Upland, California Weather Forecast Discussion

204
FXUS66 KSGX 170441
AFDSGX
Area Forecast Discussion
National Weather Service San Diego CA
941 PM PDT Tue Sep 16 2025
.SYNOPSIS...
Remnants of Tropical Storm Mario will bring increasing moisture and
clouds to the area for the middle of the week. There are chances of
showers and thunderstorms for the mountains and deserts from this
afternoon through early next week. For Wednesday through Friday
morning there are chances for showers and thunderstorms west of the
mountains. The marine layer will become shallower with the potential
for minimal or no low cloud coverage for the middle to end of the
week.
&&
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...
Update: The latest update by the Joint Typhoon Warning Center (JTWC)
on now post-tropical Mario shows it continuing to drift towards the
northwest. The remnants of this system, with the first initial bands
will begin to impact the CWA beginning by around 10 AM tomorrow
morning. The WRF shows a few storms already beginning to develop by
around that time over the mountains, or shortly after, although the
HRRR (with a drier bias) does not have convection initiating over
the mountains until closer to Noon. The first bands will arrive
right off the coast around the same time, and there is only slight
deviation between the CAMs reflecting this. It still appears that
the better chance of getting measurable precipitation for the
lower elevations throughout the afternoon and evening hours
tomorrow, with likely a few heavier showers and thunderstorms
embedded within the bands of rain that move over. During the
afternoon hours, there will also likely be some more thunderstorms
developing over the mountains and deserts. It does appear with
the post-tropical low ejecting out of the region and up to the
north on Thursday, that the flow will allow for any storms that do
develop over the mountains and deserts to drift towards the
northeast an not be of much concern to the inland areas. That
being said, there could still be some areas receiving showers and
possibly a thunderstorm, and with PWATs of 200%, it is likely that
these heavier showers/thunderstorms will put down a quick amount
of rainfall in a short period of time, and this may lead to flash
flooding concerns, especially for the burn scars. A chance of
thunderstorms will still exist during the afternoon hours on
Friday over the mountains and deserts, and this chance will
continue going into the weekend, although likely with much less
coverage by Sunday. The extended forecast going past this upcoming
weekend still looks to be on track, showing a gradual warmup
going into early next week as ridging becomes more of the dominant
feature again.
(Previous discussion submitted at 144 PM):
Satellite at 1 PM was showing areas of cumulus clouds over the
mountains, locally into the valleys. No precipitation has been
detected thus far, although convection has developed in the
mountains south of the border. The 12Z KNKX sounding was showing
1.25 inches of precipitable water with 700 j/kg of most unstable
CAPE. One inhibiting factor is a fairly substantial inversion near
700 mb. This could limit convective development, even with the
otherwise favorable conditions. The slight chances of showers and
thunderstorms over the mountains has been maintained in the
afternoon forecast package.
Chances for showers and thunderstorms become more widespread for
Wednesday afternoon through early Friday morning. It looks like
there will be enough moisture and instability tomorrow afternoon for
thunderstorms to develop over the mountains, locally into the
deserts ahead of the main moisture surge. As we move into Wednesday
evening and Thursday morning, shower chances become more widespread
west of the mountains, with the potential for isolated thunderstorms
to develop. Shower coverage overnight may be fairly scattered. For
Thursday afternoon, shower and thunderstorm chances continue for the
inland valleys, mountains, and deserts. Depending on how strong the
easterly flow is, it`s possible thunderstorms could drift closer to
the coast. Flash flooding is possible where thunderstorms develop,
especially in the mountains and deserts. Otherwise, main impacts
will be frequent lightning and the potential for urban flooding
where showers/thunderstorms develop west of the mountains. Based on
the latest high resolution guidance rainfall rates west of the
mountains will generally be 0.25"/hr or less, but could be locally
higher where thunderstorms develop; in the mountains rainfall rates
look to be around 1"/hr; and 0.50"/hr in the deserts.
High resolution guidance is indicating some lingering isolated
heavier showers or thunderstorms overnight Thursday into Friday
morning. For Friday, it looks like the best chances of showers and
thunderstorms will be over the mountains potentially into the
deserts. Chances for afternoon showers and thunderstorms will
continue for the mountains, deserts, and potentially inland valleys
into early next week.
The increase in moisture and clouds, Wednesday and Thursday will
result in high temperatures for inland areas to cool several
degrees. High temperatures on Thursday will be as much as 8 to 12
degrees below average for the mountains. High temperatures for
inland areas will warm around 10 degrees from Thursday through
Sunday with Sunday high temperatures mostly a few to around 5
degrees below average. High temperatures on Sunday will range from
the mid 70s near the coast to the upper 80s to mid 90s for the
Inland Empire with the mid 90s to around 100 for the lower deserts.
There could be another influx of tropical moisture early next week
from a second tropical system that is yet to fully develop. More
information will be available in the coming days.
&&
.AVIATION...
170245Z...Coast...Low confidence in low cloud development tonight
through Wednesday evening. Very patchy low clouds 09-16Z Wed with
bases 300-700 feet MSL. Vis restrictions (1-4 SM) wherever low
clouds develop.
Regionwide...BKN-OVC high clouds around 10000 feet MSL through
Wednesday. SCT -SHRA and ISO TSRA begin to move in from the south
after 18Z Wed with cloud bases AOA 10K ft MSL. ISO TSRA continue
into Thursday with increasing storm chances for the mountains and
deserts.
&&
.MARINE...There is a slight chance of thunderstorms Wednesday
through Friday, with the greatest chance Wednesday evening and
overnight. Any storms could briefly produce lightning, gusty winds
and choppy seas. Quieter conditions expected over the weekend.
&&
.BEACHES...There is a slight chance of thunderstorms Wednesday
through Friday, with the greatest chance Wednesday evening and
overnight. Storms may be accompanied by cloud to ground lightning
and gusty winds.
KW
&&
.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.
&&
.SGX WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&
$$
UPDATE...Stewey
PUBLIC...CO
AVIATION/MARINE/BEACHES...KW

NWS SGX Office Area Forecast Discussion