Winter Weather Advisory 10, NWS San Diego, CA

Area Forecast Discussion
National Weather Service San Diego CA
1000 AM PST Sat Dec 25 2021

.SYNOPSIS...
Scattered showers will continue today, with increasing precipitation
tonight, including areas of heavy snow in the San Bernardino and
Riverside County mountains. More storm systems will bring heavier
precipitation again late Monday into Tuesday and then possibly again
around Wednesday or Thursday, with more snow in the mountains.
Temperatures, especially during the days, will remain below normal
through next week, and snow levels could fall below 3000 feet in
some areas Tuesday into Wednesday.

&&

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

Mostly cloudy skies covered the region, except for sunny conditions
in the San Bernardino County deserts. Scattered showers continued,
mostly light. That is pretty much the picture for the rest of today,
though some locations near the coast could partially clear.

Another wave will move through tonight, with best moisture fields
between about 10 PM and 4 AM, which is when precip should be
heaviest. With snow levels of 5000 feet or a little less, it should
be sufficiently cold for snow in the mountains, especially the
SBD/RIV County mountains where up to a foot of snow could fall. NBM
is showing a lot of variability though between 10th and 90th
percentile (basically zero vs. amounts up to 2 feet), but with
relatively consistent ENS solutions, will lean towards around the
75th percentile values. Orographic precipitation should be
substantial given the cross-ridge 850 MB winds of 30-50 knots and
saturation below about 700 MB, though above 700 MB the RH is much
lower, so that could be a limiting factor in snow amounts. With the
orographics, there could be local rainfall of 2 inches on the SW
mountain slopes.

Another system should move through late Monday into early Tuesday,
and this could produce heavier snow in the mountains than the wave
tonight as the moisture will be deeper--to around 500 MB--and
extending well into the dendritic snow crystal growth layer with
temps aloft below -10 C, as well as a lowering snow levels, possibly
below 3000 feet in northern areas by early Tuesday. San Diego County
should finally get significant accumulations with that system. SBD
County mountains could get over a foot of new snow. Due to the cold
air aloft, there could be greater instability, so a thunderstorm,
especially near/over the ocean water, is not out of the question
then.

After Tuesday, confidence decreases greatly with timing and amounts
of precipitation, so effectively the big picture message stands:
more chilly weather with periods of precip, though the position of
the possibly closed low Thursday will have a big impact on
where/when precip occurs. Occasional wind will occur at times in the
mountains and deserts through most of next week.

&&

.AVIATION...
251730Z...Scattered SHRA will continue today, then become widespread
tonight, then gradually taper off Sunday morning. Bases will be
between 1000-2000 feet MSL in SHRA, with localized lowering to 500
feet MSL at times during the heavier showers. Vis reduced down to 2-
4 SM in SHRA. Otherwise, bases will be to 2000-5000 feet MSL through
Sunday morning. Mountain slopes will continue to be obscured in
clouds/fog through Sunday morning. South to southwest winds will
increase this evening over the mountains and along the desert
slopes, with gusts of 40-45 kts at times. Moderate to strong
up/downdrafts and LLWS expected over/east of the mountains.

&&

.MARINE...
Scattered showers will continue today. Small craft advisory
conditions are expected this evening through Sunday morning, due to
stronger westerly winds, especially over the outer waters. Another
Pacific storm system could bring renewed small craft advisory
conditions Monday afternoon through Tuesday, along with periods of
rain squalls.

&&

.HYDROLOGY...
Scattered showers will increase in coverage today in advance of a
low pressure system from the northwest that will bring widespread
precipitation for tonight through early Sunday. Rainfall is
expected to range from one-quarter to one-third inch near the coat
to 1 to 2 inches in the mountains with mostly one-tenth inch or less
in the deserts through early Sunday. Snow levels will lower from
around 5000 feet this evening to 4500 feet early Sunday. This will
result in heavy snow in the mountains, especially in San Bernardino
and Riverside Counties.

The next low pressure system from the northwest will bring another
round of widespread precipitation late Monday through early Tuesday
with rainfall ranging from one-third to one-half inch near the coast
to 1 to 1.5 inches in the mountains with one-quarter inch or less in
the deserts. Snow levels will lower from around 4500 feet late
Monday to 2500 to 3500 feet early Tuesday.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Winter Storm Warning from 6 PM this evening to 10 AM PST Sunday
     for Riverside County Mountains-San Bernardino County
     Mountains.

PZ...NONE.

&&

PUBLIC/HYDROLOGY...Maxwell
AVIATION/MARINE...Connolly