Area Forecast Discussion
Issued by NWS San Diego, CA

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000
FXUS66 KSGX 182225
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
225 PM PST Wed Jan 18 2017

.SYNOPSIS...
Seasonably cool with increasing clouds through this afternoon. A
series of storm systems will then move through the region tonight
through early Tuesday. Rain and mountain snow will be heaviest and
westerly winds will be strongest Thursday morning, Friday
afternoon and Sunday night into Monday, along with a slight chance
of thunderstorms Thursday and Friday. Dry weather is expected to
return on Tuesday.

&&

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

Mid and high clouds continue to increase from the northwest ahead
of an approaching storm. Radar indicates light showers now moving
into Santa Barbara and Ventura counties. Showers are likely to move
into Orange, Riverside, San Bernardino and San Diego counties
this evening, increasing in coverage and intensity overnight as
the storm moves through the region. Periods of heavy rain and
snow will affect some areas and there is enough instability for a
slight chance of thunderstorms. The snow level will drop to about
5000 feet. Strong winds will be felt in most but not all areas.
The mountains and adjacent deserts will be most impacted by the
strong winds. This system will move through fairly quickly and
showers will decrease Thursday afternoon and evening.

A second storm moves in Friday morning and is not expected to be as
fast-moving as the first. It also looks to have a deeper moist
layer so more rain and snow amounts are expected. Most of the
precip from this storm will likely fall Fri afternoon with
widespread rain and showers. Coverage and intensity of precip will
taper off Fri night into Sat. The snow level will lower to around
4000 ft Sat morning as colder air moves in. Strong and gusty west
to northwest winds will continue, mainly impacting the mountains
and adjacent deserts. Conditions improve with weaker winds and
there is a break in the precip late Sat into Sun morning as a
transient ridge between the two storms moves over SOCAL.

On Sunday, a third storm arrives to bring another round of rain
and snow that looks heaviest Sunday night. Initially some warm air
is pulled in ahead of the approaching cold front for higher snow
levels Sunday. After frontal passage, colder air moves in to
bring snow levels down to 3500 feet by Mon morning. Soils will
very likely be saturated by this time, so the flood threat will be
greatest with this third storm. Rivers and burn areas will have to
be watched closely for flooding and debris flows. Showers could
continue into Tuesday morning.

Cumulative rain amounts for all three storms will be impressive.
Here are the current estimates: Coast: 2-5 inches, Valleys: 3-6
inches, Mountains: 6-12 inches on coastal slopes, High desert: 1-3
inches, Lower deserts: 0.75-1.5 inches. Total snowfall should be
2-3 feet above 7000 feet and 10-20 inches above 5000 feet. Several
inches can be expected above 4500 feet, and even a little in the
high desert on Monday. The strong jet stream is forecast to weaken
and lift out Tuesday for fair and warmer weather heading into the
middle of next week. Stay tuned to updates in the forecasts,
discussion, and hazard products in the coming days to understand
the risks and impacts involved with these storms.

&&

.AVIATION...
182200Z...Low cloud bases, with bases 2500-3500 ft MSL, will
increase in coverage this evening west of the mountains as -SHRA
develops from NW to SE. RA will become widespread Thursday morning
and afternoon, with low cloud bases lowering to 1000-1500 ft MSL at
times. VIS could lower to around 1SM at times in heavier showers.
Isolated TSRA are also possible Thursday.

SW wind gusts of 50-60 kt will develop tonight over the mountains,
desert mountain slopes and into adjacent deserts, with gusts of 20-
30 kt possible over the coast and valleys Thursday morning and
afternoon. Severe Up/Downdrafts will be possible on the lee side of
the mountains through the period.

&&

.MARINE...
A few disturbances moving across the region will create unsettled
conditions tonight through Monday night. Small Craft Advisory
conditions will develop Thursday, then Gale Warning conditions will
develop Friday, Saturday and possibly Monday. See the Small Craft
Advisory and Gale Watch (LAXMWWSGX) currently in effect for
additional details.

&&

.BEACHES...
A series of storms will generate messy, large surf along the beaches
Thursday through Tuesday next week. Large and damaging surf is
possible late friday through saturday, and again Monday into
Tuesday. Coastal flooding and beach erosion is possible as highest
sets to 16 ft develop Friday and Saturday. Strong rip currents and
large surf will make swimming conditions very dangerous. The Beach
Hazard Statement has been upgraded to a High Surf Warning through
the period. Surf will diminish by the middle of next week.

&&

.HYDROLOGY...
A series of storms will move into Southern California over the
next 5 days. They will bring significant amounts of rain, with
snow accumulations at elevations above 5000 ft. Some areas could
receive 5 to 10 inches of total rainfall between tonight and
Monday night, possibly as much as 12 inches along the coastal
slopes. This has the potential to create an extended period of
enhanced runoff into streams and main-stem rivers. Over time, the
soil will have decreasing capacity to absorb rainfall resulting in
increasing runoff. Flash flooding will also be possible,
especially during periods of heavier rainfall. For more details,
please see the Hydrologic Outlook, located on our home page:
weather.gov/SanDiego.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory from 10 PM this evening 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 Storm Warning from 10 PM this evening to 6 AM PST
     Saturday for Riverside County Mountains-San Bernardino
     County Mountains.

     High Surf Warning from 4 PM Thursday to 10 PM PST Tuesday for
     Orange County Coastal Areas-San Diego County Coastal Areas.

PZ...Gale Watch from Friday morning through Saturday afternoon 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.

     Small Craft Advisory from 4 AM Thursday to 10 AM PST Friday 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...PG
AVIATION/MARINE/BEACHES...JJT


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