Excessive Heat and Wildfire in California, September 4 to 30, 2020

Area Forecast Discussion
National Weather Service San Diego CA
834 AM PDT Fri Sep 4 2020

.SYNOPSIS...
Building high pressure aloft will bring record heat to Southern
California this weekend. Saturday and Sunday are expected to be the
hottest days with a warm nights. Some cooling could begin along the
immediate coast late Sunday, but more likely on Monday as a coastal
eddy develops. Gradual cooling will spread inland early next week as
the marine layer slowly recovers. Next up is an offshore wind event
mid next week. Strength of this Santa Ana is uncertain at this
point, but it will keep the weather warm and dry.

&&

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

The forecast remains on track. Areas of low clouds and patchy dense
fog were dissipating over coastal areas but clouds were holding
offshore. The sfc pressure gradient is trending more offshore this
morning, but so far, NE wind reports have been light. It will be
much warmer today, especially inland with a warm night expected in
many areas as well. No forecast updates planned.

From previous discussion...

.SHORT TERM (Today through Sunday)...
High pressure over the Great Basin and southwestern states will
continue the strong warming trend into the weekend. The weekend is
expected to be very hot with a large number of record high
temperatures expected to be tied or broken with many record warm
overnight low temperatures as well.

Weak offshore flow on Saturday with gusty east winds to around 35
mph for the mountains and inland valleys in Riverside and San
Diego Counties will push the hottest temperatures toward the coast
with the hottest temperatures outside of the lower deserts
expected for inland coastal areas into the western valleys with
high temperatures of around 115 in some of these areas. If the
offshore flow were to be a little weaker, the downslope warming
would be less and those inland valley areas would not get quite as
hot as currently forecast but still above 100.

Winds are expected to be weaker on Sunday with 850 mb temperatures
continuing to slowly warm. Inland areas will continue to warm
while some of the areas that warmed the most from downslope
warming on Saturday might not be quite as hot on Sunday, but still
above 100.

One area of continued uncertainty is when a coastal eddy might
develop and bring some cooling near the coast. The local WRF with
NAM boundary conditions continues to show an eddy developing
Saturday night, but the trend has been weaker with cooling
limited to near the coast. The local extended run of the WRF with
GFS boundary conditions continues to be 24 hours later, with the
eddy developing Sunday night. In that scenario, areas near the
coast wouldn't seen any cooling before Sunday night and Monday.

&&

.LONG TERM (Monday through Thursday)...
A cold low pressure system will be moving southward from the
northern Rockies on Monday into the central Rockies and Great
Basin on Tuesday, beyond which the models continue to diverge.
That spread has narrowed somewhat and generally in the direction
of the ECMWF. But even among the ECMWF ensemble members, there
remains a large spread with about a 25 degree difference in the
warmest and coolest members for the high temperature for Ontario
for Tuesday and Wednesday for example.

High temperatures are expected to begin cooling on Monday with
greater cooling into the middle of next week. A period of moderate
to potentially stronger offshore winds is possible around Tuesday
and Wednesday of next week with the timing and strength dependent
on the location of the low pressure system to the east. A coastal
eddy could bring a return of coastal low clouds and fog for
Sunday night into Monday morning and again Monday night into
Tuesday morning before being pushed well off the coast by
developing offshore flow on Tuesday.

&&

.AVIATION...
041500Z...Coast/Valleys... Areas of low clouds with bases of 400-800
ft MSL and reduced VIS of 2-3 SM across northern portions of coastal
San Diego to persist through 16z Fri before diminishing. A few
patchy low clouds could form off the coast late tonight but ceilings
are not anticipated to affect coastal TAF sites.

Mountains/Deserts...Mostly clear today and tonight. Just a few
clouds around 10000-15000 feet.

&&

.MARINE...
No hazardous marine conditions are expected through Tuesday.

&&

.FIRE WEATHER...
Excessive heat and periods of moderate offshore flow will elevate
fire weather conditions along the coastal slopes, foothills and
eastern valley areas of San Diego and Riverside Counties. Winds of
25-35 MPH with locally higher gusts are possible in and below
offshore wind favored areas, including the San Gorgonio Pass. Lowest
daytime humidity is expected to fall to 10-15% with poor overnight
recovery. The strongest winds will be on Saturday, with some
weakening on Sunday, but some areas may be even hotter with poor
overnight recovery in RH. The most critical fire weather conditions
are expected on Saturday afternoon. A Fire Weather Watch remains in
effect for portions of San Diego and Riverside Counties both
Saturday and Sunday. A coastal eddy may bring cooling and a recovery
in humidity for coastal areas Sunday night and Monday.

A stronger offshore event is possible from late Tuesday into
Wednesday of next week. Strength and exact timing is uncertain, but
more widespread critical fire weather conditions look possible at
this time.

&&

.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 this morning to 8 PM PDT
     Monday for Apple and Lucerne Valleys-Coachella Valley-
     Orange County Inland Areas-Riverside County Mountains-San
     Bernardino County Mountains-San Bernardino and Riverside
     County Valleys-The Inland Empire-San Diego County Deserts-
     San Diego County Mountains-San Diego County Valleys-San
     Gorgonio Pass Near Banning-Santa Ana Mountains and
     Foothills.

     Fire Weather Watch from Saturday morning through Sunday
     afternoon for Riverside County Mountains-Including The San
     Jacinto Ranger District Of The San Bernardino National
     Forest-San Bernardino and Riverside County Valleys  -The
     Inland Empire-San Diego County Inland Valleys-San Diego
     County Mountains-Including The Palomar And Descanso Ranger
     Districts of the Cleveland National Forest.

     Excessive Heat Warning from 10 AM Saturday to 8 PM PDT Monday
     for Orange County Coastal Areas-San Diego County Coastal
     Areas.

PZ...NONE.

&&

PUBLIC/FIRE WEATHER...10/17
AVIATION/MARINE...Connolly/Schenk