Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 180318

Area Forecast Discussion
National Weather Service San Diego CA
817 PM PDT Sat Jun 17 2017

Very hot weather will prevail over the deserts under strong high
pressure aloft through at least next Thursday, with Tuesday and
Wednesday likely the hottest days when a few locations could exceed
120 degrees. Mild weather will occur along the coast with low clouds
and fog at times during the late nights and early mornings, locally
dense. The inland valleys will be hot at times, with peak heat there
expected to be Tuesday and Wednesday as well. Some cooling is
possible next weekend.



Satellite imagery through 8 PM PDT showed patchy marine stratus over
the Bight drifting toward shore. The 00Z Miramar sounding had a 4
degree C inversion based near 1550 FT MSL with 24-hr cooling of 1-3
degrees C below to the sfc. The sfc pressure gradients were a bit
stronger onshore with about 8 MBS KSAN to the lower deserts. Most of
the peak wind gust reports at 8 PM PDT were below 25 MPH.

It was a bit hotter far inland today, while coastal areas were a
bit cooler. Based on the sounding, better onshore flow and the
development of a weak mid-level trough offshore, the heat should not
spread farther west on Sunday, so forecast temps along the coast
and western valley areas are similar to today. Look for greater
warming on Monday for the western valley areas.

The main forecast issues will continue to be the heat far inland,
and some fog development within 10 to 15 miles of the coast each
night/early morning into next week. The fog could become dense late
tonight into early Sunday, especially over the mesas. No forecast
changes this evening.

From previous discussion...

A 592 DM high was centered over the Central Coast of California with
ridging southeast through NW Mexico. The axis of the high will
gradually shift east to Nevada-Arizona-Chihuahua through Sunday and
then strengthen through Tuesday over Arizona where it could reach
598 DM. Over our area, there will be variations of the theme, with
very hot weather with very low humidities in the desert but a marine
layer, which rarely dissipates entirely in June, along the coast,
and a strong temperature gradient from the mild coast through the
very warm/hot valleys to the extreme heat in the desert. The all-
time high at Palm Springs is 123, but right now I am thinking they
will fall a couple degrees short of that as models show 850 MB temps
in the 31-33 deg C range near the Coachella Valley, which is not
quite as hot as the 34-36 deg C range farther east and northeast
where 850 MB temps will be within about 1 deg C of records based on
the SPC sounding climatology page. Current forecast of 121 for PSP
Tue/Wed looks good and will not change that.

Closer to the coast, the location of the sharp temperature gradient
between the coast and valleys will depend on the marine layer depth,
which should get gradually more shallow Sunday through Tuesday. Some
temps over 105 could occur in the Inland Empire at times, around 100
in the far inland parts of the San Diego County Valleys and temps
could get close to 100 over far inland Orange County, mainly
Tuesday/Wednesday. Some dense fog will occur near the coast, and
possibly could be along the immediate coast Tue/Wed, if it does not
dissipate entirely. There is hint of cooling as the upper high
weakens slightly, then low-level flow, including at 850 MB, becomes
solidly westerly and strengthens Friday/Saturday for more
substantial cooling as a trough moves south over the Northwest, as
the upper high shifts southeast, though ECMWF is has that troughing
moving east versus south, so it would have less of a cooling trend.
Most likely, temperatures late next week will lower to "just" those
of this weekend, still above the hot averages that the desert has
for late June.


180300Z...Coast...IFR conditions will develop within 10-15 miles of
the coast through 15Z Sunday. Low cloud bases should be between 600
and 1,000 ft MSL, with visibility restrictions of 3-6 SM. Local
visibility reductions of 1 SM or less may in FG over the coastal
mesas. KSAN, KCRQ, and KSNA should all be affected, with a low
probability of sub 2 SM visibility and KCRQ after 06Z. Rapid
clearing is expected after 15Z Sunday, with VFR conditions and light
winds anticipated thereafter through 03Z Monday. Low clouds and fog
may return along the coast after 03Z Monday.

Valleys...Mountains...and Deserts...SKC, P6SM and light winds will
prevail through 06Z Monday.


Areas of night and morning dense fog will occur over the coastal
waters through at least Thursday. Otherwise, no hazardous marine
weather is anticipated through Thursday, with mixed seas of 4 ft or
less and light southwest-northwest winds.


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


CA...Excessive Heat Warning until 9 PM PDT Thursday for Apple and
     Lucerne Valleys-Coachella Valley-San Diego County Deserts-
     San Gorgonio Pass Near Banning.

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

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




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