Southwestern California Discussion

 
000
FXUS66 KSGX 252133
AFDSGX

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

.SYNOPSIS...
Our dry and mild offshore pattern will turn back onshore on Monday,
returning marine air and cooler weather. 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 little warmer on
Wednesday, then cool and breezy with more scattered showers possible
late Thursday through Saturday.

&&

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

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...
252100Z...Mostly clear skies and unrestricted VIS will prevail
through 15Z Mon, with SCT-BKN clouds developing after 15Z Monday
from the coast to the coastal mountain slopes with variable bases
1500-3000 ft MSL possible. Clouds will be more widespread by Monday
evening with SHRA likely with SHSN in the mountains and higher
terrain obscured.

&&

.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 winds 20-25 kt will be likely.
A small craft advisory will be 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...NONE.

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
AVIATION/MARINE...Maxwell

NWS SGX Office Area Forecast Discussion