Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 200457
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
857 PM PST Thu Jan 19 2017

...Damaging Wind Likely Friday, Capable of Downing Trees...
...Heavy Rainfall Friday, Capable of Producing Flash Flooding...

.SYNOPSIS...
A very stormy pattern will continue across Southern California
through Monday. The next in the series of Pacific storms will
move across the area on Friday with heavier rain and mountain
snow, possible flooding and stronger, damaging winds for portions
of the area along with scattered thunderstorms. On Sunday the
next system arrives and continues into Monday, possibly bringing
the heaviest precipitation of the event. Dry weather is expected
by midweek.

&&

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


...Update - High Impact Weather Expected on Friday...

A storm seen churning on satellite imagery off the Pacific NW will
barrel southeast and move across Southern California on Friday-
Friday night and is poised to bring high-impact weather to the
area.

Precipitation looks to begin just before the morning commute and
becomes more widespread and heavier by late morning and through
the afternoon. H85 winds off the latest WRF increase to 50 KT by
early afternoon. CAPE reaches 200-500 J/KG. This mix of sufficient
instability (given the excellent dynamics -- left front region of
a 200 mph Jet) along with high winds just above the surface gives
concern for damaging winds in addition to the flash flooding
potential. Going to be a very active weather day on Friday and
then again on Sunday into Monday. Graphic posted on social media
highlights the hazards and precip amounts for Friday...
@NWSSanDiego on Twitter.

We will closely be monitoring area rivers, especially the flashy
San Diego River. The forecast currently is below flood stage at
Fashion Valley, but we definitely cannot rule out flooding along
this river by late Friday afternoon or evening. /Gregoria

...Previous Discussion (Issued at 539 PM PST Thu Jan 19 2017)...

The first in a series of storms has moved through the region today,
bringing moderate and locally briefly heavy showers mainly to
areas over and west of the mountains. Over the last 18 hours, the
coast and valleys have seen generally 0.50-0.75 inches, while the
coastal slopes have seen about 0.75-1.5 inches, and about
0.05-0.15 inches in the deserts. Radar shows that showers have
diminished for the most part, with just some isolated post-
frontal light showers, with the snow level around 5500-6000 ft.
Snow levels will likely stay at around that level today. Latest
HRRR shows mainly isolated showers continuing through the rest of
the day.

On water vapor satellite, the current trough can be seen moving
east across the region. On Friday a strong short-wave trough and
accompanying 130 kt jet noses its way into Southern California,
bringing a round of heavier precipitation than we`ll have today.
For this storm, showers start to increase early Friday morning,
with widespread moderate to heavy precipitation mid-Friday morning
through Friday afternoon. Showers then diminish gradually Friday
night into Saturday morning except in mountains where some showers
may linger through the day Saturday in westerly flow aloft. As far
as thunderstorm chances goes, the NAM12 and WRF show 500-1000
J/KG of most unstable CAPE and up to 30 kt of low level wind shear
late Friday morning and afternoon. Given these parameters, would
not be surprised if we see a few strong thunderstorms producing
severe wind gusts tomorrow. In addition, 850 mb winds increase to
35-50 kt out of the southwest late Friday morning and afternoon,
which will not only increase the potential for higher orographic
precipitation along the coastal mountain slopes, but also bring
strong winds not only to the mountains and deserts but also the
coast and valleys. Hi-res models show sustained winds reaching
20-30 kt over the coastal and valley areas Friday morning and
afternoon as the cold front moves through, which translate to wind
gusts of 35-45 mph and locally stronger. This, in addition to the
likely strong convective downburst that will move through the
area, has led to the issuance of a High Wind Warning for the
coasts, valleys and Santa Ana Mountains/foothills, with damaging
wind gusts possible. Meanwhile, the San Diego county deserts look
to have a bit stronger mountain wave activity Friday and Saturday
than the other desert locations, so the High Wind Watch was
converted to a High Wind Warning for those areas while a Wind
Advisory was issued for the other deserts. Meanwhile, strong wind
gusts of 70 mph or greater will be possible along the mountain
ridges and desert slopes.

A large, broad trough will move down off the coast of California
on Sunday, directing an atmospheric river into Southern
California, with precipitable water increasing to 1.4 inches. This
moisture, in addition to southwest 850 mb winds increasing to 50
kt, will bring widespread moderate to heavy precipitation, higher
snow levels, and strong winds to the region. The trough then drops
down into Southern California on Monday, bringing colder air with
it, and less intense precipitation. For rain and snow forecast
amounts and flooding potential for the next couple storms, please
see the HYDROLOGY section below. Long-range models indicate some slight
instability on Monday, so perhaps thunder will be a possibility
that day as well.

There`s the possibility that another short-wave will move through
the region on Tuesday, with the slight chance for some showers
that day.

Warming and dry weather expected starting next Wednesday as a
ridge aloft builds over the west.

&&

.AVIATION...
200400Z...Coast and Valley...SCT-BKN025-040 tonight. Widespread rain
and strong winds Friday with BKN-OVC010-020 lowering to OVC005-010
in strongest cells. Vis 2-4SM in Rain and Fog locally below 1/2 mile
in RA+. Sustained winds 20-30 kt between 18-23Z impacting all
coastal and valley TAF sites. A strong/severe thunderstorm possible
with squalls to 50 kt.

Mountains and Deserts...SW to W wind gusts of 50-60 kt over the
mountains and down the desert mountain slopes and into adjacent
deserts Friday. Severe UDDFS will be possible on the lee side of the
mountains through the period. Mountains obscured at in clouds
and pcpn Friday.


&&

.MARINE...
Several storms moving across the region will continue unsettled
conditions through Tuesday. A Small Craft Advisory continues over
the waters through early Friday morning, then it will be upgraded to
a Gale Warning from midday Friday through Saturday. A Gale Warning
may also be issued gain on Monday. See the Small Craft Advisory and
Gale Warning (LAXMWWSGX) currently in effect for additional details.

&&

.BEACHES...
A High Surf Warning in effect through Tuesday. Large damaging surf
is likely late Friday through Saturday, and again Monday into
Tuesday. Coastal flooding and beach erosion possible as highest sets
to 16 ft occur Friday night through Saturday coincident with times
of high tide. High tide is 4.6 ft at La Jolla at 435 AM Saturday.
The extreme surf and strong rip currents will make swimming
conditions very dangerous. High 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 bringing the heaviest precipitation seen
here since 2010. The storms will bring significant amounts of
rain, with heavy snow accumulations at elevations above 5000 ft.
Some areas could receive 5 to 10 inches of total rainfall between
through 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 the Friday storm, snow levels will likely be around 5500-6000 feet
or so, with 1-2 feet of snow possible above 6000 feet and 6
inches to a foot between 5000 and 6000 feet. A Winter Storm
Warning continues for the San Bernardino and Riverside county
mountains through 6 am Saturday...and a Winter Storm Warning was
issued for the San Diego county mountains for areas above 5000
foot elevation. Storm total precipitation forecast for Friday
looks to be:

Coast and Valleys: 1-2 inches
Mountains: 2-4 inches
High Deserts: 0.15-0.40 inches
Lower Deserts: 0.10-35 inches

A Flash Flood Watch has been issued for areas over and west of
the mountains on Friday due to the likely heavy rainfall rates and
thunderstorms.

For the Sunday-Monday storm, it will be a warmer storm initially
so snow levels look to be initially high around 7000-8000 ft
Sunday then lowering quickly to 4000 ft Monday. Storm total
precipitation forecast for Sunday-Monday looks to be:

Coast and Valleys: 2-3 inches
Mountains: 3-6 inches
High Deserts: 0.50-1.5 inches
Lower Deserts: 0.25-1 inch

With precipitation amounts this heavy, more flooding issues look
likely, especially considering the precipitation amounts and
saturated Ground from previous rainfall.

When all is said and done, we could see a total of:
Coast: 3-5 inches
Valleys: 4-6 inches
Mountains: 6-12 inches
High Deserts: 1-3 inches
Lower Deserts: 1-2 inches

&&

.SKYWARN...
Skywarn activation is requested Friday morning-afternoon and
again on Sunday. Threats: Damaging winds and flash flooding and
extreme mountain snowfall.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Flash Flood Watch from 7 AM PST Friday through Friday evening
     for Orange County Coastal Areas-Orange County Inland Areas-
     Riverside County Mountains-San Bernardino County Mountains-
     San Bernardino and Riverside County Valleys-The Inland
     Empire-San Diego County Coastal Areas-San Diego County
     Mountains-San Diego County Valleys-San Gorgonio Pass Near
     Banning-Santa Ana Mountains and Foothills.

     Wind Advisory until 4 AM PST Friday for San Diego County Deserts-
     San Diego County Mountains.

     Winter Storm Warning from 4 AM Friday to 6 AM PST Saturday for
     San Diego County Mountains.

     High Wind Warning from 4 AM Friday to 10 PM PST Saturday for San
     Diego County Deserts.

     Wind Advisory until 10 PM PST Saturday for Apple and Lucerne
     Valleys-San Gorgonio Pass Near Banning.

     Winter Storm Warning until 6 AM PST Saturday for Riverside
     County Mountains-San Bernardino County Mountains.

     High Wind Warning from 7 AM to 10 PM PST Friday for Orange
     County Coastal Areas-Orange County Inland Areas-San
     Bernardino and Riverside County Valleys-The Inland Empire-
     San Diego County Coastal Areas-San Diego County Valleys-
     Santa Ana Mountains and Foothills.

     Wind Advisory from 10 AM Friday to 10 PM PST Saturday for
     Coachella Valley.

     High Surf Warning until 10 PM PST Tuesday for Orange County
     Coastal Areas-San Diego County Coastal Areas.

PZ...Small Craft Advisory until 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.

     Gale Warning from 10 AM Friday to 4 PM PST Saturday 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)/Harrison (Previous Discussion)
AVIATION/MARINE...Moede



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