Area Forecast Discussion
Issued by NWS San Diego, CA

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453
FXUS66 KSGX 210944
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
244 AM PDT Wed Jun 21 2017

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

.SYNOPSIS...
Very hot conditions 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. It will be a
little cooler west of the mountains to end the week as onshore
flow increases, extending the marine influence farther inland. The
foothills and lower coastal slopes will remain seasonably hot
through the weekend. Inland temperatures closer to seasonal
averages could return towards the middle of next week.

&&

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

Following the record-setting heat of yesterday afternoon,
temperatures are in the 90s to low 100s across the lower deserts
even at this hour! This is 5-10 degrees higher than at this time
yesterday. Near the coast, temps are currently in the 60s as the
marine layer low clouds and fog continue to spread inland. By 7
am, low clouds and fog could spread up to 10 miles inland in some
areas. This will bring patches of dense fog to the mesas and other
elevated locations near the coast.

The upper level ridge of high pressure will continue to dominate
our weather through the coming weekend. As a result, inland
temperatures will continue to be 10-20 degrees above seasonal
averages while a shallow but persistent marine layer will restrict
temps near the coast to only a few degrees above seasonal
averages. The persistence of the marine layer is due in large part
to water temps that remain in the 60s. Sfc pressure gradients are
likely to remain onshore, allowing the maritime influence to
spread inland to the western portions of the inland valleys.

The latest numerical model solutions indicate a slight weakening
of the upper high today which will likely result in temps
generally a few degrees lower than yesterday, but this will be
noticeable only to thermometers. The models also indicate a
strengthening of the onshore flow for Thu through early next week.
This will allow the marine layer influence to spread farther
inland, with a moderating effect on temps west of the mtns.

An upper-level disturbance moving westward on the south side of
the upper level high could bring some elevated moisture to SoCal
on Sat and Sun. This is unlikely to have much of an effect on
conditions since any associated clouds will be mostly thin and
above 10000 ft.

More noticeable cooling is possible for the middle of next week as
the upper ridge weakens in response to a low pressure trough
moving inland to our north.

&&

.AVIATION...
210905...Coast/Valleys...Low stratus/fog continuing within 5-10 sm
inland (and locally 15 sm inland) through 1500 UTC, with bases 200-
500 ft msl, tops around 800-1000 ft msl, areas of vis 1-5 sm and
local vis 1/4 sm or less. KCRQ may have a period of 1/4-1/2 sm vis,
while vis at KSAN and KSNA should stay at or above 1 sm, although
cigs will likely be low (around 300-500 ft agl). Clearing taking
place during the 1500-1700 UTC time-frame. 1700-22/0300 UTC,
primarily P6SM vis and mostly SKC except for a few patches of low
clouds 400-1000 ft msl possibly lingering near and along the
beaches. 22/0300-1500 UTC, low stratus/dense fog likely re-
developing 5-15 sm inland with bases around 300-600 ft msl and tops
around 1000-1200 ft msl.

Mountains/Deserts...P6SM vis and mostly SKC through Thursday
morning. High density altitude at KPSP and KTRM with max
temperatures 118-122 deg F.

&&

.MARINE...
A shallow marine layer will continue to bring some dense fog (with
visibilities of 1 NM or less) to the coastal waters this morning,
with improving visibilities likely in the afternoon. However, there
is a chance that dense fog could re-develop tonight/Thursday morning
and again Thursday night/Friday morning. A Marine Weather Statement
is in effect through Friday morning.

&&

.BEACHES...
A southerly swell of 3-4 ft/13-15 seconds 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 (with local sets to 6 ft)
at Orange county beaches and San Diego county beaches north of
Oceanside. Also, the aforementioned high tides will likely reach
around 7+ ft during the mid-evening hours Thursday through Saturday.
Tides this high may result in minor tidal overflow at low lying
beaches. The Beach Hazards Statement continues through Friday, and
may need to be extended into Saturday.

&&

.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 expected 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 Apple and
     Lucerne Valleys-Coachella Valley-San Diego County Deserts-
     San Gorgonio Pass Near Banning.

     Heat Advisory until 9 PM PDT this evening 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.

PZ...NONE.

&&

$$

PUBLIC/FIRE WEATHER...PG
AVIATION/MARINE...Harrison



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