Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 142054

Area Forecast Discussion
National Weather Service San Diego CA
154 PM PDT Mon Jun 14 2021

A strong area of high pressure will dominate the weather pattern
this week, leading to a prolonged heat wave for much of the
region. High temperatures will climb 15 to 20 degrees above
average for inland areas, along with very warm overnight
temperatures inland as well. A shallow marine layer will result in
less warming for coastal areas, where patchy low clouds and fog
can be expected nights and mornings. The large area of high
pressure is expected to weaken and spread cooling inland by early
next week.



As you can probably start to feel today, the heat is on as a
strong area of high pressure centered over the Four Corners region
starts to expand into Southern California. Forecast models
continue to show this feature maximizing in strenght around 600
DM later in the week, quite strong, especially for this time of
year. Tuesday will be the hottest day of the heat wave for many
across the valleys west of the mountains. Portions of the eastern
valleys of San Diego County into the Inland Empire will see highs
from 100 to 110 degrees, which are prompting these areas to be
under Excessive Heat Warnings through the early part of the
weekend. This will not be an event where the mountains see relief
from the heat, as many areas in the mountains will be in the 90s
to slightly over the century mark. Forecast models continue to
pick up on some high level moisture making it into the area as
southwest flow aloft around the high pressure takes hold over the
next few days. This may bring some areas of high level cloudiness,
which may slightly inhibit high temperatures but not by much.
Mountains and deserts will continue to see breezy afternoon and
evening winds, leading to elevated fire weather conditions.

The coastal areas will be the only ones to escape the brutal heat
as a shallow marine layer will keep temperatures moderated.
Beaches can expect highs in the 70s with 80s just a couple miles
further inland. This shallow layer will lead to possible low
clouds and fog late nights and mornings intermittently throughout
the week. Inland valleys of Orange County will still be in the 90s
for much of the event, where a Heat Advisory has now been issued.

As the high pressure slightly weakens later in the week, the
monsoonal flow will soon be cut off, leading to a very dry air
mass. This will lead to the hottest day being in the desert by
Friday. Some places across this area will reach over 120 degrees,
which would be getting close to all-time high temperature records.
Not only will high temperatures be hot, but also overnight low
temperatures across the lower deserts will be quite warm with lows
in the upper 80s to near 90 from Palm Springs to Borrego Springs.
The deserts will see very hot temperatures through much of the
week, but cluster analysis points to the ridge finally breaking
down by early next week, which will lead to some relief from the
heat across Southern California.


142000Z...Coast...Areas of low clouds currently limited to the
beaches. Cloud bases are about 900 ft MSL with no impacts at TAF
sites. Patchy low clouds 09Z-16Z Tuesday. Cigs for any prolonged
duration not likely, but intermittently possible vcnty KSAN/KCRQ.
Bases would be around 400-700 feet MSL and vis around 3-6SM, locally
below 1 mile.

Otherwise...SCT clouds at or abv 20000 ft MSL and unrestricted
vis through tonight.


No hazardous marine conditions are expected through Friday. However,
northwest winds occasionally gusting over 20 knots are expected this
evening and Tuesday evening over the outer coastal waters.


Very hot temperatures through much of the week and low daytime
relative humidity for the deserts, mountains, mountains and also
into the inland valleys early this week. High temperatures will
be around 15 to 20 degrees above average for inland areas with
lowest humidity in the deserts falling to around 10 percent each
day. The driest day will be Tuesday, where some locales across the
mountains and desrts will drop into the single digits.

The low humidity combined with gusty southwest to west winds each
afternoon and evening in the mountains and deserts will bring
elevated fire weather conditions each afternoon for the deserts
onto the desert slopes of the mountains. This will also lead to
very high mixing heights each morning and afternoon, which will
increase the potential for plume-driven fire growth.

The area of high pressure will begin to break down by early next
week, with cooler weather and higher humidity spreading inland.


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


CA...Excessive Heat Warning until 9 PM PDT Sunday for Coachella
     Valley-San Diego County Deserts.

     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 until 9 PM PDT Saturday for Apple and
     Lucerne Valleys-San Gorgonio Pass Near Banning.

     Excessive Heat Warning from 10 AM Tuesday to 9 PM PDT Friday for
     San Diego County Valleys-Santa Ana Mountains and Foothills.

     Heat Advisory from 10 AM Tuesday to 9 PM PDT Friday for Orange
     County Inland Areas.



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