Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 190453
AFDSGX

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

.SYNOPSIS...
A series of storm systems will move through the region tonight
through early Tuesday, possibly resulting in the heaviest
precipitation event since 2010. Rain and mountain snow will be
heaviest and westerly winds will be strongest Friday and then
again Sunday through Monday. There is even a risk of strong
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...

...Update - Possibly the Heaviest Precipitation Event Since 2010
Underway and to Continue Through Monday...

Precip has made its way into Southern California this evening and
will continue spreading southeast. The heaviest period of precip
with this wave looks to be during the morning commute Thursday
and continuing through late morning before tapering off. A quick
0.50-0.75 inches is forecast west of the mountains to the coast
and 1-2 inches along the coastal mountain slopes Thursday.
Isolated thunderstorms will be possible as well...so will be
monitoring this for the potential of isolated higher rainfall
rates. It looks as though the greater potential for thunderstorms
and potential flash flooding comes with the next storm system
poised to hit Southern California on Friday...beginning Friday
morning and continuing through the afternoon while becoming
heavier. A Flash Flood Watch may be needed for portions of the
area Friday and will be assessed in the coming shifts.

Strong wind gusts capable of downing trees may be possible Friday
as low level winds really crank up then, so will be watching for
that potential as well. Then a short break on Saturday before the
next in the series of storms arrives Sunday morning and lasts through
Monday...and could be the strongest of this multi-day storm event.
This could be the heaviest precipitation event across Southern
California since 2010! Stay tuned. /Gregoria

...Previous Discussion (Issued at 225 PM PST Wed Jan 18 2017)...


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...
190410Z...BKN-OVC clouds 2500-3500 ft MSL increasing this evening
west of the mountains as -SHRA develops from NW to SE. RA will
become widespread Thursday morning and afternoon with bases lowering
to 1000-1500 ft MSL. Local CIGS blo 1000 ft and vis 2 SM or less in
heavier showers. Showers tapering off with a slight chance of TSRA
Thursday afternoon and evening.

SW to W wind gusts of 40-50 kt will develop tonight over the
mountains, desert mountain slopes and into adjacent deserts and
continue through Thursday evening. Severe UDDFS will be possible on
the lee side of the mountains through the period.

&&

.MARINE...
Several storms moving across the region will create unsettled
conditions tonight through Monday night. Small Craft Advisory
conditions will develop Thursday, then Gale force winds conditions
may develop Friday, Saturday and possibly again on Monday. See the
Small Craft Advisory and Gale Watch (LAXMWWSGX) currently in effect
for additional details.

&&

.BEACHES...
High surf is likely along the beaches Thursday through Tuesday.
Biggest and potentially 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. A High Surf Warning remains in
effect 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, possibly brining the heaviest precipitation seen here
since 2010. The storms 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 until 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 until 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...Gregoria (Update)/PG
AVIATION/MARINE...SS


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