Area Forecast Discussion
Issued by NWS San Diego, CA

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000
FXUS66 KSGX 170359
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
900 PM PDT Fri Jun 16 2017

.SYNOPSIS...
Strong upper-level high pressure building over the Southwest will
create hot to very hot weather inland through next week, with
Tuesday and Wednesday likely the hottest days. A shallow marine
layer will keep coastal areas cooler through the period, with
areas of night and morning low clouds and fog. Otherwise, skies
will be mostly clear.

&&

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

A few patches of marine stratus held on over the Bight through
sunset, but satellite trends show wholesale clearing just to the
north along the Central Coast this evening. By 9 PM PDT...more
marine clouds were forming and spreading north along the south coast
and NW Baja. The 00Z Miramar sounding showed continued modest
warming, and even some weak easterly flow in the mid-levels. Surface
pressure gradients were 6-7 MBS onshore KSAN to the lower deserts,
and weakly onshore to the NE. Peak wind gusts were under 25 MPH at 8
PM PDT, except through the San Gorgonio Pass where some peak gusts
over 35 MPH have been recorded.

Although the near-shore sea surface water has begun to warm,
temperatures were mostly in the 60s. This will continue a large
temperature gradient inland to the desert each day. Slight
variations in the sea breeze front and marine layer depth will make
the difference for coastal and western valley areas. For now, the
pattern suggests that the marine layer will remain intact over the
Bight, allowing the thermally driven sea breeze to cool areas
for several tens of miles inland each day. Still, nailing the high
temp each day may be problematic, especially in that transition
zone from marine to continental air.

Several heat products have been issued. See LAXNPWSGX for details.
Another concern will be for grass fires each afternoon as gusty
westerly winds develop inland. The grasses have cured after a good
growing season, so there is plenty of fuel. No changes to the
forecast planned this evening.

From previous discussion...


The weather pattern through next week will be dominated by a
strong ridge aloft. Water vapor satellite shows the ridge to the
south starting to build into the region, bringing 590 dm 500 mb
heights. The 12Z Miramar sounding shows a few degrees of warming
taking place from the surface through most of the atmospheric
profile compared to yesterday`s 12Z sounding. This indicates the
anticipated warming and presence of the ridge taking shape. As of
1 pm, temperatures are several degrees higher than 24 hours ago.
Expecting highs today in the low 110s in the lower deserts, upper
90s/100 in the Inland Empire and High Deserts, upper 80s/low 90s
in the San Diego county valleys, inland Orange county and the
foothills of the mountains, 70s/low 80s at the coast and in the
higher elevations. This morning`s sounding also shows a marine
layer inversion of around 8 deg C at around 700-800 feet msl, and
areas of dense fog were reported within 3 to 8 miles of the
coast. Fog was able to clear by mid-morning, and visible satellite
shows clear skies across the region this afternoon.

The ridge will remain at around 591 dm 500 mb heights on Saturday
with little change in temperature. Although the ridge remains at a
similar strength Sunday, models show 850 mb temperatures
increasing a bit, so Sunday will likely be a bit warmer. The ridge
then strengthens next Monday and Tuesday, with 500 mb heights
reaching near 597 dm over us on Tuesday. Thus, expect significant
warming Monday and further warming on Tuesday. Although the ridge
appears to be strongest on Tuesday, 850 mb temperatures are
actually the highest on Wednesday, with the GFS and ECMWF showing
widespread 31 deg C all the way to the coast. So, wouldn`t be
surprised if Wednesday ends up being the hottest day of the heat
wave. The ridge weakens by a couple dm on Thursday, which should
result in temperatures about 5 degrees lower than Wednesday,
although it will still be hot, especially in the deserts. With the
position of the ridge moving more to the south over the SE
Arizona/SW New Mexico/Mexico border by Thursday, southerly flow
may bring enough moisture into the area for some cumulus build-ups
during the afternoon and perhaps a shower or two over the
mountains. This moisture could also serve to lower temperatures a
bit more than expected, but confidence in that occurring is low at
the moment. With the marine layer remaining very shallow due to
the ridge, dense fog will be possible along the coast each night
and morning, including tonight/Saturday morning. The presence of
the marine layer and onshore flow should keep temperatures at the
coast moderated.

For next weekend, GFS and ECMWF really don`t show the ridge going
away altogether. In fact, the ECMWF re-strengthens the ridge late
next Sunday, meaning that the heat wave may continue all the way
through next weekend!

&&

.AVIATION...
170300Z...Patchy and intermittent low clouds/fog at the coast
through 16Z Saturday. Bases around 400 feet msl with vis 3-5sm,
locally down to as low as zero on coastal mesas. Otherwise, clear
skies through Saturday evening.

&&

.MARINE...
Areas of fog with visibilities 1 NM or less mainly nights and
mornings at least through the weekend, possibly longer. Otherwise,
no hazardous marine weather is anticipated through Wednesday.

&&

.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 Wednesday for Coachella
     Valley-San Diego County Deserts-San Gorgonio Pass Near
     Banning.

     Heat Advisory from 11 AM Saturday to 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.

     Excessive Heat Warning from 11 AM Saturday to 9 PM PDT Wednesday
     for Apple and Lucerne Valleys.

     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.

PZ...NONE.

&&

$$

PUBLIC...JAD/Harrison
AVIATION/MARINE...MM



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