Area Forecast Discussion
Issued by NWS San Diego, CA

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000
FXUS66 KSGX 182150
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
150 PM PST Sun Feb 18 2018

.SYNOPSIS...
Onshore flow will be increasing tonight, becoming strong and gusty
over the mountains and deserts as a cold low pressure trough drops
south into the Great Basin. Scattered, light rain and snow showers
will pop over and west of the mountains late tonight into Monday as
the marine layer builds. Expect the coldest days of the winter so
far on Monday and Tuesday. Dry and a little warmer for Wednesday and
Thursday. Then cooler again into Friday, with a chance for some more
scattered showers as another cold low pressure system drops into the
Great Basin from the north. Dry and a little warmer into next
weekend as temperatures edge back toward average.

&&

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

Just a few mid-level clouds were visible in satellite imagery over
SoCal at midday, otherwise it was clear. Temperatures were running 5
to 10 degrees lower than yesterday. A rapid acceleration is the
surface pressure gradient to 14 MBS KSAN to SW NV portends the
strong winds to come overnight. But as of Noon PST...Peak wind gusts
over 50 MPH were confined to the San Gorgonio Pass, and Volcan Mtn
in San Diego County. The sfc gradient will maximize this evening and
increasing winds aloft overnight will combine to create the
strongest gust potential, first over northern areas of the CWA, then
south across San Diego County. A secondary wind max is likely on the
highest elevations of the San Diego County Mts on Monday as the
powerful upper-level jet core traverses the area. A few peak gusts
of around 70 MPH may be observed in these areas.

A longwave trough will dive into the Great Basin on Monday, grazing
SoCal with strong onshore winds and scattered light precipitation.
It will also bring the coldest air of the season, plunging maximum
temps on Monday into the 15 to 20 degrees below normal range. given
the extended period of well above normal temps this winter, it will
be a profound change.

This wave is actually the beginning of a more sustained cooler
weather pattern for the SoCal as the longwave is maintained for
sometime in the model runs. Various shorter wavelength disturbances
will periodically drop into the longwave, creating gusty onshore
flow over the coastal waters, mts and deserts, and chances for light
precip. So for the foreseeable future, daytime temperatures will
be mostly below average.

A variety of Warnings and Advisories are in effect from frost/freeze
to wind and winter conditions. Please visit our webpage for more
detail on all of these. Of most significance will be the strong
westerly winds late tonight into Monday. Gusts over 70 MPH are
expected on the upper desert slopes and ridge tops, while on the
desert floor, it will be breezy, but potentially damaging gusts look
to be more isolated and near the foothills late tonight into Monday
morning. windy conditions are also expected for coastal San Diego
County on Monday. Precipitation will be light and scattered, but it
will be cold enough so that some light snow accumulations are
forecast above 4000 FT. Freezing conditions on the passes, could
create a hazard to travelers.

As this system moves east, another, weaker trough slips south across
the area midweek with little weather. Temps will moderate some, then
a stronger disturbance drops south for Friday. this one could
generate more light showers, and gusty winds over the mts/deserts/
coastal waters. Slightly warmer and dry into the weekend, then both
the 12Z GFS/ECMWF runs are advertising a stronger system early next
week. Who knows, we may be able to salvage some precip before the
end of the month.

&&

.AVIATION...
182100Z...Mostly SCT-BKN clouds at or abv 6000 ft MSL and
unrestricted vis will continue through this afternoon. Low clouds
currently northwest of San Clemente Island not likely to impact TAF
sites before 00Z Mon. Low clouds will increase rapidly after 02Z
west of the mtns, with bases mostly 2000-3000 ft MSL, obscuring
higher terrain. Timing of cigs blo 3000 ft MSL at TAF sites is
uncertain. -SHRA could develop late tonight through Monday morning.
Clearing at coastal TAF sites could occur between 12Z and 14Z
Monday, but low clouds inland will continue to obscure higher
terrain through 18Z Mon.

Southwest to west winds 25 to 35 mph with gusts to 55 mph will
develop over mtn ridges this evening, and continue through Monday
afternoon. The winds will produce strong up/downdrafts and LLWS over
and east of the mtns. Strong and gusty southwest to west winds could
also impact TAF sites in San Diego County Monday morning through
Monday afternoon.

&&

.MARINE...
No hazardous weather is expected through this afternoon. West-
northwest winds will increase to 20-30 kt tonight and continue
through Monday night, with gusts 35 to 40 kt possible in the outer
waters Monday morning. Combined seas of 9-12 ft with steep waves are
likely through the period. Conditions will improve Tuesday.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Winter Weather Advisory from 6 PM this evening to 10 PM PST
     Monday for Riverside County Mountains-San Bernardino County
     Mountains-San Diego County Mountains.

     Wind Advisory from 2 PM this afternoon to 6 PM PST Monday
     for Coachella Valley-San Diego County Deserts-San Gorgonio
     Pass Near Banning.

     High Wind Warning from 2 PM this afternoon to 6 PM PST Monday
     for Apple and Lucerne Valleys.

     Frost Advisory from 10 PM Monday to 9 AM PST Tuesday for San
     Diego County Valleys-San Gorgonio Pass Near Banning.

     Wind Advisory from 8 AM to 6 PM PST Monday for San Diego County
     Coastal Areas-San Diego County Valleys.

     Frost Advisory from 10 PM Tuesday to 8 AM PST Wednesday for San
     Bernardino and Riverside County Valleys-The Inland Empire.

     Freeze Warning from 10 PM Monday to 9 AM PST Tuesday for San
     Bernardino and Riverside County Valleys-The Inland Empire.

PZ...Gale Warning from 10 PM this evening to 6 AM PST Tuesday for
     Waters from San Mateo point to the Mexican Border Extending
     30 to 60 nm out including San Clemente Island.

     Small Craft Advisory from 10 PM this evening to 6 AM PST Tuesday
     for Coastal Waters from San Mateo Point to the Mexican
     Border and out to 30 nm.

&&

$$

PUBLIC...jad
AVIATION/MARINE...PG



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