Excessive Heat and Heat in California, July 7 to 8, 2017

Area Forecast Discussion
National Weather Service San Diego CA
845 PM PDT Thu Jul 6 2017

.SYNOPSIS...
High pressure will bring hot weather inland through Saturday,
followed by minor cooling early next week. Moist southeasterly
flow aloft could allow for a few thunderstorms to develop in
the mountains during the afternoons. Marine layer low clouds will
be patchy and restricted to the coastal areas, with little if any
low clouds for the weekend.

&&

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

...Update - Heat Advisory Expanded to Include Inland Orange County
Friday-Saturday...

A hot dome of high pressure will build across the area Fri-Sat
with dangerous heat across the Inland Empire and the deserts. It
will also get very hot in the lower mountain locales, inland San
Diego County and inland Orange County. Temps across inland Orange
County east of I-5 look to soar to around 100F degrees Fri-Sat
with perhaps temps even approaching 105F degrees in the far
inland locales, such as Brea and Yorba Linda. So the Heat Advisory
was expanded to include Inland Orange County.

We had a few towering cumulus clouds develop across the mountains
and one report of light rain along Highway 78 on the desert slopes
of the San Diego County mountains. No lightning strikes across
our service area. A similar pattern prevails through the weekend,
but we cannot discount a risk of a thunderstorm or two developing
over the mountains, so will maintain the slight chance of
thunderstorms in the mountains and into portions of the deserts
each afternoon. See previous discussion below for further forecast
details.

...Previous Discussion (Issued at 158 PM PDT Thu Jul 6 2017)...

Only patchy Marine Layer low clouds remain over the coastal waters
this afternoon, and the coastal eddy has dissipated. Low clouds
will be even less widespread tonight into Friday morning, then for
the most part absent over the weekend.

Cumulus clouds have developed over the San Bernardino and San
Diego County mtns. An upper-level disturbance moving to the
northeast over northern Baja. This disturbance is triggering tstms
20-100 miles south of the border, but chances are good that it
will enhance the activity over S.D. County and produce isolated
tstms. Any storms that develop will likely have high bases with a
dry sub-cloud layer. This will result in mostly dry lightning
strikes with locally gusty and erratic winds in the vicinity of
the storms.

The warming trend will continue as the upper level high
strengthens over the Four Corners region. Mid and high level
moisture is increasing under south-southeast flow aloft. For
Friday and Saturday...the upper high shifts west and remains quite
strong, bringing hotter conditions inland. Saturday looks to be
the hottest day, when highs could climb into the 115-120 range in
the low deserts, and 105-110 in the high deserts and Inland
Empire, locally up to 115 in the hottest spots. Highs in the San
Diego County Valleys will be around 95-105.

As the high expands over Southern California, the deeper layer
moisture will be shunted southward with mid-level drying,
especially on Saturday. Precipitation chances decrease each day,
but there remains enough instability that a couple showers and
thunderstorms over the mountains are still possible.

The high begins to weaken and lift northeast just enough for the
flow aloft to turn more southeasterly, although models are showing
decreasing moisture on Sunday and Monday. This will bring some
cooling with the potential for thunderstorms over the mountains
and high deserts decreasing. An inverted trough rotates northward
around the west side of the ridge, though at this time it appears
any lifting and instability with this wave will be too far west
to bring us nocturnal thunderstorms. Flow aloft turns northwest
Tue and Wed, ushering in drier air. There may be enough residual
moisture Tuesday for some cu buildups and isolated showers over
the mountains Tuesday, but Wed looks dry. Otherwise expect minimal
day to day variations in temperatures and a return of coastal low
clouds during the nights and mornings.

&&

.FIRE WEATHER...
There is the potential of a few thunderstorms developing over the
mountains each afternoon through Monday, bringing a risk of dry
lightning and locally gusty outflow winds. Hot and dry conditions
near the surface accompanied by gusty south to southwest winds on
the desert mountain slopes each afternoon and evening will
increase fire weather concerns, especially on Friday and Saturday.
Sunday and Monday will be cooler with increasing humidity in the
mountains, however isolated dry lightning and gusty outflow winds
are still a threat, but less so on Tuesday.

&&

.AVIATION...
070345Z...Coast/Valleys...Only patchy stratus/fog is expected late
tonight/early Friday morning. If a CIG reaches KSAN or KCRQ, it will
likely only occur for a few hours between 10Z and 14Z Fri. The
chance is very small for a CIG at KSNA, with no stratus in most of
the valleys. CIGS will be 600-800 ft MSL with tops to 1000 ft MSL.
Local vis 1-3 miles will be possible over higher coastal terrain,
including the San Diego County Mesas. Otherwise, occasional SCT
clouds at/above 10000 ft MSL will occur through Friday evening.

Mountains/Deserts...Isolated -TSRA likely over the San Bernardino,
Riverside and San Diego county mountains this afternoon, with bases
around 9 Kft MSL, CB tops to 40 Kft MSL, strong up/downdrafts and
locally gusty surface winds. Occasional SCT clouds at/above 10000 ft
MSL and unrestricted VIS will occur through Friday morning. There is
a slight chance of TSRA Friday afternoon/early evening over
mountains of San Diego, Riverside and San Bernardino County, with
bases 9000 ft MSL and tops to 35000 ft MSL, strong up/downdrafts and
local gusty surface winds.

&&

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

&&

.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 11 AM Friday to 9 PM PDT Saturday
     for Apple and Lucerne Valleys-Coachella Valley-San
     Bernardino and Riverside County Valleys-The Inland Empire-
     San Diego County Deserts-San Gorgonio Pass Near Banning.

     Heat Advisory from 11 AM Friday to 9 PM PDT Saturday for Orange
     County Inland Areas-Riverside County Mountains-San
     Bernardino County Mountains-San Diego County Mountains-San
     Diego County Valleys-Santa Ana Mountains and Foothills.

PZ...NONE.
&&

PUBLIC...Gregoria
AVIATION/MARINE...Maxwell