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Area Forecast Discussion (AFD)

Issued by San Diego, CA (SGX)

FXUS66 KSGX 182150

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

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.



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.


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. 


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


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.