Area Forecast Discussion
Issued by NWS San Diego, CA

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000
FXUS66 KSGX 221124
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
324 AM PST Sun Jan 22 2017

.SYNOPSIS...
The third and most powerful in the sequence of storms to impact
Southern California will bring heavy rain...heavy mountain snow...
and strong damaging winds for late this morning through this
evening with showers continuing at times through Tuesday. Warmer
and dry weather will follow for Wednesday into next weekend with
periods of offshore flow and gusty northeast winds late in the
week.

&&

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

.SHORT TERM (Today through Tuesday)...
The third and most powerful of the storms to impact Southern
California in this sequence of storms is centered off the Pacific
Northwest coast early this morning. Adjustments to the current
forecast are mostly to speed up by a few hours the arrival of the
more significant impacts today.

Radar shows lighter...warm advection precipitation already
overspreading Southern California. The GFS 300K isentropic surface
better depicts this area of moisture and lift than do the model
precipitation fields. Precipitation chances have been increased
farther south sooner today to account for this mostly light
precipitation. A plume of seasonally high precipitable water
values (1.35 inch...or 300 percent of normal) will be directed
into Southern California through early evening. This moisture will
be lifted strongly along south facing mountain slopes this
afternoon...and southwest slopes through early evening with
significant orographic enhancement of precipitation...especially
in the San Gabriel and San Bernardino Mountains. Peak three-hourly
rainfall could exceed 3 inches.

Snow levels near 5000 feet early this morning will rise to around
6500 feet during the afternoon and early evening. Snow levels will
fall quickly from the northwest during the evening and overnight...
to around 4000 to 4500 feet by Monday morning. Snow levels will
fall to around 3000 to 3500 feet for Monday night and early
Tuesday.

Following the passage of the cold front...showers will follow for
tonight into Tuesday. The coldest mid level air...with 500 mb
temperatures around -30 C...will arrive from the northwest during
the day on Monday. This will further destabilize the atmosphere
with a slight chance of thunderstorms for later on Monday through
early Tuesday with small hail possible.

Forecast rainfall through Tuesday...

Coast..............2 to 2.5 inches.
Valleys............2.5 to 3.5 inches.
Mountains..........3.5 to 6 inches. Isolated 8+ inches.
Upper deserts......0.75 to 1.5 inches.
Lower deserts......0.25 to 1 inch.

Very strong prefrontal south winds are expected to develop
today...with strongest winds for the afternoon and early evening.
Following the passage of the front...periods of strong and gusty
west winds will continue through Monday.

Both Monday and Tuesday will be very cool days with high
temperatures well below average with high temperatures at the
lower elevations mostly in the upper 50s to around 60.

&&

.LONG TERM...
Drying and a little warmer for Wednesday. Continued dry with a
warming trend for Thursday into next weekend with warming to
near seasonal levels for next weekend. Periods of gusty northeast
winds are possible for Friday into next weekend.

&&

.AVIATION...
221100Z...Light precip has already moved into the area and will
become more widespread and heavier later this morning into the
afternoon. BKN-OVC CIGS layered 2000-15000 ft MSL with increasing
showers coming in from the west through 18Z this morning. Expect
cigs and vis lowering to 1000-1500 ft MSL and 1-3SM, during heavier
bursts of rain. Mountain obscurations likely to become widespread
after 15Z this morning. These conditions will persist through
tonight. Southerly winds of 12-20KT gusting to 30-35KT will develop
along the coastal areas after 15Z this morning, becoming westerly
around 12Z Mon.

As wind speeds increase Sun morning, expect mostly mdt
up/downdrafts and LLWS over the mtns and adjacent desert areas.

&&

.MARINE...
Winds are picking up again early this on the coastal waters...for
example the San Clemente Buoy is now reporting SCA level winds and
combined seas 15` or higher.  These conditions are likely to
continue through Monday night and with winds gradually picking back
up to near gale force once again later today and continuing through
Monday. In addition to the winds and seas, heavy rain squalls will
develop later today and likely continue through Monday afternoon,
when there will also be a slight risk of thunderstorms. A Gale
Warning continues through Monday night.

&&

.BEACHES...
A High Surf Warning remains in effect. Very large west swells will
produce surf heights of 8-12 feet with sets to 16 ft at times. Surf
this large could damage structures and produce beach erosion. Early
morning high tides will only be around 5 ft, so the coastal flooding
threat will be mitigated somewhat.

In addition, there will be a slight chance of thunderstorms Monday
afternoon and evening at the beaches.

The swells will diminish a little this morning, but high surf
conditions will persist into Tuesday. It is not until Wednesday that
surf and swell conditions will lower below high surf thresholds.

More high surf is expected next Friday as a large long-period swell
(9 ft/19 seconds) from 290 degrees moves into the coastal waters and
surf zone.

&&

.HYDROLOGY...
Expected rainfall through Monday could bring additional flooding
along the San Diego River. The latest CNRFC forecast is for the
river to reach Flood Stage at 2 AM PST Monday morning. The Santa
Margarita River is expected to exceed Action Stage by 10 PM PST
this evening...which would have impacts to surface streets along
the River on Camp Pendleton. Other areas of concern will be the
Whitewater Basin in the Coachella Valley and the Tijuana River
Basin along the International Border. These waterways will likely
experience flows that may have impacts to transportation...
people...and other interests in their respective flood plains.

Even the slower-to-respond San Luis Rey River is expected to have
significant flow which could impact low water crossings in the
upper watershed...and the normally dry Mojave River could see
some water covering most of the channel for a time Sunday night
into Monday.

Flash flooding in other parts of our forecast area is possible as
well. The system will start out with a period of moderate to heavy
rain this afternoon and early evening...then become more showery
with some of the showers having briefly more intense rainfall.
Isolated thunderstorms are also possible...mainly Monday afternoon
through early Tuesday.

Snow levels near 5000 feet early this morning will rise to around
6500 feet during the afternoon and early evening. Snow levels will
fall quickly from the northwest during the evening and overnight...
to around 4000 to 4500 feet by Monday morning. Snow levels will
fall to around 3000 to 3500 feet for Monday night and early
Tuesday.

Forecast rainfall through Tuesday...

Coast..............2 to 2.5 inches.
Valleys............2.5 to 3.5 inches.
Mountains..........3.5 to 6 inches. Isolated 8+ inches.
Upper deserts......0.75 to 1.5 inches.
Lower deserts......0.25 to 1 inch.

&&

.SKYWARN...
Skywarn activation is requested for this afternoon and evening
from noon to 10 PM.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Winter Storm Warning from noon today to 6 PM PST Monday for
     Riverside County Mountains-San Diego County Mountains.

     Flash Flood Watch from this afternoon through Monday evening for
     Coachella Valley-San Diego County Coastal Areas-San Diego
     County Deserts-San Diego County Mountains-San Diego County
     Valleys.

     Wind Advisory from 6 AM to 6 PM PST Monday for San Diego County
     Deserts-San Gorgonio Pass Near Banning.

     High Wind Warning from 10 AM this morning to 6 PM PST Monday for
     Apple and Lucerne Valleys.

     Flash Flood Watch from 10 AM PST this morning through Monday
     evening for Apple and Lucerne Valleys-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 Gorgonio
     Pass Near Banning-Santa Ana Mountains and Foothills.

     High Wind Warning from 2 PM this afternoon to 6 PM PST Monday
     for San Diego County Coastal Areas-San Diego County Valleys.

     Winter Storm Warning from 8 AM this morning to 6 PM PST Monday
     for San Bernardino County Mountains.

     High Wind Warning from 10 AM this morning to noon PST Monday for
     Orange County Coastal Areas-Orange County Inland Areas-San
     Bernardino and Riverside County Valleys-The Inland Empire-
     Santa Ana Mountains and Foothills.

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

PZ...Gale Warning until 4 AM PST Tuesday 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/HYDROLOGY...17
AVIATION/MARINE/BEACHES...JMB


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