Monrovia, California 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

044
FXUS66 KLOX 131942
AFDLOX

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
1242 PM PDT Sun Apr 13 2025

.SYNOPSIS...13/319 AM.

More marine influence and stronger onshore flow will continue a
gradual cooling trend. Night through morning low clouds and fog
with patchy drizzle at times will be a staple of the forecast. Low
clouds will push into the coastal slopes of the mountains by
Tuesday. A cold upper level low pressure system will bring
additional cooling and a chance of showers to the area between
late Thursday and early Saturday. An afternoon or evening
thunderstorm cannot be ruled out during that time.

&&

.SHORT TERM (TDY-TUE)...13/830 AM.

***UPDATE***

Classic marine layer pattern this morning with good onshore flow
and a weak troffiness aloft. The marine inversion deepened to
around 1400 ft at VBG and to around 2400 ft at LAX. Low clouds and
fog were extensive this morning from the coast to most adjacent
vlys. Low clouds also moved up the Salinas River Vly into northern
SLO County. The low clouds are expected clear back to the coast
by midday, with many beaches likely to remain socked in most of
the day thanks to strong onshore pressure gradients (e.g., NAM
fcst +7.1 mb LAX-DAG at 00Z). Otherwise and elsewhere, mostly
sunny skies with a few hi clouds at times can be expected across
SW CA thru this afternoon.

Locally gusty NW to N winds were noted this morning, mainly over
SW SBA County and in the L.A. County mtns, otherwise winds were
generally weak. All areas should have breezy to gusty W to SW
winds this afternoon into early this evening.

Temps across the area today are forecast to be a couple degrees
cooler than yesterday overall, with coast and vlys near normal to
a few degrees below normal while interior areas remain several
degrees above normal. The warmest vlys and lower mtns should top
out in the mid 70s to low 80s, except lower to mid 80s in the
Antelope Vly.

***From Previous Discussion***

A general cooling trend should continue into Tuesday as a
stronger onshore flow and more cooler marine air will move in. A
brief break in the cooling trend could develop on Monday as a
shortwave along the North Coast of California digs, cuts off from
the flow pattern, and retrogrades from the California coastline.
This will weaken the onshore push slightly and bring a few degrees
of warming on Monday.

By Tuesday, broader cyclonic flow will move back into the region
as another trough sitting near 30N and 127W will phase with the
new cutoff low off the Central Coast of California and lift out
into southern California. This will bring substantially deepening
to the marine layer depth and the possibility of night through
morning drizzle. The low cloud field will expand into the coastal
slopes of the mountains between Monday night and Tuesday as the
marine layer depth deepen to near 2500 feet deep. With onshore
pressure gradients approaching 8 mb between KLAX and KDAG, clouds
will likely to struggle to clear from the beaches on Tuesday and
may struggle to clear from the coast and valley areas across the
Southland. Future shifts will need to take a closer look at this
as the scattering out of the low clouds may be inhibited greatly
by the strong onshore gradients.

.LONG TERM (WED-SAT)...13/426 AM.

A colder upper-level trough near 40N and 155W will drop into the
region between Tuesday and Wednesday and combine with the cutoff
trough off the Central Coast. The resultant trough will deepen to
near 560 dam, as it approaches the California coast. Increasing
onshore flow and a deeper marine layer will bring cooler
conditions with the threat of night through morning drizzle
occurring. A few mountain showers cannot be ruled out for
Wednesday afternoon and evening as the trough passes over the
region.

Shower chances will increase for late week as a cold trough near
the Aleutian Islands this morning will move over the top of the
ridge near British Columbia and dig down into the region. This
trough will much be colder, piping colder air in from Canada, as
it takes an inside track. The trough will bottom out near Point
Conception and is likely to pick up some Pacific moisture in the
process to work with for precipitation. EPS precipitable water
value means increase to around 0.75 inch on Thursday and a
majority of the ensemble members have precipitation for sometime
between late Thursday and early Saturday. There is a non-zero
chance for thunderstorms, especially during the afternoon and
evening hours as the colder air mass could turn unstable with
daytime heating. While there is much uncertainty with this trough,
there is a lesser chance that a drier forecast could result with
continued night through morning low clouds and fog with drizzle.

A warming trend should develop for next weekend with weakening
onshore flow and less low clouds and fog. Cluster analysis favors
stronger troughing pushing to the east and allowing for a ridge
over the Pacific Ocean to nose in.

&&

.AVIATION...13/1933Z.

At 1844Z at KLAX, the marine layer was around 600 feet deep. The
top of the inversion was around 2600 ft with a temp of 14 C.

Low to moderate confidence in all TAFs, except high confidence in
desert TAFs. Cigs may struggle to clear from KSBA, KOXR, KSMO,
KLAX, and KLGB through the day, and scatter and reform frequently
before settling in for the night after 06Z. Timing of flight cat
changes may be off by +/- 3 hours. Cigs may drop one flight cat
lower than forecast overnight.

KLAX...Low confidence in TAF. There is a 30-40% chc that cigs
will not scatter out this afternoon. There is a 20% chc that cigs
will arrive as early as 00Z. There is a 20% chc of LIFR cigs
tonight. Good confidence that any E wind component will remain
under 6 kt.

KBUR...Low to moderate confidence in TAF. There is a 30% chc that
cigs will arrive as early as 04Z Mon.

&&

.MARINE...13/717 AM.

Across the outer waters, winds will drop below Small Craft
Advisory (SCA) levels this morning. Seas may linger at SCA levels
into the afternoon. Then, SCA conds are not expected thru Thu
night

For the inner waters N of Pt Conception, the Santa Barbara
Channel, and the southern inner waters, SCA conds are not
expected thru Thu tonight.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...Small Craft Advisory in effect until 3 PM PDT this afternoon
      for zones 670-673-676. (See LAXMWWLOX).

&&

$$

PUBLIC...Hall/Sirard
AVIATION...Lund
MARINE...DB/Lund
SYNOPSIS...Hall

weather.gov/losangeles

Experimental Graphical Hazardous Weather Outlook at:
https://www.weather.gov/erh/ghwo?wfo=lox