Mira Loma, California Weather Forecast Discussion

475
FXUS66 KSGX 220339
AFDSGX
Area Forecast Discussion
National Weather Service San Diego CA
839 PM PDT Sun Sep 21 2025
.SYNOPSIS...
Widespread clouds and scattered light showers will continue
this evening, gradually shifting south and east into Monday and
out of the area by early Monday afternoon. Precipitation Chances
decrease for Monday night into Tuesday but could return for
Tuesday afternoon into early Wednesday, along with a slight chance
of thunderstorms mainly for areas north of San Diego County.
Drier weather returns for Thursday through next weekend, though
there could be isolated thunderstorms in the mountains on Saturday
afternoon. Temperatures will be mild this coming week, with
decreasing coastal low clouds.
&&
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...
Evening update...
A band of light showers continue to move across San Diego County
and into the low deserts this evening with more isolated coverage
northward. Rainfall rates have been limited to a few hundredths
inch per hour. Despite Precipitable Water of 1.65" on the 00Z KNKX
sounding, the better forcing remains well off the coast closer to
the upper low, and a pretty substantial subcloud dry layer exists,
hence the light precipitation we`re seeing. This band of
precipitation will slowly shift south and eastward as the upper
low drops southward, then ending across San Diego County by late
monday morning or early Monday afternoon. A few isolated showers
could develop over the mountains Monday afternoon (15-20% chance),
but that is highly depending on any clearing of mid/high clouds
and how much residual moisture will remain. Though a marine
inversion remains on the sounding, the thick cloud cover will
disrupt stratus formation, with only patchy coverage of low clouds
and fog, if any, tonight in the coastal areas.
Previous discussion....
Numerical models are in good agreement through Tuesday, and the
closed upper low will likely move southward becoming briefly cut-
off on Tuesday about 360 miles southwest of San Diego. Showers,
and possibly thunderstorms, could develop over the mtns Tue pm and
continue into Wed over portions of Orange, western Riverside and
western San Bernardino Counties.
Based on current model consensus, the low will likely move in an
arc - first northwards then southeast - moving directly over
SoCal next Saturday. Most guidance has now reduced the chances for
showers/tstms on Tuesday as the best instability and forcing will
be to the west and north. Rainfall amounts will generally be less
than a tenth of an inch, although, there could be higher amounts
along the west-facing slopes of the foothills, and over the
mountains, where there could be amounts ranging from 0.10-0.25
inches. Any thunderstorms that develop may also result in locally
higher amounts. The marine layer will also be disrupted by this,
and will attempt to reform and move back inland by much later in
the night.Temperatures will remain on the cooler side, with the
cut-off low over the region, and even slightly below the seasonal
average for this time of year, especially for the inland areas and
deserts.
By Thursday, the position of the upper low will allow for a bit
of a lull in terms of the chances of precipitation as the region
becomes more "dry-slotted", and temperatures will also be slightly
warmer as well. The upper low will then begin to transition
slowly towards the southeast.
By Friday, model solutions begin to diverge, and it will all
depend on where the closed low goes will determine how much of a
chance of showers and thunderstorms there will still be with us on
Friday and Saturday, for the mountains during the afternoon
hours. While there is no consensus of solutions, a number of them
do indicate slight chances for showers/tstms in the mtns on Sat
afternoon. High temperatures will remain on the cooler side as
overall troughing remains persistent over the region going into
the following weekend, and ensembles also confirm this, with the
relatively cooler and drier conditions locked in through then.
&&
.AVIATION...
220245Z....Coast/Valleys...Intermittent -SHRA/-RA  through about 17Z
Mon. FEW-SCT coastal low clouds with bases 500-900 ft MSL and mid to
high level clouds AOA 7500 ft MSL tonight. Low cloud coverage is
expected to be patchy overnight. Any low clouds that develop will
clear by 17z Mon. Local vis restrictions 2-4SM BR/HZ possible 14-
16Z.
Mountains/Deserts...Mid and high level SCT-OVC clouds AOA 10,000
ft MSL into Monday. Intermittent SHRA/-RA expected through
tonight. San Diego County mtns could see showers into Mon
afternoon. VIS and CIGs will briefly lower any shower in the mtns.
&&
.MARINE...
Light rain showers possible into Monday morning which could briefly
lower visibly. There is as 5-10% chance of a thunderstorm developing
Tuesday. Any storm may produce lightning, gusty and erratic winds,
and choppy seas. No hazardous marine conditions are expected
Wednesday through Friday.
&&
.BEACHES...
Thunderstorm chances return on Tuesday, peak Tuesday afternoon
(10% chance), and may prevail into early Wednesday (5% chance).
&&
.SGX WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&
$$
PUBLIC...SS/PG
AVIATION/MARINE/BEACHES...CO

NWS SGX Office Area Forecast Discussion