Area Forecast Discussion
Issued by NWS San Diego, CA

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000
FXUS66 KSGX 252009
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
109 PM PDT Sun Jun 25 2017

.SYNOPSIS...
Hot weather will continue inland into Monday under high pressure
aloft. The high will weaken Monday through Wednesday, as low
pressure drives inland to the north. This will ramp up the onshore
flow, and begin a cooling trend that will send daytime temperatures
back below average by midweek. Westerly winds will be strong and
gusty across the mountains and deserts each afternoon and evening,
increasing the danger of wildfires. Typical summer weather is
expected for the upcoming Holiday weekend with a shallower marine
layer and seasonal temperatures.

&&

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

At Noon PDT, marine stratus remained along the coast of San Diego
and south Orange Counties and offshore. Elsewhere it was mostly
clear , except for patchy high clouds drifting north over Baja
CA. The sfc pressure gradients were weak and winds generally
light. It was considerably warmer in the valleys at Noon local
time with Ramona and Riverside at 100, Ontario with 95, and Yorba
Linda at 92.

The marine layer has thinned a bit, so max temps have surpassed
values observed yesterday at the same time in the valleys. The sea
breeze has begun to spin up though, so it should provide some relief
inland later today and keep coastal areas mild. Please see LAXNPWSGX
for details. All of the models suggest the marine stratus will be
very limited tonight, and mostly along the immediate coast. Tomorrow
may be a tough forecast with indications the marine layer will thin
to nothing in the morning allowing temps to rise rapidly before
leveling off with a stronger sea breeze. Count on another very warm
day inland, and perhaps even near the coast late morning.

The strong high pressure area centered over the SW for the past few
days will begin to weaken on Monday and slip south as low pressure
off the Central Ca Coast moves NE. The high will eventually
retrograde west and reamplify over the EastPac near 140W later in
the week. This scenario allows for cooling to begin on Mon, then
increase into Wed as onshore flow ramps up, eventually deepening the
marine layer. The new position of the high will favor troughing
along he West Coast for the upcoming weekend and through the Fourth
of July. This should keep the weather seasonal with periods of low
clouds and for extending into the valleys each night.

Before we get to that point, we will have another hot day on Monday,
so the Excessive Heat Warnings remain in place over the deserts
through Mon, but the Advisories for the mts and points west
are set to expire this evening.

Gusty westerly winds will increase over the mts through Mon, and
remain strong through Tue, with gusts of 50 MPH or more in the
windier desert slope/pass areas at times each afternoon through late
evening. Wind Advisories may be needed, and there will be an
increased threat of wildfires due to the combination of low humidity
and strong winds.

&&

.AVIATION...
241956Z...Coast/Valleys...Through 26/0600 UTC, mostly SKC and P6SM
vis except for lingering BKN cloud decks at/near the beaches in the
300-1000 ft msl layer. 26/0600-1500 UTC, patchy low stratus/dense
fog may develop within 5-10 sm of the coast, with bases 200-400 ft
msl, tops around 500 ft msl, and local vis 1/4-2 sm possible.
However, confidence is low in occurrence and timing of low
stratus/fog at coastal airports (including KSAN, KCRQ and KSNA).
Otherwise, mostly P6SM vis and SKC will continue. Any low
stratus/fog should clear out during the 26/1500-1700 UTC time-frame.

Mountains/Deserts...A few cumulus with bases around 10000 ft msl may
develop over the mountain ridges during the afternoon, dissipating
by sunset. Otherwise, mostly SKC with P6SM vis through Monday
morning.

&&

.MARINE...
A shallow marine layer could bring patchy dense fog (visibility 1
nm or less) to the coastal waters tonight into Monday morning.

Northwest wind gusts near San Clemente Island may reach 20-25 kt
during the afternoons and evenings Sunday through Tuesday,
potentially resulting in conditions hazardous to small craft.
However, the locations in the outer waters that may reach 25 kt
appears to encompass too small an area at the moment to issue a
Small Craft Advisory.
&&

.FIRE WEATHER...
Onshore flow will be increasing over the next two days across the
mts/deserts. Winds will be strongest along the ridges, through the
passes, on the desert slopes, and locally onto the desert floor
late Monday. Winds are expected to decrease slightly on Tue, but
remain gusty. On Monday west winds of 25 to 30 MPH are expected,
with gusts of at least 40 to 50 MPH in these areas. Several hours of
these conditions are possible over San Bernardino County, but these
winds are expected to be more persistent for eastern San Diego and
Riverside Counties from late Mon afternoon through Mon night.

In addition to the wind, it will be hot with temps from 100 to 105
in the afternoon, falling through the 90s in the evening with RH
in the single digits. The combination of very low RH and the strong
winds will produce near, to critical wildfire conditions for the
desert slope and foothill areas, including elevations above the
Morongo Pass and all desert slopes and adjacent desert foothill
areas of Riverside and San Diego Counties.

An extended period of these conditions is possible beginning Monday
afternoon and continuing through the night. Some improvement is
likely as winds drop off Tuesday morning, but by afternoon, these
areas may again turn critical for several hours as winds increase
and remain strong into the late evening with low RH. At this time no
Fire Weather products have been issued.

&&

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

     Heat Advisory from 10 AM this morning to 9 PM PDT this evening
     for Riverside County Mountains-San Bernardino County
     Mountains-San Bernardino and Riverside County Valleys-The
     Inland Empire-San Diego County Mountains-Santa Ana
     Mountains and Foothills.

     Excessive Heat Warning until 9 PM PDT this evening for Apple and
     Lucerne Valleys.

PZ...NONE.

&&

$$

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



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