681
FXUS66 KSGX 070417
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
917 PM PDT Tue May 6 2025

.SYNOPSIS...
A dry and warming trend sets in Wednesday and beyond with the
marine layer becoming shallower. High temperatures on Saturday
will be 10 to 20 degrees above average for inland areas. For
Sunday through Tuesday, a low pressure system moving along the
California coast brings a cooling trend with night and morning
coastal low clouds spreading farther inland and onto the lower
coastal slopes of the mountains by Monday or Tuesday.

&&

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

Update: The majority of any shower activity has now exited the CWA,
with the exception of a few lingering showers still possible over
portions of the high deserts. This is expected to all come to an
end by midnight. The marine layer has finally thickened up enough
to provide some light drizzle along some of the coastal and inland
areas this evening. This will likely continue into the early
morning hours tomorrow for a lot of the same locations, and then
begin to eventually break up and scatter out, which will give way
to mostly clear skies. There could still be some mostly cloudy
skies possible right along the extreme coastal areas of southern
San Diego County. It does appear that the marine layer will begin
to redevelop and push inland by later in the evening tomorrow,
although it doesn`t appear that this will be thick enough to
provide drizzle for tomorrow night, and ceilings will be lower.

As high pressure continues to build back in over the region
throughout the week and into the weekend as the ridge strengthens, a
warming trend will begin by Thursday and continue to get much warmer
and well above the seasonal average for this time of year. By
Saturday, highs are expected to be anywhere from 10 to 15 degrees
above average, with temperatures approaching (or possibly exceeding)
the 100 degree mark within the Imperial Empire. This could lead to
the need of there being possible heat advisories. The only caveat
which could slightly hinder temperatures from getting quite so hot
will be a weak upper level low transitioning over the region, as
deterministic models suggest. Ensembles still continue to display a
significant cooling trend after such a hot weekend, which will begin
by the early part of next week as a longwave trough begins to
propagate in from the northwest. There is more indication from the
models that the shortwave trough that could possibly provide precip
by late Tuesday into Wednesday will now likely remain too far to the
north to provide much of anything other than increased westerly
winds, however, this is still quite far out in the forecast period
and subject to change.

(Previous discussion submitted at 159 PM):

Visible satellite currently shows a developing cu field over the
High Desert and mountains, expanding as an upper level low swings
down from Nevada. Some precipitation is expected this afternoon as
this low propagates down, but with the greatest instability well to
the east of the region, only rain showers are expected with the
thunderstorm threat expected to remain out of the area. Latest
hi-res guidance has started showing some heavier rain rates for
this afternoon and evening across the High Desert, up to
0.25-0.35"/hr, with bodes well with PW values up to 0.80-1.00".
The rain generally will reach from Running Springs eastward, with
the heavier bands expected to be on the desert foothills of San
Bernardino County. Total rainfall amounts are expected to be
light, mostly below 0.10", but some heavier rain bands could
result in amounts up to 0.40" through late tonight. Any and all
precipitation should be out of the area by midnight.

The marine layer remains deep into Wednesday morning with fairly far
inland spread, but drier air near the surface may limit any drizzle
potential for Wednesday morning along the coast. By the afternoon,
the aforementioned low will be moving into eastern Arizona and New
Mexico with upper level ridging building overhead. Heights continue
to increase through the weekend, with temperatures 3 to 10 degrees
above normal by Thursday and 10 to 20+ degrees above normal by
Saturday. Saturday may be the hottest day of the year thus far, with
highs 105-110 for the low deserts and Coachella Valley, 97-101+ for
the Inland Empire, 90s for the valleys and High Desert, 80s for the
mountains, and upper 70s to mid 80s along the coast. Spread on the
models for the high temperatures is very small, with the 25th
percentile to 75th percentile within 2-3 degrees, indicating high
confidence in this heat. With decent consensus on the warm up,
future forecast packages may need to include Heat Advisories for
some of the hotter locations, somewhat unusual for early to mid
May.

The thermal ridge axis shifts slightly eastward on Sunday to set up
over Arizona, but highs on Sunday look to remain well-above normal,
potentially only 3-7 degrees below Saturday`s highs. The pattern
changes significantly by Monday as the next system, a large upper
level low off the coast of OR pushes down, and heights fall
quickly into mid week. Any associated precipitation with this next
system remains to be seen, but cooler weather should return as
early as Monday with highs falling to near normal, and continue
cooling into mid week.

&&

.AVIATION...
070430..Coasts/Valleys/Coastal Slopes...BKN-OVC cloud layers between
2000-3500ft MSL with tops to 5000ft covering the coastal basin this
evening, with localized SCT layer 1000-1500 ft MSL. VIS restrictions
limited to coastal foothill obscurations in FG and DZ. Low clouds
will remain in place across the coastal basin tonight. Expecting
scattering to the coast around 18z Wednesday.

Desert Mtn Slopes/Deserts...SCT-BKN debris clouds around 10,000ft
MSL present across the mountains and deserts this evening from TSRA
to the northeast. Winds gusting 30-35kts on mountain peaks and
through passes this evening...weakening overnight.

&&

.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.

&&

$$

UPDATE...Stewey
PUBLIC...Zuber
AVIATION/MARINE...PG