Area Forecast Discussion
Issued by NWS San Diego, CA

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000
FXUS66 KSGX 051700
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
900 AM PST Fri Mar 5 2021

.SYNOPSIS...
Offshore winds today will turn back onshore tonight and Saturday as
a weak trough passes to the north. This will set us up for a
noticeable weather change from Sunny, warm and dry weather today, to
partly cloudy, breezy, and cooler weather over the weekend. The
marine layer will deepen into Monday, and may produce some patchy
drizzle Monday and Tuesday mornings as a cold trough sinks slowly
south off the West Coast. The storm will spread rain and snow
showers across Southern California midweek, and keep daytime
temperatures well below average.

&&

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

Skies were clear this morning over all of SoCal this morning.
Locally gusty offshore winds were reported, and especially on those
wind-prone coastal slopes of the San Diego County mts. Peak NE gusts
were mostly 35-45 MPH. It was rather chilly as well in those wind-
sheltered valleys and coastal areas, where temperatures were in the
40s at 7 AM PST. The 12Z Miramar sounding had a sfc-based inversion,
with light and variable winds below 15K FT. Warming of 5-10C was
indicted below 10K FT, so it will be a much warmer day inland, and
maybe a bit warmer at the coast, but the sea breeze will keep a lid
on it. No forecast changes this morning.

After a mild and sunny day today, a strong shortwave trough will
swing through northern and central CA on Saturday, accompanied by
rain and mountain snow there, and an increase in westerly winds over
the mts/deserts of SoCal. This will bring in the marine air and
perhaps some low clouds and patchy fog tonight and Saturday morning.
A much more persistent troughy pattern will develop as a large and
cold longwave trough drops slowly south off the West Coast. This
will be accompanied by a gradual increase in the marine layer depth
and even the chance for some patchy drizzle along and west of the
mts Mon/Tue mornings. It will be noticeably cooler as well, with
daytime temperatures forecast to be some 10F below average inland.

The latest operational runs of the global models are close on the
track and intensity of the upper low dropping south off the coast
this coming week and all of the ensemble members are wet. Moderate
precipitable water values of 1/2-3/4 inch are forecast to accompany
this system as it arrives over SoCal by Wednesday. Precipitation
looks to linger through Thursday as well. Snow levels are forecast
to be in the 4000-4500 FT range through much of the event so
substantial snowfall could accumulate at the resort levels and
higher. Precipitation overall looks to be moderate, with moderate
rates as well based on the PW values. With significant cooling and
resultant instability though, some heavier showers look possible,
especially on Wednesday, when lapse rates are steep.

Preliminary precipitation totals are mostly around one inch or less
for most areas west of the mountains. More will be possible in the
mountains where significant snow accumulations are possible with
fairly low snow levels. There is the potential for more, so stay
tuned.

&&

.LONG TERM...(Monday through Thursday)...
Ensemble forecast models are in fair agreement of a large upper
level low pressure system pushing south from the Gulf of Alaska
toward the West Coast. This system looks rather cold, with
daytime highs struggling to reach 60 degrees by the middle of the
week. Some ensemble model guidance supports temperatures even
colder, so we will have to keep track of this as the time gets
closer. Strong west winds will once again be felt from the
mountains into the deserts with widespread gusts 35-45 MPH.
Tuesday will be the windiest day overall, where gusts from the
coast to the inland valleys may reach near 25 MPH.

As for precipitation, the marine layer will deepen with the incoming
trough, but precipitable water struggles to get to a half inch by
Monday, so should stay relatively dry. As the system pushes
southward, areas of light showers will push into the region on
Tuesday. We can expect the heaviest precipitation to fall Tuesday
night into early Wednesday morning. The trough axis will progress
slowly over SoCal Wednesday into Thursday, bringing a continued
chance for light to moderate showers. In total, much of the coast
and western valleys will generally see around one half inch to an
inch, with portions of the coastal slopes just over an inch. Some
parts of the lower desrts may receive up to one tenth of an inch of
rainfall. As for snowfall, snow levels will generally be 3000-4000
feet. Locales above 6000 feet may see 2-5 inches of fresh powder.

&&

.AVIATION...
051615Z...Mostly clear skies with unrestricted vis today. After 06Z
areas of low clouds are expected along the coast. Low clouds will
spread inland with moderate confidence in cigs forming at KSAN and
KCRQ around 09Z, with lower confidence of cigs at KSNA. Bases will
be 800-1500 ft MSL. Decreased vis of 2-4SM possible in areas of low
clouds. Low clouds will scatter out around 16Z Sat.

&&

.MARINE...
Northwest winds with occasional gusts to around 20 knots in the
outer waters during afternoons today through mid next week, with a
lull on Sunday.

&&

.BEACHES...
An increased west swell will result in elevated surf and a high rip
current risk at the beaches over the weekend. Surf will mainly range
from 3-5 feet, with sets to 6 feet possible in southern San Diego
County.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...NONE.
&&

$$

PUBLIC...10
AVIATION/MARINE/BEACHES...CO



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