Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 180619
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
1000 PM PST Tue Jan 17 2017

.SYNOPSIS...
A series of storm systems will move through the region late
Wednesday night through early next week and bring rain, heavy at
times, and snow in the higher mountains. The first storm will bring
mostly moderate rain with a slight chance of thunderstorms and some
snow in the mountains Thursday, and then a second stronger storm
will bring some heavy rain and possibly a thunderstorm Friday with
heavier snow in the mountains, along with strong winds in the
mountains and deserts. Showers will continue over the weekend before
a warmer storm system moves through late Sunday night and Monday,
with more rain and less areal coverage of snow in the mountains.
Fair weather will likely not return until Tuesday or Tuesday night.


&&

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

Storm systems are still on track to bring substantial rainfall
Wednesday night through at least Monday, with heavy snow in parts of
the mountains. Models are pretty consistent with total precipitation
amounts through Monday but differ somewhat on timing of each
individual storm. The first storm will arrive with mostly light
precipitation Wednesday night with an initial wave, becoming
moderate Thursday with a slight chance of thunderstorms due to some
CAPE values over 500 J/kg which could bring locally heavier
rainfall. Overall, the first storm will bring 3/4 to 1 inch west of
the mountains with 1 to 2 inches in the mountains with deep enough
moisture to spill some precip into the deserts. Snow levels mostly
appear to be 5000 to 5500 feet, except a little higher in San Diego
County. Winter weather advisory is in effect for the mountains late
Wednesday night through Thursday night with mostly 4-8 inches above
about 6000 feet. Moderately strong winds will occur in some of the
deserts, especially the high deserts and desert mountain slopes.

The system for Friday and Friday night is stronger with most areas
west of the mountains getting at least one inch, and possible strong
orographic amounts of 2 to 4 inches on the coastal mountain slopes.
Snow levels will be slightly lower overall, mostly around 5000 feet,
though varying somewhat during the storm as some warm air advection
will precede the second storm and could bring snow levels briefly
above 6000 feet early Friday. A winter storm watch is in effect for
the northern mountains as snowfall could be well over a foot in that
area. Winds will be strongest with this storm and could last into
Saturday in the same mountain/desert wind prone areas. Some flooding
will be possible with this storm, though the current CNRFC forecast
for the San Diego River at Fashion Valley has it in monitor stage
around 9 feet for Saturday, compared to flood stage of 11.3 feet.
After the main Friday/Friday night period of the storm, some showers
will continue over the weekend due to moist onshore flow, which
could include some additional snow in the mountains.

The final storm arrives sometime around Sunday night with a strong
warm air advection in the 850-700 mb layer, so snow levels could
jump to 7000 feet or higher Sunday and Sunday night, which would
quite obviously decrease the total snow amounts at least until
colder air moves in from the west/northwest Monday. Earlier model
runs were a little weaker with this storm, but 00Z GFS has the storm
stronger again, so substantial rainfall will be possible. Soils will
very likely be saturated by this time, so the flood threat will be
greatest with this storm. A few showers could linger into Tuesday,
with model solutions indicating a return to fair weather and
possible offshore flow midweek next week.

Total predicted rainfall amounts through Monday are as follows:

Coast:  2-5 inches
Valleys:  3-6 inches
Mountains:  6-12 inches on coastal slopes,
            with locally higher amounts
High Deserts: 1-3 inches
Lower Deserts: 0.75-1.5 inches

Total snowfall could reach 2 to 3 feet around and above 7000 feet,
though some compaction at times or even rainfall around Sunday night
could result in lower snow depths at the end of the series of storms.

&&

.AVIATION...
180420Z...Patchy low clouds with bases 1000-1500 ft MSL developing
along the coast this evening and spreading into the western valleys
overnight. Local vis 1-3 SM in BR in the valleys overnight. Low
clouds becoming SCT after 16Z, then increasing cloud layers AOA 3000
ft MSL and unrestricted vis through Wednesday evening. Coastal
slopes of the mountains becoming obscured in clouds/fog Wed evening
as well.

&&

.MARINE...
A series of Pacific storms will bring strong winds, rain and large
rough seas Thursday through Tuesday. Conditions hazardous to small
craft are likely Thursday, then gale force wind gusts to 35 kt and
very large combined seas of 14-18 ft are likely Friday and Saturday.
Winds and seas will diminish Sunday, then gale force conditions
possible again late Sunday through Monday as another system moves
across the area.

&&

.BEACHES...
Swell will gradually increase Thursday into Saturday creating
periods of high surf. The highest surf will occur Saturday when
waves could be large enough to cause coastal damage, coastal
flooding and beach erosion. Highest sets will be around 12-15 ft.
Swell and surf will decrease Sunday before increasing again Monday.

&&

.SKYWARN...
Skywarn activation will not be needed tonight.


&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory from 10 PM Wednesday to 4 AM PST Friday for Apple
     and Lucerne Valleys-San Diego County Deserts-San Diego
     County Mountains-San Gorgonio Pass Near Banning.

     High Wind Watch from late Thursday night through Saturday
     morning for Apple and Lucerne Valleys-San Diego County
     Deserts-San Diego County Mountains-San Gorgonio Pass Near
     Banning.

     Winter Weather Advisory from 10 PM Wednesday to 4 AM PST Friday
     for Riverside County Mountains-San Bernardino County
     Mountains.

     Winter Storm Watch from late Thursday night through late Friday
     night for Riverside County Mountains-San Bernardino County
     Mountains.

&&

$$

PUBLIC...Maxwell
AVIATION/MARINE/BEACHES...SS



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