Area Forecast Discussion
Issued by NWS San Diego, CA

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933
FXUS66 KSGX 260200
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
559 PM PST Sun Feb 25 2018

.SYNOPSIS...
A cold low pressure system from the north will bring periods of
rain and snow showers with low snow levels Monday night through
Tuesday. Dry and a little warmer on Wednesday, then cool and
breezy with more showers possible Thursday evening through
Saturday.

&&

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

...Updated forecast...

Updates were made to issue the winter weather advisories for the
mountains and the high desert of San Bernardino County. Have also
made some adjustments to the forecast in general for the Monday
night-Tuesday storm system.

...Previous forecast discussion...

Skies were clear over SoCal midday. Some marine clouds were visible
in satellite imagery, but were well offshore. Offshore surface
pressure gradients were weakening from the deserts and the gusty NE
winds this morning were subsiding in offshore wind-prone areas.
Under sunny skies at Noon PST, temperatures were running several
degrees F over values observed yesterday.

Surface high pressure over the Great Basin is weakening this
afternoon, along with the offshore flow. The dry air extends well
offshore so no marine layer clouds are expected overnight, This will
set us up for another cool night, but not quite as cool as last
night. Still, some valleys will likley experience some patchy frost
before sunup.

On Monday...the onshore flow will pick-up and allow marine air to
return with some lower clouds developing through the day. It will be
cooler as well. By evening, clouds will be thickening along the
foothills and westward toward the coast. Scattered showers may
break out after dark, and become more numerous after Midnight as a
cold front sweeps east over the coastal waters making landfall by
around 4 AM PST. Depending on the timing of the front, the coverage
of the showers may be largest before 8 AM Tue.

The various model suites vary in timing and track of the incipient
cut-off low, but it will develop close by on Tue, before heading
east on Tue night. This will generate plenty of instability showers,
and the moisture is deep enough to reach the deserts. A few showers
could have some thunder and be briefly heavy. Due to the low snow
levels of between 3K to 3.5K FT for this event, several inches
of snow is expected in the mts, and there may even be an inch or so
over portions of the high desert. The higher portions of the Cajon
Pass and I-8 corridor may get icy as well. Expect Winter Weather
Advisories to be issued as certainty increases regarding track and
intensity of this storm.

Overall liquid precip amounts are expected to be mostly below one
inch in the mts, and below one-half inch along the coast and in the
valleys.

Dry and continued cool weather Wednesday and Thursday. By late Thu
and into the weekend, another cold, low pressure system will drop
into the PacNW. As sfc pressure falls over the Great Basin, and
onshore flow ramps up the marine layer again over the region, more
light precip is possible. The heavier precip should remain to the
north, unless subsequent models runs show more energy coming south.
At this point, Sunday and early next week look dry for SoCaL.

&&

.AVIATION...
260200Z...VFR and P6SM VIS through Monday afternoon. Increasing low-
level clouds late Monday afternoon, with mountain tops being
obscured at times. Showers will begin to develop across the area
Monday evening, then increase in coverage Monday night. VIS will
lower to 1-3 SM in showers, with low cloud bases around 2000 ft MSL.

&&

.MARINE...
No hazardous marine weather is expected through Monday morning.
Winds and seas will increase over the SoCal Bight late Monday
through Tuesday morning as a storm system moves through the region.
Combined seas of 6-9 ft and northwest wind gusts of 20-25 kt are
likely. A Small Craft Advisory remains in effect for the inner and
outer coastal waters from 2 PM Mon-noon Tue. Isolated thunderstorms
are possible Tuesday.

&&

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


&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Winter Weather Advisory from 7 PM Monday to 10 PM PST Tuesday
     for Riverside County Mountains-San Bernardino County
     Mountains-San Diego County Mountains.

     Winter Weather Advisory from 10 PM Monday to 10 PM PST Tuesday
     for Apple and Lucerne Valleys-Santa Ana Mountains and
     Foothills.

PZ...Small Craft Advisory from 2 PM Monday to noon 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...jad/Brotherton
AVIATION/MARINE...JJT



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