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Forecast Discussion for San Diego, CA

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911
FXUS66 KSGX 140442
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
930 PM PDT Sun Jun 13 2021

.SYNOPSIS...
High pressure will strengthen and expand westward over the Desert
Southwest through the week. This will bring a long duration heat
wave to much of the region, with well-above average temperatures
peaking Tuesday through Saturday. A sea breeze and shallow marine
layer will keep temperatures moderated in the coastal areas, along
with some patchy night and morning low clouds and fog at times. The
ridge over the Southwest will keep it hot well inland into next
weekend, with slow cooling expected across the entire region by
early next week.

&&

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

Update...

Low clouds covered parts of the San Diego County coast with only a
few high clouds elsewhere. Some stratus will continue along the
coast overnight, extended 5-10 miles inland at times, though it will
be patchier in Orange County. Regarding the heat wave, we are still
on track for very high temperatures, possibly up to 120 in the lower
deserts, with the ECMWF ensemble MOS showing the maximum solutions
of 122 for Friday at both PSP and TRM. Since some southeasterly flow
will occur around the interior upper high, some moisture could
result in some cumulus over mainly the mountains and could slightly
moderate temperatures at times. Surface moisture will be mostly
limited, but if a surge of moisture near the surface reaches the
lower deserts from the Gulf of California, there could be a drop in
daytime temperatures, though not necesssarily a drop in the heat
index due to the effects of the higher humidity. Latest indications
are for no cooling until at least next Sunday when the high could
weaken a bit as a trough of low pressure moves through the northern
parts of the Western US. No changes were made to the forecast this
evening.

From previous discussion...

Southern California is currently sandwiched between a broad upper
level trough to the northwest and a strengthening upper level
ridge of high pressure to the east today. The area of high
pressure will begin to nudge its way west through the week,
peaking around 600 DM, which is quite strong! This feature will
bring a prolonged heat wave with temperatures over 20 degrees
above average for some of our cities across the region. Excessive
Heat Warnings are now in effect for all of the deserts, mountains,
and eastern valleys from the Inland Empire to the San Diego
valleys. Temperatures will climb from 100 to 110 degrees across
the high desert and Inland Empire. The mountains will be
especially hot with daytime highs in the 90s to near 100 for many
communities. If you must be outside or decide to hike, try to do
so during the early morning or evening hours as temperatures will
be quite hot for the afternoon. As the strong ridge moves west,
850 mb temps reach their maximum values later in the week over the
lower deserts. Temperatures will soar to near 120 degrees each
afternoon in some spots across the low deserts including Palm
Springs for much of the work week.

The only area that will see a bit of relief from the heat will be
the spots near our pristine SoCal coastline as the cool waters of
the Pacific provide a thin marine layer, which will keep these
coastal areas moderated. The warmest day looks to be Tuesday with
most beaches in the 70s and 80s a few miles inland. A coastal eddy
continues to be depicted from local hi-resolution models at this
time, which would bring cool southerly winds off the ocean and
provide some cooling starting on Wednesday. There is still quite a
bit of uncertainty on this occurring, so temperatures across this
region may increase in later forecasts. This region will also see
patchy low clouds and fog for some parts of the week as the thin
marine layer remains in place.

Long range models are in agreement of the strong high pressure
system slowly breaking down into early next week. This will lead
to breezier winds across the mountains and deserts, along with
more cooling spreading across the region. We will continue to
track how long this heat event will last as we get closer to next
weekend.

&&

.AVIATION...
140400Z...Coast...Areas of stratus along the San Diego County coast
will spread north and 5-10 mi inland overnight, reaching KSAN and
KCRQ between 05-08Z and KSNA around 10-12Z. Bases will be near 700-
1000 ft MSL. Vis locally reduced to 3-5 SM over higher coastal
terrain. Clouds and vis restrictions clearing 15-17Z. After 17Z, FEW-
SCT clouds AOA 15,000 ft MSL and unrestricted vis through Monday
night.

Valleys/Mountains/Deserts...Clear with unrestricted vis through
Monday morning, followed by FEW-SCT clouds AOA 15,000 ft MSL Monday
afternoon through Tuesday.

&&

.MARINE...
Northwest winds gusting 15-20 knots will develop over the outer
coastal waters each afternoon and evening through Tuesday. No
hazardous marine conditions are expected later in the week.

&&

.FIRE WEATHER...
Increasing temperatures and low daytime RH values will result in
elevated fire weather conditions in the afternoons and evenings
for the valleys, mountains, and desert slopes. The driest day
looks to be on Tuesday, as RH values drop to the single digits for
many of these geographic regions with subtle increases expected
through the week. The main concerns will be with the inland push
of the sea breeze in the afternoons and evenings, especially
through and below the desert passes...and the coming extreme heat
(temperatures 15-20 degrees above average), resulting in very high
mixing heights, creating the potential for large, plume-
dominated fires over the far inland valleys and mountains.

These elevated fire weather threats while not critical, will be
greatest Tuesday through at least Friday. Gradual cooling and
increasing humidity will help to ease conditions over the weekend
and into next week.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Excessive Heat Warning from 10 AM Monday to 9 PM PDT Saturday
     for Apple and Lucerne Valleys-Coachella Valley-San Diego
     County Deserts-San Gorgonio Pass Near Banning.

     Excessive Heat Warning from 10 AM Tuesday to 9 PM PDT Saturday
     for Riverside County Mountains-San Bernardino County
     Mountains-San Bernardino and Riverside County Valleys-The
     Inland Empire-San Diego County Mountains.

     Excessive Heat Warning from 10 AM Tuesday to 9 PM PDT Friday for
     San Diego County Valleys.

     Excessive Heat Watch from Tuesday morning through Thursday
     evening for Orange County Inland Areas-Santa Ana Mountains
     and Foothills.

PZ...NONE.
&&

$$

PUBLIC/FIRE WEATHER...Maxwell (Update)/APR (Previous Discussion)
AVIATION/MARINE...Gregoria

Previous Forecast Discussions may be found at
NWS San Diego, CA (SGX) Office Forecast Discussions.
(Click 'Previous Version' there to view past versions successively.
Some may differ only in time posted.)

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