090
FXUS66 KSGX 160429
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
930 PM PDT Sun Sep 15 2024

.SYNOPSIS...
Light rain will occur over and west of the mountains late tonight
and Monday, mainly in the morning. It will continue to be cooler
Monday with strong gusty winds in parts of the mountains and
deserts. Brief warming will occur Tuesday before cooler windy
weather returns by Thursday, along with a chance of light rain.
Warmer weather will return Friday through next weekend as ridging
builds in from the west.

&&

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

Update... Marine layer clouds are already widespread and extend up
to over 5000 feet in parts of the mountains, especially in San
Bernardino County. The 00Z NKX sounding already had 0.82" for
precipitable water, and this was almost entirely from the marine
layer. Mid-level water vapor satellite imagery shows the low a bit
west of San Francisco, and that low will gradually move into the
Great Basin. Height gradients will gradually increase overnight with
tight gradients aloft through Monday night, though the trough axis
will not actually move through until very early Tuesday morning.
Moisture will be sufficient for light rain, and orographic precip
will occur along the SW mountain slopes, especially in San
Bernardino County where 850 mb winds will be 15-25 knots and nearly
perpendicular to the mountain crests in a nearly saturated
atmosphere. While MOS is running quite low for precip chances due to
limited statistical data for stratiform rain in the dry season,
ensemble precipitation chances are quite high, with most solutions
from GEFS, ENS and HREF showing measurable precipitation from the
coast to the mountains Monday morning, albeit mostly 0.10" or less,
except on orographic slopes where 0.25" to locally over 0.50" is
expected. Therefore, I increased the PoPs to "likely" category,
generally 60-70% over most of the area. With that trough axis not
moving through until early Tuesday morning and a continued deep
marine layer, there could be a bit more rain, albeit light, as late
as a little after sunrise Tuesday.

Temperatures will be well below normal with upper 60s-lower 70s west
of the mountains and only in the 80s in the lower deserts Monday. In
many areas these will be the lowest highs since early June, with the
deserts having possibly the lowest highs since May. Monday night
could have lows near freezing at Big Bear, though snow chances are
minimal as the freezing/sub-freezing air will be above the moisture.

Thursday still has a chance of rain, and GEFS/ENS are showing a lot
of ensemble members with at least light precipitation. Instability
could be a little greater as the closed upper low could be more over
SoCal versus central California, so the rain could be more showery
then.


From previous discussion...

An upper level low pressure system digging south into California
from the Pacific Northwest will increase onshore pressure gradient
for Monday. The increased onshore flow will deepen the marine layer
to 5500-6000 ft and will produce areas of light showers west of the
mountains and on the coastal mountain slopes. For the coastal areas
and valleys, rainfall totals will generally be 0.05 to 0.15 inches.
High-res guidance is still suggesting higher amounts on the south
facing slopes of the San Bernardino Mountains. The HREF has a 50-70%
chance of total rainfall accumulations greater than 0.50 inch for
portions of the coastal slopes of the San Bernardino mountains, and
a 20% chance of greater than 1 inch. Much of the shower activity
will occur Monday morning. More isolated shower activity is possible
into Monday afternoon along the coastal slopes of the mountains. An
embedded shortwave will move across the area Monday night into early
Tuesday. That may spark another round of isolated showers Monday
night into early Tuesday, most likely in San Diego County.

Southwest to west winds will begin to increase this afternoon and
evening for the desert mountain slopes, deserts, and passes. Peak
wind gusts for the desert slopes of the mountains and into the
deserts will be 35 to 45 mph, with a few local gusts to 50 mph
possible through the San Gorgonio Pass. Winds are expected to
strengthen for Monday afternoon and evening. The strongest winds are
expected through passes and into the deserts. There is a 60 to 70%
chance of gusts greater than 55 mph below the San Gorgonio Pass and
along I-8 near the Imperial County border. Forecast peak gusts are
be 45 to 55 mph, with a few local gusts to 60 mph possible.

Deep boundary layer moisture will still be in place for the middle
of the week, but with a weakened/non-existent marine inversion cloud
coverage may be patchy. Brief warming will occur Tuesday with a
minor cooling on Wednesday as another trough digs south along the
California coast. There remains spread amongst the ensembles in
exact timing of the trough passage. This introduces some uncertainty
in the forecast, particularly surrounding high temperatures on
Wednesday and Thursday as well as timing of any precipitation and
gusty winds. Current forecast has minor change in temperature from
Wednesday to Thursday and best chance of precipitation Wednesday
night into early Thursday for areas in and west of the mountains.
The forecast also has the strongest winds Wednesday night into
Thursday for desert mountain slopes, deserts, and through passes. As
the trough moves east ridging will build in from the west for Friday
into the weekend for warmer and fair weather. There is the potential
for weak offshore flow sometime next weekend as an upper low/inside
slider moves into the Great Basin, but confidence is low. If any
offshore flow develops, it would contribute to additional warming
west of the mountains.

&&

.AVIATION...
160320Z...Coasts/Valleys...Hazy conditions continue in the inland
valleys due to smoke from the Line and Bridge Fires in the San
Bernardino Mountains.

Low clouds with bases varying 2000-3000ft MSL and tops to 4000ft MSL
continue to increase in coverage at this hour, covering all of San
Diego and Orange counties west of the mtns. By 15Z expect all areas,
from the coast to the coastal mtn slopes, to be covered by low
clouds. CIG impacts are likely at all TAF sites except KPSP and KTRM.
Light rain/drizzle could develop after 09z, with very low
accumulation expected. Similar to previous nights, smoke from the
Line Fire may be trapped by the marine later and result in VIS
reductions down to 3SM for KSBD with the only other VIS reductions
seen where marine clouds intersect with terrain. Clearing on Monday
will likely be similar to what was observed today.

Mountains/Deserts...Smoke from the Line and Bridge fires will
produce localized VIS restrictions VCNTY San Bernardino County
Mountains, and the Airport fire will produce similar conditions in
the Santa Ana Mountains through Monday. West-northwest winds
strengthen this evening, with gusts up to 40kts possible through the
mountain passes, continuing into Monday. Light rain will be possibly
tonight into Monday for the coastal slopes of the mountains but
little impacts expected at TAF sites KONT and KSBD.

&&

.MARINE...
Expect light precipitation late tonight through Monday morning, with
less than 0.10" expected. Winds will strengthen from the west to
northwest in the outer coastal waters Monday and Monday night, with
gusts up to 20kts likely. Otherwise, no hazardous marine conditions
are expected through Friday.

&&

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


&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory through 9 PM PDT Monday for
     Apple and Lucerne Valleys.

     Wind Advisory from 11 AM Monday to midnight PDT Monday night for
     San Diego County Deserts.

     Wind Advisory through midnight PDT Monday
     night for San Gorgonio Pass Near Banning.

PZ...None.

&&

$$

PUBLIC...Maxwell (Update)/CO (Previous Discussion)
AVIATION/MARINE...PG