429
FXUS66 KSGX 300357
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
857 PM PDT Sun Sep 29 2024

.SYNOPSIS...
Hot temperatures weather will occur through the week continue
through the coming week for inland areas with a shallow marine
layer keeping temperatures at the coast moderated, with low
clouds and patchy dense fog each morning. High pressure will slowly
ease its grip by the end of the week, with a subtle decrease in
temperatures.

&&

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

Evening update...
Low clouds stuck to the San Diego County coast again today, but
have started to retreat this evening. The 00Z KNKX sounding shows
the marine inversion based near 900 ft MSL, down slightly from
this morning. While some stratus development is expected
overnight, less inland extent will occur as surface pressure
gradients trend offshore (mainly to the north) and the marine
layer remains more shallow. Patchy dense fog is possible where
clouds develop over higher coastal terrain and on the coastal
mesas. Otherwise skies will be clear tonight, except for areas of
smoke in the vicinity of the Line Fire.

Previous discussion...
.SHORT TERM (Through Wednesday)...
Our inversion has held strong today, keeping low clouds near the
beaches early this afternoon. Slow lower level mixing will allow
for clearer skies to occur over the next couple of hours. A weak
low is crossing our region overhead, leading to the coolest day
for most areas away from the coast as head into the work week.
High pressure off the coast today will move eastward across the
desert southwest, bringing increased heat impacts to our region
this week. Temperatures will increase near 5 to 10 degrees from
what we`ve seen today for inland valleys west of the mountains
into the high desert with a moderate to high chance in seeing
triple digit readings in the Inland Empire. The marine layer will
become slightly more shallow, keeping low clouds and areas of
dense fog closer to the coastal areas by Monday morning. This
coastal fog pattern will continue into at least Wednesday.

The ridge of high pressure will strengthen further into Tuesday
and Wednesday when the hottest weather is expected overall as 850
mb temps rise near 28-30C. Latest forecast guidance continues to
trend hotter, so forecast highs have been increased to account
for this as our confidence grows in this forecast. We have also
issued another Excessive Heat Warning across the Lower Deserts and
Heat Advisories for the Inland Empire and San Diego Co valleys as
major heat risk is expected from Tuesday through at least
Thursday. As of now, Wednesday looks to be the hottest day across
the region. NBM probabilities are moderate to high in seeing
temperatures of at least 105 degrees across the Inland Empire.
Probabilities for triple digit heat across San Diego county
foothills like Ramona have similar chances as well. 80s and 90s
will be common away from the immediate coast in Orange and SD
Counties. Please be safe in the heat by avoiding outdoor
recreation in the afternoon hours and drink plenty of fluids if
you must be outside.

&&

.LONG TERM...(Thursday into Next Weekend)...
By Thursday, models are in good agreement of the high weakening
some as a weak trough lingers farther off the coast. This should
help increase onshore flow, leading to subtle cooling for areas
west of the mountains. Confidence is still low on how much we are
able to cool, but one thing is certain that temperatures will
continue to remain much above normal through the end of the week.
The Pacific trough that will near that PacNW that we have been
mentioning the past few days that may help us cool off...well,
that is looking less likely. Long range models are now coming into
fairly good agreement of the ridge anchoring over the desert
southwest into the end of the week. We may be able to be far
enough away from center of the high pressure and closer to the
weak trough off the coast, which would help us cool off a bit
more by late week, but confidence is low on where the ridge and
the trough set up.


&&

.AVIATION...
300330Z...Coasts/Valleys...Low stratus clouds have developed along
the coast of San Diego County this evening, bases around 800-1000 ft
MSL, making their way inland up to 10 mi. Cigs will lower overnight
to 500-700 ft MSL with tops to around 1200 ft MSL. Clouds will
likely be patchy with intermittent cigs and periods of scattering
through the night due to a weak low pressure system moving into the
region. Similarly intermittent FG across the western valleys and
coastal terrain with VIS down to 2-5SM, locally dense areas down to
1SM or less for the higher coastal terrain. Low clouds clearing to
beaches by 15-17Z.

Mountains/Deserts...Mostly clear with generally unrestricted VIS
through Monday. Localized areas of VIS reductions possible near and
to the north of the Line Fire in the San Bernardino Mountains due to
FU.

&&

.MARINE...
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...Heat Advisory from 10 AM Tuesday to 8 PM PDT Wednesday for San
     Bernardino and Riverside County Valleys-The Inland Empire-
     San Diego County Valleys.

     Excessive Heat Warning from 10 AM Tuesday to 9 PM PDT Thursday
     for Coachella Valley-San Diego County Deserts.

PZ...None.

&&

$$

PUBLIC...SS/APR
AVIATION/MARINE...CSP