Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 190349
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
749 PM PST Thu Jan 18 2018

.SYNOPSIS...
Partly to mostly cloudy skies will prevail through Friday morning
with low clouds, patchy fog, and drizzle possible overnight. A
fast-moving low pressure system will move across the area Friday
through Saturday bringing much cooler temperatures, a few
showers, light snow in the mountains, and gusty winds in the
mountains and deserts. The storm system will depart the area
early next week with a return to dry weather and a gradual
moderation in temperatures.

&&

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

...Update...

The forecast remains on track with the main weather story being an
incoming upper level trough from the Pacific. This will be a cold
system with noticeably colder weather for Southern California
Friday and through the weekend. Temps will continue on the cool
side even into the first part of next week while some gradual
moderation of temps occurs.

The system will be fast moving and moisture will be limited. GFS
shows max PW at only around 0.85 inches with continued dry air in
the mid levels. This will limit precip amounts projected to be
0.15 inches or less west of the mountains. Some high res model
runs show many areas west of the mountains not even receiving
measurable precip, which seems possible. However, decided not
to lower QPF as it is already forecast to be low. The bulk of the
precip will be in the mountains where around 0.25-0.50" of precip
is forecast along with a few inches of snow at higher elevations.
The snow levels will fall to about 3500 FT by Friday night.

No changes were made to the forecast this evening.

...Previous Discussion (Issued at 302 PM PST Thu Jan 18 2018)...

An aggressive afternoon seabreeze blowing through the valleys
this afternoon is dropping temperatures significantly and bringing
increased haze along the foothills. A coastal eddy spun up
earlier today generating low clouds along the San Diego County
coastline. This activity has again pushed into the coast...further
north into northern San Diego County and southern Orange County.

Deeper marine layer will develop later tonight with patchy drizzle
developing near the coast and mesas as the cold Pacific trough
approaches the region. By Friday afternoon this trough will
move through Southern California and bring a period of strong
westerly winds over the mountains, and light precipitation for the
coast, valleys, and mountains. The deserts will see gusty winds
but not likely to reach wind advisory criteria and little or no
precipitation. Overall precipitation amounts will be low with this
one without good moisture transport parameters.

The main concern with the incoming trough, besides the gusty
winds, will be light rain causing wet road concerns for the
cities, and accumulating snowfall down to around 3500 feet Friday
night through Saturday morning (pass level) which could create
some slick spots on the roads. Although only a few inches of snow
are expected, the slick roads will have the potential to bring
significant travel hazards and delays.

A couple of weak troughs will move through next week but the
models are indicating weak moisture transport at best so there are
no chances for precip in the forecast through next week. The first
trough will come through on Monday with a decaying front and will
keep temperatures near normal. Another trough will come through
around Thursday with another period of gusty winds and cooler
temperatures but no precip expected at this time.

&&

.AVIATION...
190300Z...COAST/VALLEYS...Patches of low clouds and fog along the
coast not currently impacting TAF sites. Cloud bases are around 1000
ft MSL and the fog is locally restricting vis to 3-5 miles in a few
locations. Expect low clouds and fog to increase in coverage after
06Z. BKN-OVC cigs 1000-1500 ft MSL could impact coastal TAF sites
til 12Z. Aft 12Z, ceilings will likely lift to 2000-2500 ft
MSL through about 21Z then cigs lift to 3000 ft MSL or higher.

Mountains/Deserts...Variable high clouds into tonight then
increasing clouds by midday Friday, with higher terrain obscured.
Strong southwest winds will develop Friday afternoon, producing mdt-
strong up/downdrafts and LLWS over and east of the mtns.

&&

.MARINE...
Patches of fog with visibility 1 NM or less this evening, improving
overnight. A storm arriving Friday will bring periods of gale-force
winds and hazardous seas late Friday through Saturday. Refer to the
Gale Watch for details.

&&

.BEACHES...
Northwest swell and surf will increase tonight through Friday. High
surf will peak Friday night into Saturday morning before diminishing
late Saturday. Consult the High Surf Advisory for details.

&&

.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 5 PM Friday to noon PST Saturday
     for Riverside County Mountains-San Diego County Mountains.

     Winter Weather Advisory from 2 PM Friday to noon PST Saturday
     for San Bernardino County Mountains.

     High Surf Advisory until 8 PM PST Saturday for Orange County
     Coastal Areas-San Diego County Coastal Areas.

PZ...Gale Watch from Friday afternoon through Saturday evening for
     Coastal Waters from San Mateo Point to the Mexican Border
     and out to 30 nm-Waters from San Mateo point to the Mexican
     Border Extending 30 to 60 nm out including San Clemente
     Island.

&&

$$

PUBLIC...Gregoria (Update)/Brotherton (Prev Discussion)
AVIATION/MARINE...PG


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