Excessive Heat Warning 4, NWS San Diego, CA

Area Forecast Discussion
National Weather Service San Diego CA
930 AM PDT Tue Jun 2 2020

.SYNOPSIS...
High pressure aloft over the Southwest will bring a warming trend
through Wednesday. Temperatures in the lower deserts could reach 110
Wednesday and Thursday. South flow aloft and a low pressure system
off the coast will bring high clouds at times through Friday.
Onshore flow will strengthen Thursday through Friday as the low
pressure system moves through Friday, which will bring more night
and morning low clouds west of the mountains and gusty winds in th
mountains and deserts Friday. Cooler weather will occur west of the
mountains Thursday and over the entire region Friday. Temperatures
will be a little below normal this weekend but could warm early next
week.

&&

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

Widespread high clouds were continuing to move north over the region
as a low pressure system was about 600 miles west of San Diego. Only
patchy low clouds occurred this morning. The NKX sounding this
morning showed mostly 1-2 deg C warming from just above the surface
to 700 MB, and while the high clouds could moderate temperatures
somewhat, they should still be a few degrees above early June
normals today. There could be some cumulus over the mountains this
afternoon and evening due to surface based CAPE up to around 1000
J/kg according to the HREF, and for tonight and Wednesday morning,
some instability aloft will occur and could interact with some of
the elevated moisture to produce a few sprinkles or light showers,
but for now we will leave thunderstorms out of the forecast. The
high clouds should mostly inhibit low clouds and fog tonight.

The upper high over AZ/NM will remain through Thursday and bring hot
conditions to the deserts. The models are pretty consistent for
Wednesday with showing around 110 in the lower deserts, as 850 MB
temps will be in the 26-27 deg C range, and ensembles are fairly
tight with their spread. A few locations in the Inland Empire could
reach 100. By Thursday, an eddy could form along the coast, and all
model solutions have shown cooling west of the mountains then, but
the deserts will likely be almost as hot Thursday. With the upper
low off the coast finally making landfall over SoCal Friday,
substantial cooling aloft will take place and likely make the
deserts stay at or below 100 degrees. The stronger onshore flow and
desert-valley temperature gradients will likely bring some gusty
west winds Friday in the mountains/deserts. We will need to keep an
eye out for any precip that could occur Friday, but moisture will be
quite limited, though with some instability likely. ECMWF ensembles
show less than 20 percent of the solutions showing precip at any
time period Friday, with pretty much zero for the GFS ensembles. The
longwave trough over the Pac NW will be what ulimately drives the
upper low inland Friday through Saturday, but the mean troughing
pattern could bring a deep marine layer and possible drizzle or even
light rain Saturday too, though generally any PoPs would be below 20
percent. Cool weather will prevail Saturday but some warming could
occur early next week though likely with a continuation of night and
morning coastal low clouds.

&&

.AVIATION...
021625Z...Along the coast...patchy low clouds are possible again
between 08Z and 15Z Wednesday. Any low clouds that form would have
bases 700-1200 feet MSL but cig/vis impacts at TAF sites is
uncertain.

Otherwise variable high clouds AOA 15000 feet through tonight.

&&

.MARINE...
No hazardous marine conditions are expected today through Saturday.
&&

.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 afternoon through Thursday
     evening for Coachella Valley-San Diego County Deserts.

PZ...NONE.

&&

PUBLIC...Maxwell
AVIATION/MARINE...PG