115
FXUS66 KSGX 150947
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
247 AM PDT Sun Sep 15 2024

.SYNOPSIS...
Slightly cooler with a deep marine layer and breezy winds in the
deserts. A low pressure system moving through on Monday will
bring gusty winds, more significant cooling, and light showers
along and west of the mountains. A slight warming trend will
start on Tuesday but another low will enter southern California
by Thursday and could bring more light precipitation and cooler
weather. Warmer Friday into the weekend as ridging builds in from
the west.

&&

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

Marine layer low clouds have spread into the foothills of the
mountains and is creeping through the Cajon Pass this morning.
Patchy drizzle is possible this morning, mainly across the valleys
where the cloud deck appears to be thicker. Clouds will slowly
clear back towards the coast by the early afternoon. Above the
marine layer, mostly clear skies will prevail.

An upper level low dropping south through the Pac NW will help
amplify the trough along the West Coast, bringing slight cooling
with high temperatures 5 to 10 degrees below normal, and as much
as 15 degrees normal in the valleys. Onshore flow will begin to
strengthen this afternoon producing gusty west winds across
portions of the desert mountain slopes into the deserts where
gusts of 35-45 mph will occur.

The upper low moves south and then east across Central CA into
the Great Basin late tonight into Monday night. The initial short
wave embedded in the mean trough is forecast to move through So
Cal tonight into Monday morning. This will help deepen the marine
layer to around 5500-6000 ft, producing light showers for the
coast to the coastal slopes of the mountains. For the coastal
areas and valleys, rainfall totals will generally be less than
one-tenth inch. High-res guidance still suggesting higher amounts
on the south facing slopes of the San Bernardino Mountains. The
HREF has a 50% chance of accumulations greater than 0.50" in this
area, and a 20% chance of greater than 1". The second and final
short wave associated with this low will move across Monday
evening as low-level flow turns northwesterly, which may help
showers persist into Monday night, mainly across San Diego
County. Showers should end by early Tuesday morning. Monday will
also be the coldest day of the week with highs 15-25 degrees below
normal inland, and around 5-10 degrees below normal near the
coast.

Winds will also strengthen Monday afternoon and evening, strongest
below the San Gorgonio Pass and along I-8 near the Imperial County
border where there is a 70-80% chance of gusts over 50 mph. The
Wind Advisory may need to be expanded later today to cover the
San Gorgonio Pass and San Diego County Deserts for Monday
afternoon into Monday night. The Wind Advisory for the high desert
remains in effect from 7 Pm this afternoon through 9 PM Monday.

Brief warming will occur Tuesday with little change in
temperatures for Wednesday as the upper low departs but weak
troughing remains across the West Coast. Deep boundary layer
moisture will still be in place, but with a weakened/non-existent
marine inversion any cloud coverage will be rather patchy.
Another closed low dives down the coast and moves across So Cal
sometime late Wednesday into Thursday, though there remains spread
amongst the ensembles in exact timing. This low will bring
cooling, another round of breezy conditions in the mountains and
deserts, and potentially more light precipitation for the
mountains westward. The trough moves east with ridging eventually
building 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...
150930Z...Coasts/Valleys...Hazy conditions continue in the inland
valleys due to smoke from the Line and Bridge Fires in the San
Bernardino Mountains. Stratus with varying bases around 2600-3000ft
MSL and tops to 3200-3500ft MSL have filled in across all
coasts/valleys this morning. Similar to previous nights, FU 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 higher elevations. Clouds start
to clear in the San Diego County valleys 16z-17z, in the Inland
Empire 17-18z, and for the Orange County coasts by 19z. Clouds may
not break across the San Diego County coasts through the afternoon
(40-60% chance) but may experience intermittent scattering at times.
Stratus pushes back inland this evening after 01z with bases likely
a few hundred feet higher than this morning. Light rain showers
are also possible after 03z.

Mountains/Deserts...Smoke from the Line and Bridge fires will
produce localized VIS restrictions near the San Bernardino County
Mountains, and the Airport fire will produce similar conditions in
the Santa Ana Mountains. This will continue through at least Monday
morning. Strong west-northwest winds develop this afternoon/evening,
with gusts up to 40kts possible through the mountain passes,
continuing into Monday.

&&

.MARINE...
Expect light precipitation Sunday night 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 20-25 knots likely. Otherwise, no hazardous marine conditions
are expected through Thursday.

&&

.FIRE WEATHER...
The marine layer is around 3500-4000 ft deep this morning with
dry conditions prevailing above that elevation. For the Bridge and
Line Fires, relative humidity will drop as low as 15-25% late this
morning through the early afternoon with gradual improvements the
remainder of the afternoon with the onset of the sea breeze.
Stronger southwest to west winds this afternoon will be through
and below the Cajon and San Gorgonio Passes, with gusts generally
15-20 in the immediate vicinity of the fires, up to 25 mph on the
ridges. Cooler with much higher relative humidity and a 20-40% chance
of showers tonight into Monday evening. Rainfall amounts will
range from around 0.10 inch for the coasts/valleys to 0.25 inch
along the coastal mountain slopes with isolated amounts in excess
of 0.50 inch along the south facing slopes of the San Bernardino
Mountains. Onshore winds strengthen on Monday afternoon and
continue into Monday, especially across the ridges and desert
slopes of the mountains where gusts of 30-40 mph are likely.

&&

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


&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory from 7 PM this evening to 9 PM PDT Monday for
     Apple and Lucerne Valleys.

PZ...None.

&&

$$

PUBLIC/FIRE WEATHER...SS
AVIATION/MARINE...Munyan