Heat in California, August 30 to 31, 2022

Area Forecast Discussion
National Weather Service San Diego CA
928 PM PDT Mon Aug 29 2022

.SYNOPSIS...
A long duration heat wave will impact Southern California
beginning Tuesday and lasting through the Labor Day weekend.
This high impact heat wave will be a result of a strong upper ridge
building over our region. A surge of monsoonal moisture will bring
increasing humidity beginning Thursday, along with a chance of
isolated thunderstorms over the mountains. Intense heat will elevate
the wildfire risk inland as well.

&&

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

Except for some patchy low clouds along the San Diego County Coast,
it was mostly clear over SoCal at 9 PM PDT. The 00Z Miramar sounding
had a 7C inversion near 2K FT MSL. Warming of 5C+ was observed at
4500 FT MSL with a general drying of the column. Light winds were
observed below the inversion, and at most sfc reporting sites. The
forecast remains on track with the heat, so no updates are in order
this evening.

From previous discussion...

KEY POINTS:

* High impact heat wave to impact SoCal beginning Tuesday
* Potential of very high heat risk coast/valleys this weekend

A ridge is currently building over Southern California/SW U.S. and
this ridge will strengthen through the weekend, resulting in a
long duration heat wave region wide. High confidence on this.
Moderate confidence on the temperature forecast.

There are model differences with the EC keeping a strong ridge
dominance over SoCal while the GFS weakens the ridge with more
influence from an inverted trough moving from east to west over
northern Mexico late this week. However, the ensemble consensus
of all models points to a heat wave of long duration and
potentially extreme for the coast and valleys by the upcoming
holiday weekend if the hotter EC verifies. We are leaning towards
the EC over GFS and continuing to go slightly hotter than NBM
given past precedence with a cool bias during anomalous heat waves
locally.

Have upgraded to an Excessive Heat Warning area wide, including
the mountains below 6000 feet and locations all the way to the
coast. Deserts are in this as well, though temperature departures
there will only be +2-5 degrees from normal, while coastal
communities will see temperature departures by this weekend on the
order of +10-20 degrees from normal.

The heat will take longer to reach the coast but 90s are possible
along the coast this weekend. It will be humid as well near the
coast and into the inland San Diego County valleys, Orange County
and in the lower deserts. These regions will see dewpoints well
into the 60s with even lower 70s/tropical especially starting
Friday through Labor Day. Lower deserts could see a moisture surge
from the 90-degree Gulf of California water on Friday-Saturday
with dewpoints well into the 70s to even near 80 degrees possible.
This higher humidity across the aforementioned regions will make
it feel even hotter than the actual air temperature, increasing
the heat stress.

Lows at night for 7+ nights will be in the 70s almost area wide,
except in the 80s to lower 90s over the lower deserts. Some lows
near 80 are possible west of the mountains, most likely in the
Inland Empire communities.

Record high temperatures are very high this time of the year so
they mean something! Records highs aren't forecast until possibly
Sunday. Still, successive days of heat, humidity and warm
overnight temperatures are the big concern, especially for
vulnerable populations, such as children, elderly and those
without air conditioning.

There will be some monsoonal moisture increase as easterly flow
develops late this week through the weekend. We are favoring a dry
scenario, but there is a 10-20% chance of afternoon thunderstorms
developing over the mountains each day beginning on Thursday.
Only a 5-10% chance across the deserts.

There are signals that Tuesday, September 6 may be the last really
hot day with cooling occurring after.

&&

.FIRE WEATHER...

Heat will be building Tuesday and lasting through the upcoming Labor
Day weekend. Hot and dry conditions, along with minimum relative
humidity of 15-25 percent each day inland, will lead to the
potential of large vertical plume growth for any fires that develop,
particularly in the foothills and in the mountains. This will be a
long duration heat wave, lasting through early next week. There is
an outside risk of a thunderstorm developing over the mountains
beginning on Thursday (less than 20% chance).

&&

.AVIATION...
300420Z...Coast/Valleys...Patchy low clouds with bases 1000-1500 ft
MSL developing along portions of the San Diego County Coast tonight,
locally spreading 10-15 mi inland. CIGS likely to be intermittent at
KSAN and KCRQ. Moderate confidence in now CIGS at KSNA. Clouds
clearing by 17Z Tue. Little if any low clouds redeveloping Wednesday
night.

Mountains/Deserts...Mostly clear skies with unrestricted vis will
continue through Tue night

&&

.MARINE...
Northwest winds gusting near 20 kt in the outer coastal waters
Tuesday afternoon and evening. otherwise no hazardous marine weather
is expected through Saturday.

&&

.SKYWARN...
Skywarn activation will not be needed through Tuesday.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Excessive Heat Warning from 10 AM Tuesday to 8 PM PDT Monday for
     Apple and Lucerne Valleys-Coachella Valley-Orange County
     Coastal Areas-Orange County Inland Areas-Riverside County
     Mountains-San Bernardino County Mountains-San Bernardino
     and Riverside County Valleys-The Inland Empire-San Diego
     County Coastal Areas-San Diego County Deserts-San Diego
     County Mountains-San Diego County Valleys-San Gorgonio Pass
     Near Banning-Santa Ana Mountains and Foothills.

PZ...None.

&&

PUBLIC...10/Gregoria
AVIATION/MARINE...SS