Excessive Heat Watch 4, NWS San Diego, CA

Area Forecast Discussion
National Weather Service San Diego CA
854 AM PDT Tue Jul 6 2021

.SYNOPSIS...
High pressure over the southwest states and Great Basin will
continue the slow warming trend into the weekend. This will bring
a heat wave for inland areas for the latter part of the week into
early next week with the heat peaking over the weekend. The
marine layer and weak onshore flow will moderate the warming for
the coast into the valleys with areas of night and morning coastal
low clouds and fog spreading into western portions of the San
Diego County valleys late each night. There might be sufficient
moisture on Thursday for isolated thunderstorms near the
mountains in the afternoon.

&&

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

Morning water vapor satellite shows a broad area of high pressure
over the southwest US. The 12Z KNKX sounding had a 10C marine
inversion based near 1400 ft MSL with dry air aloft. Low clouds
are already rapidly clearing across San Diego County but are much
slower to erode across Orange County. Beaches should see at least
a few hours of sun today. Elsewhere clear skies will prevail.

Minor warming expected today for inland areas with little change
near the coast, where highs will remain near to slightly below
normal. The upper level high shifts slightly eastward through
Thursday and strengthens slightly. The position of the high will
allow some mid level moisture to return to So Cal by Thursday
while high temperatures continue to creep upwards. Some low-level
moisture surging into the low deserts could actually cause temps
to lower by a couple degrees on Thursday, but the humidity and
warmer nights will not offer any relief. There is some very weak
and shallow surface based instability over the mountains on
Thursday, but a subsidence inversion near 15 kft and southeast
flow of 15-20 kt in the layer above the LFC will limit the
potential for thunder, with a few light and brief showers the most
likely scenario, mainly along the sea breeze/east flow convergence
boundaries near the mountain crests.

For Friday into early next week, once a short wave passes across
the top of the ridge across the Pac NW into the northern Rockies,
the high shifts back towards NV and CA and strengthens further,
peaking at h500 somewhere near 598-600 dm. Some mid-level moisture
remains, but given the proximity of this very strong ridge, the
subsidence inversion will be too strong to allow for any
convection, with only a few shallow cu over the mountains each
afternoon. The main concern will be the heat, especially over the
high deserts where the ECMWF Extreme Forecast Index is pointing to
well above normal temperatures. As far as the ridge itself, there
is very little variability between the ensemble members over So
Cal as shown in the WPC cluster analysis. Confidence is fairly
high in temperatures over the Mojave Desert and areas west of the
mountains. However, the presence and amount of any surface-based
moisture in the mountains and low deserts will determine just how
hot it gets in those areas. Current local high res guidance as
well as NBM showing dew points in the 50s and 60s in portions of
the low deserts. Current NBM 10th and 90th percentile high
temperatures for Palm Springs this weekend are around 112 and 120,
respectively. If low-level moisture manages to mix out, highs will
be closer to the 120 end of things, but if SE flow/higher dew
points prevail through afternoon then it will be closer to the 112
end. Much less variability in the high desert, where temperatures
will easily climb to 108 or more, a solid 10-15 degrees above
normal. An Excessive Heat Watch remains in place for the high
deserts Wednesday through Monday, but will need to be considered
for the low deserts for this weekend into early next week, mostly
due to very warm nights. Mountains and parts of the Inland Empire
may need to be added as well, though those look more Advisory
level.

Through all of this, persistent onshore flow over our mid-upper
60s ocean temps should keep the marine layer firmly in place.
Temperatures in the coastal areas and western valleys will remain
near normal. Night and morning low clouds and fog will continue
across the coastal areas and up to about 10-15 mi inland each
morning.

&&

.AVIATION...
061530Z...Coast/Valleys...Low clouds with bases 1300-1600 feet MSL
will continue to recede toward the coast this morning, with many
areas clearing after 17Z. Low clouds with similar bases will
redevelop after 04Z Wednesday and slightly thereafter in Orange
County with moderate to high confidence. Cigs may lower near 1000 FT
MSL after 10Z.

Mountains/Deserts...Mostly clear skies with unrestricted vis through
tonight.

&&

.MARINE...
No hazardous marine weather is expected through Saturday.

&&

.BEACHES...
A south swell from 190-200 degrees will continue to bring elevated
surf and high rip current risk to Orange County beaches today. Surf
of 3-5 feet, with local sets to 6 feet is expected. Highest surf
will be on south facing beaches. A Beach Hazards Statement remains
in effect through this evening. Surf will gradually decrease
Wednesday.

&&

.FIRE WEATHER...
Elevated fire weather conditions will continue for this afternoon
for the deserts onto the desert slopes of the mountains and for
the Apple and Lucerne Valleys on Wednesday afternoon. Lowest
humidity this afternoon will fall to around 10 percent in the
deserts. On Wednesday afternoon it will fall to 10 to 15 percent
for the Apple and Lucerne Valleys with a greater recovery in the
lower deserts. Those low humidities combined with gusty southwest
to west winds will result in elevated fire weather conditions each
afternoon.

The chance for thunderstorms for the mountains and portions of the
Lucerne Valleys for Thursday afternoon is no more than 10 percent.
Any thunderstorms that do develop could produce dry lightning
strikes and strong gusty winds.

The intense daytime heating and higher mixing heights for late in
the week into early next week in the Apple and Lucerne Valleys
into the adjacent mountains will increase the potential for
plume-dominated fire growth each day from late morning to early
evening.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Excessive Heat Watch from Wednesday morning through Monday
     evening for Apple and Lucerne Valleys.

     Beach Hazards Statement until 8 PM PDT this evening for Orange
     County Coastal Areas.

PZ...NONE.

&&

PUBLIC/FIRE WEATHER...SS
AVIATION/MARINE/BEACHES...APR

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SAN DIEGO CA
1105 AM PDT TUE JUL 6 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1200 PM     WILDFIRE         3 N SANTA YSABEL        33.15N 116.67W
07/03/2021                   SAN DIEGO          CA   FIRE DEPT/RESCUE

            THE YSABEL FIRE STARTED AS AN RV FIRE, AND
            SPREAD INTO VEGETATION IN THE AREA OF
            HIGHWAY 79 AND SCHOOL HOUSE CANYON RD IN SAN
            DIEGO COUNTY, RESULTING IN A BRIEF CLOSURE
            OF HWY 79 AND EVACUATIONS FOR RESIDENTS
            ALONG SCHOOL HOUSE CANYON RD. THE FIRE
            BURNED 173 ACRES AND IS 90% CONTAINED.

&&

BR