321
FXUS66 KSGX 152105
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
205 PM PDT Tue Jul 15 2025

.SYNOPSIS...
High pressure will weaken as a weak upper level low pressure
system off the central coast of Baja moves into northern Baja
Thursday night and into southern California on Friday. There will
be cooling for today and Wednesday with a chance of afternoon
showers and thunderstorms, mainly for the mountains and mainly on
Thursday and Friday with decreasing chances on Saturday afternoon.
Then drier and a little warmer inland for Sunday into early next
week. Night and morning coastal low clouds will spread into the
western valleys late each night.

&&

.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...

Tonight through Friday...

Currently, high pressure conditions have continued to dominate the
region, with troughing upstream allowing for a break down in the
ridge and better onshore flow. This has resulted in a cooldown,
which will continue into tomorrow, with high temperatures within the
Imperial Empire (IE) running around 10 to 15 degrees cooler than
the seasonal average for this time of year. Latest GOES-19 Water
Vapor Imagery reveals mid level moisture is still confined to the
southeast of the CWA for the time being. This will remain to the
south through tomorrow, with relatively drier still in place for
the mid-levels. As far as the marine layer is concerned, there
will be a slightly further eastward resurgence by later this
evening as ceilings are slightly lifted with the advancement of
troughing and lower heights aloft. There will also be some patchy
fog for inland areas, and where the marine layer intersects the
mountain ranges. Temperatures tomorrow will be even slightly
cooler from where they were today.

By Thursday, the U/L low is going to slide to the north and be
located roughly just south of San Diego County by early morning,
which is going to allow for moisture at the 700-500 mb level to be
advected up over the region from the southeast. This could do either
one of two things; this could keep things too cool and not allow the
convective temperature to reach the value needed to initiate
thunderstorm development and prevent it if these mid-level clouds
move overhead too early in the morning, or if it moves over a little
later, it could help to add more moisture and destabilize the
mid- levels, to allow for more thunderstorm development,
especially over the mountains of San Diego County. That being
said, some fg the high res models were pinging a vort max by
later in the afternoon which has a vort max over the Mt. Palomar
area by the afternoon, which moves over into the Escondido area,
which might materialize into a storm and may move off the mountain
into the inland valleys, but likely fall apart shortly after
given how much more stable it will be west of the mountains. By
Friday, as the moisture in place remains and the U/L low
propagates further towards the northeast and minor troughs
circulating around the low, along with more a conditionally
unstable environment and MUCAPE values approaching 1000 J/kg,
there will likely be a lot more thunderstorm development, mainly
over the San Bernardino Mountains. Given the weak flow aloft,
these storms will also be slow-moving, and could result in a flash
flooding potential, especially for the Line and Bridge Burn Scar
areas. Also, with the mid to upper level low being out of the
northeast around the U/L low, there could also be a few of these
storms spilling over the the I.E. and portions of the

Saturday through early next week...

As the U/L low continues to eject into the Mojave Desert and
northeast of the region, it will allow for the mid-level flow to
return to being southwesterly and drier, which will hinder the
development of thunderstorm development during the afternoon hours.
There will be some residual moisture moving around the U/L and over
the upper deserts and within proximity of the San Bernardino
Mountains, which will be the most likely areas for any chance of
development. By Sunday, even drier air is advected in with even
further stability, so it is becoming more apparent that there will
be little to no development of any showers/storms during the
afternoon on Sunday over the mountains and deserts. There will
also be a gradual warming trend as the ridge begins to slightly
strengthen again, and this will continue into early next week.

&&

.AVIATION...
152030Z....Coast/Valleys...Low clouds will push inland after 00Z and
gradually almost fill the coastal basin overnight into Wednesday.
Bases will be 1000-1500 feet MSL with tops to 2000 feet. Vis will be
2-5SM under the inland clouds. Scatter out Wednesday by 17-19Z, but
only partially along the coast.

Mountains/Deserts...Mostly clear skies with unrestricted VIS through
early Wednesday. Increasing clouds above 15000 feet MSL Wednesday,
with cumulus at 10000 feet forming over the mountains 18-01Z.

&&

.MARINE...
No hazardous marine conditions are expected through Sunday.

&&

.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.


&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.

&&

$$

PUBLIC...Stewey
AVIATION/MARINE...KW