Area Forecast Discussion
Issued by NWS San Diego, CA

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000
FXUS66 KSGX 210430
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
930 PM PDT Tue Jun 20 2017

...Heat Wave Continues Inland with Excessively Hot Conditions Over
the Deserts...

.SYNOPSIS...
Blazing heat will continue across the deserts this week under a
strong high pressure ridge. A shallow marine layer will continue to
moderate conditions along the coast and into the western valleys,
along with areas of nocturnal clouds and fog, lingering into the
afternoons at some beaches. Summer officially begins at 9:24 this
evening.

&&

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

The extreme heat over the deserts established a new all-time record
for San Diego County at Ocotillo Wells, where the temperature
reached 124 degrees F. Compare that reading with the 65 logged at
Del Mar Beach, and at one point this afternoon there was a 59 degree
F gradient across the County! Other desert areas hit 120+ degrees in
the Coachella Valley with 122 at Palm Springs and Thermal, which
tied their high temperature records for the date.

The Holcomb Fire continues to affect communities and some roads in
the San Bernardino Mts. The smoke plume was visible in satellite
imagery this evening, streaming north, over sparsely populated areas
of the high desert. Elevated fire weather conditions will continue
for the next few days. See Fire Weather discussion below.

Low clouds had cleared the Orange County Coast early this evening,
but held along the coast of San Diego County, and were expanding
back to the north at 9 PM PDT. The 00Z Miramar sounding had 9 degree
C inversion based near 1150 FT MSL with a slight increase in
dewpoints below the inversion. Winds were NW up the column through
6K FT MSL.

The northwest flow across the coastal waters appears to have put a
lid on the sfc water temps at the beaches, which were all in the 60s
this afternoon. the combo of cool water and elevated dewpoints in
the shallow marine layer will support more marine stratus/fog
overnight into Wed morning. No forecast changes at this time.

From previous discussion...

Global models continue to show the ridge strength lessening a tad
Thursday, and then actually re-strengthening a bit Fri and into
the weekend. This will keep the heat wave for longer duration for
the deserts. Granted, temps will lower a few degrees through the
end of the week, but will remain excessive. For this reason, have
extended in time the Excessive Heat Warning for the lower deserts
(Coachella Valley and the San Diego County deserts) through Sunday.
The Excessive Heat Warning for the High Deserts was extended in
time through Friday. The heat by Sunday may become borderline for
a warning for the lower deserts, but it was decided to go all the
way through the weekend given the fact that this is an excessive
heat wave event and the long duration can have cumulative effects
on the body.

For the hot areas of the Inland Empire, mountains and inland
valley region, the heat will continue through Wednesday, but then
it will tame somewhat for the latter part of the week with slightly
lower temps above the surface (H85) and a better marine influence
further inland. Still though continuing above average.

A weak perturbation at upper levels (above H50) moves around the
ridge periphery this weekend (moving in from the S-SW). Moisture
increases aloft with this wave, so will have to monitor to see if
perhaps an isolated shower or even thunderstorm may occur, but at
this point the chance is really too low to place in the forecast.
We think right now there is a high likelihood of it remaining dry
across our service area through the weekend. This upper level wave
will likely bring in some mid-high level cloudiness for a brief
time late Sat-Sun.

&&

.AVIATION...
210415...Coast...BKN-OVC002-006 will spread 10 miles inland tonight
with areas of VIS 1/2 mile in dense fog through 16Z Wednesday.
Limited clearing at the coast Wednesday afternoon. At KSAN the vis
remained above 3 miles the last two nights, and atmospheric profile
has not changed so anticipate similar vis tonight.

Valleys/Mountains/Deserts...P6SM SKC through Wednesday afternoon.
High density altitude at KPSP and KTRM with max temperatures 118-122
deg F.

&&

.MARINE...
Areas of dense fog with visibility of 1 nm or less through Friday
during the nights and mornings due to a shallow marine layer. A
Marine Weather Statement is in effect through the period.

&&

.BEACHES...
A southerly swell of 3-4 ft/13 sec from 165-170 degrees combined
with a short-period westerly swell and large tidal swings (high
tides of 6.5-7 ft with low tides around -1.0 ft), will result in
strong and dangerous rip and longshore currents through Thursday.
This swell will also produce surf to 5 ft at Orange county beaches
and San Diego county beaches north of Oceanside. A Beach Hazards
Statement is in effect through 10 PM Friday for minor coastal
flooding during the evening high tides.

&&

.Fire Weather...

The hot weather, coupled with very low RH will make it difficult
to extinguish any wildfires that develop in wind-prone areas for
the next few days, but forecast conditions remain generally below
criteria for a Fire Weather Watch or Warning.

Elevated Wildfire conditions will generally prevail along the
mtn/desert interfaces, where the combination of heat, gusty winds
and low RH will exist. Of greatest concern will be the ridges and
north slope of the San Bernardino Mts, where southwest winds of 15
to 20 MPH with gusts of 30 MPH or more, and RH values of 10% or less
are forecast each afternoon/evening.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Excessive Heat Warning until 9 PM PDT Sunday for Coachella
     Valley-San Diego County Deserts-San Gorgonio Pass Near
     Banning.

     Heat Advisory until 9 PM PDT Wednesday for Orange County Inland
     Areas-Riverside County Mountains-San Bernardino County
     Mountains-San Bernardino and Riverside County Valleys-The
     Inland Empire-San Diego County Mountains-San Diego County
     Valleys-Santa Ana Mountains and Foothills.

     Excessive Heat Warning until 11 PM PDT Friday for Apple and
     Lucerne Valleys.

PZ...NONE.
&&

$$

PUBLIC...JAD/Gregoria
Fire Weather...JAD
AVIATION/MARINE...Moede



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