Area Forecast Discussion
Issued by NWS San Diego, CA

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000
FXUS66 KSGX 181729
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
929 AM PST Wed Jan 18 2017

.SYNOPSIS...
Seasonably cool with increasing clouds today. 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...

Temperatures in the 40s and 50s this morning, with areas of low
clouds in the coastal areas and valleys ahead of an approaching
storm. Radar indicates showers still north and west of Point
Conception, but will likely move into Southern California this
evening, increasing in coverage and intensity overnight as a storm
system moves into 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 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 some preliminary
forecast totals: 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...
181700Z...SCT/BKN low clouds, with bases 1400-1700 ft MSL and tops
near 2300 ft MSL, will continue to develop along the coast and
spread inland towards the mountains this morning. Clouds should
become mostly FEW/SCT by late morning. Low cloud bases will rise
this evening and obscure coastal mountain slopes overnight. -SHRA
will also develop this evening and overnight from NW to SE, then
become RA tomorrow morning and afternoon. 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, high surf along the beaches
Thursday through Saturday. Coastal flooding and beach erosion is
possible as highest sets to 16 ft develop Friday and Saturday.
Strong rip currents and high surf will make swimming conditions very
dangerous. See the Beach Hazards Statement (LAXCFWSGX) currently in
effect for additional details. Elevated surf is possible again on
Monday as one last system moves across the region.

&&

.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.

&&

.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 Weather Advisory from 10 PM this evening 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.

     Beach Hazards Statement from Thursday morning through late
     Monday night 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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