Area Forecast Discussion
Issued by NWS San Diego, CA

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000
FXUS66 KSGX 181546
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
846 AM PDT Wed Oct 18 2017

.SYNOPSIS...
A deepening trough over the West will bring a cooling trend and a
deepening marine layer through Friday. Expect night and morning
marine layer clouds over the coast and western valleys tonight and
Thursday morning and then over the coast, valleys and coastal
slopes with possible drizzle Thursday night and Friday morning.
Strong and gusty west winds also likely in the mountains and
deserts Thursday and Friday. Santa Ana winds and a building strong
ridge of high pressure will bring significant warming and drying
to the region Saturday through Monday. Hot conditions possible all
the way to the coast Sunday and Monday, with weather almost as hot
on Tuesday except at the coast where some cooling may occur.

&&

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

A weakening upper level short-wave trough moving through San
Bernardino county is noted on the morning water vapor satellite
imagery, bringing some mid and high level clouds and not much
else. Models have continued to back off on the instability,
moisture and lift associated with this trough, so we are no longer
expecting any elevated showers/thunderstorms in our forecast area
today. Humidity observations at the coast indicate dew points in
the 50s/60s, which shows that the marine layer has finally
returned. The 12Z Miramar sounding shows a surface based marine
layer inversion and a moist layer from around 700 mb to 400 mb.
Pressure gradients continue to trend onshore this morning, with
the San Diego to Thermal gradient up to 3.4 mb onshore, while 24
hours ago it was only 1.3 mb onshore. With the return of onshore
flow and large scale trough deepening over the Pacific Northwest,
expect today to be the start of a cooling trend for Southern
California (although highs today will still be several degrees
above normal). Local WRF shows that marine layer stratus/fog will
likely develop tonight in the coast and western valleys, with
help from a coastal eddy.

The cooling trend will continue through Friday as the trough
continues to deepen and move inland, with possibly enough lift and
moisture for some drizzle and/or light showers over and west of
the coastal slopes Friday morning. Expect highs 5-10 degrees
below normal by Friday. Onshore flow will strengthen as well, with
onshore pressure gradients forecast to reach around 10 mb from San
Diego to Las Vegas on Friday, so expect west winds gusting 35-45
mph with local gusts to 50 mph in the mountains and deserts
Thursday and Friday. The marine layer will continue to deepen as
well (up to 4000-4500 ft msl), with stratus filling the coastal
basin and likely moving into the passes and coastal slopes
Thursday night/Friday morning and again Friday night before
offshore winds clear clouds out from the north Saturday morning.

On Saturday, surface high pressure over the Great Basin will build
in the wake of the trough that moves over that area. This will
create offshore pressure gradients at around 7 mb from San Diego
to Tonopah, resulting in weak Santa Ana winds that day. A building
ridge from the southwest, in addition to the Santa Ana winds, will
result in significant warming that day in the coast and valleys,
as well as clearing out the marine layer. The ridge will will
continue to build over the region Sunday through Monday, becoming
anomalously strong by Monday with 500 mb heights reaching 597 dm
according to the ECMWF and Canadian models (GFS is weaker at
around 595 dm). Although they do not look strong at the moment
(offshore pressure gradients from San Diego to Tonopah increasing
to 10 mb with 850 mb winds of only 10-15 kt), the Santa Ana winds
will continue as well, bringing hot weather all the way to the
coast. The ECMWF also shows 850 mb temperatures reaching 25 deg C
on Monday, which translates to 103+ degree surface temperatures
west of the mountains. Once again, the GFS isn`t as warm, showing
850 mb temperatures around 23-24 deg C. Although we start to trend
back onshore on Tuesday (which means it should be cooler at the
coast that day), the ridge could be almost as strong which means
that inland areas may be almost as hot as they were on Monday, and
in the case of the mountains and deserts they may be even hotter.

&&

.AVIATION...
181545Z...SCT clouds above 15000 ft will continue today, then mostly
decrease tonight.

Near the coast, stratus will likely return this evening and continue
until Thursday morning. Bases will likely be somewhere between 500
and 1000 ft MSL, though cig height and timing are somewhat low
confidence. There is a chance of fog with visibilities below 3 miles
over higher coastal terrain late tonight and early Thursday.

&&

.MARINE...
No hazardous marine weather is expected through Thursday. Winds will
increase to 20-25 kt in the outer coastal waters late Thursday night
and Friday, decreasing by Saturday.

&&

.BEACHES...
A south-southwest swell from 200 degrees from 3-4 ft/16-17 seconds
will continue today. This will produce 3-6 ft surf with sets to 7 ft
over Orange County and northern San Diego County. Highest surf will
be on southwest facing beaches. Strong rip and longshore currents
will accompany the elevated surf. Swell and surf will lower Thursday
but some surf to 6 ft with strong rip currents will continue. The
Beach Hazard Statement continues for this return until 5 pm
Thursday.

&&

.FIRE WEATHER...
Increasingly cool and more humid Today through Friday as a trough
moves in over the West. Gusty westerly onshore winds in mountains
and deserts Thursday and Friday, with gusts 30-40 mph and local
50 mph.

Another round of possibly moderate strength Santa Ana winds
likely Saturday through at least Monday, with day-time humidity
falling down to at or below 10 percent Sunday and Monday (with
very poor overnight recoveries) along with hot conditions and
gusty northeast winds (15-25 mph gusting 35-45 mph). This would
likely result in critical fire weather conditions through and
below the mountain passes and into the coastal slopes and
foothills Sunday through Monday, and possibly into Tuesday as
well.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement through Thursday afternoon for Orange
     County Coastal Areas-San Diego County Coastal Areas.

PZ...NONE.

&&

$$

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


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