus, skies should be mostly clear through the period. As for
temperatures, expect inland areas to continue to exhibit a slight
warming trend while temperatures across the coastal plain remain
persistent or cool very slightly (based on the whims of the marine
layer).

.LONG TERM (FRI-MON)...26/156 PM.

Overall, 12Z models continue to be in decent synoptic agreement
through the period. Main synoptic features will be the upper level
high over the central part of the country. High will weaken
slightly this weekend and into early next week while slowly
shifting to the west.

Forecast-wise, the overall pattern should be a rather typical one
for the area. With the high H5 heights, the marine inversion
should remain on the shallow side with stratus/fog generally
confined to the coastal plain and lower coastal valleys. On Friday
and Saturday, the upper level flow turns southeasterly which may
allow some mid/high level moisture to move into the area. So,
skies, outside the stratus, could be mostly clear to partly
cloudy. Additionally, there may be enough moisture/instability to
generate some isolated showers over far eastern LA county during
the afternoon hours. By Sunday and Monday, the upper level high
will be over the area, keeping things dry and mostly clear. As
for temperatures, expect relatively persistent temperatures Friday
through Sunday then some slight cooling on Monday.

&&

.AVIATION...27/0006Z.

At 2355Z at KLAX, the marine layer was 1500 feet deep. The top of
inversion was at 3200 feet with a temperature of 21 degrees
Celsius.

Generally low confidence in the 00z TAF package. Extensive mid
level cloud coverage and scattered showers continue to disrupt
the marine layer this afternoon, and expecting chaotic low cloud
development tonight, mainly occurring across coastal areas.
Confidence in low cloud timing and ceilings remains low for
tonight. The shower threat is expected to diminish by later this
evening across most of the forecast area.

KLAX...Low confidence in the 00Z TAF. There is a 10% chance of a
shower through early evening. There is increasing confidence of low
clouds forming tonight, but confidence remains low in timing and
ceiling heights. East winds are expected to remain below 5 knots
overnight.

KBUR...Low confidence in the 00Z TAF. There is a 10% chance of a
shower through early evening. There is a 20% chance of IFR cigs
after 09Z tonight.

&&

.MARINE...26/817 PM.

There is a slight chance of a thunderstorm across the outer waters
along the Central Coast through this evening, otherwise the threat
for thunderstorms has ended. For Tuesday morning, there is about a
10% chance at most of thunderstorms over the waters nearest the
Los Angeles County coast. If a thunderstorm were to develop, there
could be brief gale force winds and rough seas and dangerous
lightning.

Winds and seas are expected to remain below Small Craft Advisory
(SCA) levels across most of the coastal waters through Saturday.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...NONE.

&&

.HAZARD POTENTIAL OUTLOOK (THU-MON).

Monsoonal thunderstorms are possible starting Friday and lasting
through the weekend.

&&

$$

PUBLIC...Gomberg/Thompson
AVIATION...Gomberg
MARINE...Sweet/Stewart
SYNOPSIS...RAT/Stewart

weather.gov/losangeles


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