285
FXUS66 KSGX 201623
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
923 AM PDT Fri Sep 20 2024

.SYNOPSIS...
A low pressure system moving across southern California will
bring a slight chance of mostly light showers this morning. There
are greater chances for showers along with a chance of
thunderstorms for this afternoon and early evening for
southwestern San Bernardino and western Riverside Counties with
the greatest potential for any heavier rainfall in the San
Bernardino Mountains. For Saturday through Monday, dry with a
warming trend with high temperatures warming to around 5 degrees
above average for inland areas on Monday. Generally fair weather
with seasonal temperatures will continue for Tuesday through
Thursday, but with greater spread in the temperature guidance for
the coast and valleys.

&&

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

This morning...The low pressure system over SoCal is presenting a
scenario whereby we have quite different moisture and stability
profiles over San Diego County versus southwestern San Bernardino
County. This variation in profiles is manifested in the sensible
weather. In San Diego County, low-level convergence in relatively
stable conditions where most of the moisture is in the boundary
layer is resulting in light, isolated showers. In the Mojave
Desert of western San Bernardino and Kern Counties, an organized
band of tstms will move to the east-southeast increasing in
coverage this afternoon. Shower and tstm activity will reach the
lower Colorado River valley around 8 pm today. Once the low has
moved to the east, conditions will become generally more stable
and the large field of marine layer low clouds over the water
will again move inland, reaching the coastal mtn slopes tonight.

Temperatures today will remain well-below seasonal averages, as much
as 15 to 20 degrees below average for the mountains and inland valleys,
with high temperatures for the Inland Empire mostly in the 70s.

Dry and warmer conditions are expected this weekend. On Saturday,
greater warming is expected for the mountains and deserts with
slower warming for the coast and valleys. Warming will continue on
Sunday with the greater warming for the coast and valleys. High
temperatures on Sunday will return to around seasonal averages,
with high temperatures in the mid 80s to mid 90s for the Inland
Empire and around 100 for the lower deserts.

The warming is expected to continue into Monday, with high
temperatures for inland areas around 5 degrees above average,
mostly in the 90s for the Inland Empire and around 105 for the
lower deserts.

The marine layer will persist and low clouds will cover much of
the area west of the mtns each night and morning.

Generally fair weather with seasonal temperatures will continue
for Tuesday through Thursday, but with greater spread in the
temperature guidance for the coast and valleys.

&&

.AVIATION...
201500Z...Coast to Mountain Slopes...Low stratus remain in place
this morning, with the greatest coverage against the coastal
mountain slopes, the eastern Inland Empire, and San Diego County
hills/valleys, while the coastal basins are somewhat patchy. Bases
are predominantly 3500-4500ft MSL, but intermittent CIGs around 1500-
2500ft MSL are possible at times until 16-18z, especially along the
coasts. Patchy, intermittent -RA or -DZ is possible along the coast
and up against the coastal mountain slopes at times until 18Z.
Clouds generally becoming SCT for most areas after 18z, while the
coastal slopes and parts of southern SD Co. may see higher coverage
linger into the afternoon. Low stratus clouds rebuild again between
03-06z this evening, with bases around 1500-2500ft MSL lowering
slightly overnight. Clouds clearing to the coast 17-19Z Sat.

20 Fri-02Z Sat, there is a 25-45% chance of TSRA over the eastern
Santa Ana Mtns and Inland Empire. Erratic winds, local moderate
up/downdrafts, and reduced vis in locally heavy rain expected near
thunderstorms.

Mountain Ridges to Deserts...50-60+% chance for RA and TSRA over the
San Bernardino mtns, Riverside mtns, and High Desert 19Z-02Z this
afternoon/evening. CU/CB based around 7-10k ft MSL. Erratic winds,
local moderate up/downdrafts, and reduced VIS in locally heavy rain
expected near thunderstorms. Skies clearing late this evening into
the overnight.

&&

.MARINE...Patchy, intermittent rain showers this morning.
Otherwise, no hazardous marine conditions are expected through
Tuesday.

&&

.HYDROLOGY...
A low pressure system moving across southern California will
continue to bring isolated to scattered mostly light showers this
morning. There are more organized showers, along with a chance of
thunderstorms for this afternoon and early evening, to move
southeastward across southwestern San Bernardino and western
Riverside Counties. The greatest potential for any heavier
rainfall is in the San Bernardino and Riverside County Mountains.
Hourly rainfall rates of one-tenth to four-tenths of an inch will
be possible, with totals of up to one-half in the San Bernardino
Mountains by about 8 pm.

The most likely location for any mud and debris flows from recent
fires would be with the Line Fire in San Bernardino County. While
much of the higher resolution guidance shows peak hourly rainfall
rates to around four-tenths inch, there is greater uncertainty in
the timing of any heavier rainfall during the afternoon. Some of
the guidance has the peak earlier in the afternoon while some of
the more recent HRRR model guidance is later in the afternoon,
between about 4 and 7 pm.

The current local NAM-bounded WRF model solution is showing an
overall slight slowing of this low pressure system. Indications
are now for showers/tstms persisting in extreme southwestern San
Bernardino County and western Riverside County until 7-8 pm today.

&&

.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...PG
AVIATION/MARINE...CSP