Heavy Snow in California, December 24 to 27, 2021

Area Forecast Discussion
National Weather Service Medford OR
356 AM PST Fri Dec 24 2021

.DISCUSSION...

The first round of snow has moved through the region. A cold
front sits offshore ready to move in and bring in the next round
of precipitation later this morning. Right now, we're in a
relative lull in the weather. This is generally shown on radar as
of 3 am as not much is happening, except for some returns over
north eastern California and off the coast near Florence.

Otherwise, the precipitation will pick up again once this next
cold front moves onshore later this morning. Snow levels are
forecast to start out on the lower side around 2000 feet this
morning. This is believable given temperatures are currently
sitting in the low to mid 30's along some of the passes along I-5
this morning. For curious minds interested in snow totals with
this Friday segment of weather, one can check out our homepage and
view the "Christmas Eve Storm" graphic for snow totals through
Friday night. Impacts from this Friday event will be major,
especially in the Cascades where we're forecasting up to 2 feet of
snow near Crater Lake and Diamond Lake. We did check out snowfall
rates and it appears there is a high probability of 1" an hour
rates in the high Cascades a little after 12 pm, which will be
around the time the front pushes through.

Cold moist flow will continue behind the cold front as snow levels
are forecast to lower through the overnight hours of Friday into
Saturday morning. We do have snow levels down the to valley floors
west of the Cascades when the precipitation becomes showery. One
could consider this period a relative lull in between the systems.
However, precipitation should be ongoing, albeit at a lower
intensity.

Heading further into Saturday, an upper level trough will still be
present overhead with temperatures around -5C around 4500 feet
above sea level. Given hold cold this air will be, there should
be conditionally unstable air with the ocean surface in the upper
40's. This should create unstable air with lapse rates round
9C/km. What we're trying to get at here is showers should be a
little more intense along the coastal range Saturday. Areas of
concern will be in the sections of the Illinois Valley and Camas
Mountain as these locations could see some intense snowfall at
times. Model data seems to hint at this as well with the coastal
range under some of the heavier precipitation.

The cold unstable air continues to flow into southern Oregon and
northern California by Sunday. One could analyze another short
wave rotating in around this time, yet it appears to be just cold
unstable air continuing to push into the region.

Sunday night into Monday morning will be the most impactful period
during this multi day snow event. During this period, a second
upper level disturbance will rotate through the forecast region
behind the first wave on Sunday. The reason why this is
meaningful is the first wave cools us down enough to allow for
"significant" snow accumulation ahead of the second wave. This is
generally a pattern recognition type scenario for valley snow
here as the second wave digs in a bit over the ocean allowing it
to grab a little bit more moisture over the ocean. NBM
probabilities also back up late Sunday into Sunday night will be
significant. THe probability for > 4 inches of snow over 24 hours
from Sunday morning to Monday morning in Medford is about 80
percent and near 90 percent in Roseburg. We think the bulk of this
will occur overnight Sunday.

The last topic we wanted to discuss was about the watches,
warnings and advisories(WWAs). We essentially extended the WWAs
from Friday into Saturday simply for a simpler message. Saturday
is kind of a relative lull in between the two systems with weaker
dynamics, although snow will continue to accumulate at many
elevations. However, we felt pretty good about the impacts Sunday
night into Monday, thus we just pulled out most of the WWAs into
early Monday. We also added some of the coast range into another
Winter Storm Watches as those low snow levels Sunday night could
produce issues close to the coast.

Finally by Monday evening, conditions are forecast to improve
with the upper level low departing the region and heading towards
the south east. The story then turns to some pretty cold
temperatures filtering into Oregon. Overnight lows will fall down
to 0 in some locations east of the Cascades. Wind chills might
become a concern during this time.

-Smith

&&

.AVIATION...24/12Z TAFs...The back edge of a frontal band with light
to locally moderate snow and MVFR conditions will continue across
Lake and Modoc counties early this morning. Elsewhere, conditions
are mainly dry with a mix of VFR and MVFR and areas of mountain
obscuration, with a few isolated showers expected. A brief period of
valley IFR is possible this morning. In those locales, the next
front should bring a lifting of the stratus deck to MVFR during the
morning. Otherwise, The next front will bring deteriorating
conditions to southwest Oregon, northern Klamath County, and western
and central portions of Siskiyou County this morning through the
afternoon with rain/snow, mountain obscuration, and mainly MVFR and
freezing levels around 3000 feet MSL.

During the afternoon into tonight, the front will continue across
the East Side with deteriorating conditions becoming mainly MVFR
with snow while west side precipitation becomes more showery with
the freezing level falling to around 2000 feet MSL. -DW/BPN

&&

.MARINE...Updated 200 AM PST Friday, 24 December, 2021...Gusty
west winds and very steep and hazardous seas in the northern waters
this morning will spread to the rest of the southern Oregon coastal
waters by this afternoon as a front approaches through the area. A
brief period of gale-force gusts are possible just ahead of the
front.

Winds will diminish once the front moves onshore this afternoon, but
seas will remain very steep into tonight due to ongoing large
northwest swell. Steep seas then accompany a series of unusually
cold disturbances during the weekend, with rain possibly changing
over to snow across the waters Sunday and Monday. The pattern is
expected to remain active early next week, with additional periods
of very steep seas possible with each passing system.  -BPN/DW

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 4 AM PST early this morning for
     ORZ029>031.
     Winter Weather Advisory from 10 AM this morning to 10 AM PST
     Sunday for ORZ029>031.
     Winter Storm Watch from Sunday morning through Monday morning
     for ORZ029-030.
     Winter Weather Advisory until 4 AM PST early this morning for
     ORZ027-028.
     Winter Storm Warning from 6 AM this morning to 10 AM PST Monday
     above 2000 feet in the for ORZ025>028.
     Winter Storm Watch from Sunday afternoon through Monday morning
     above 500 feet in the for ORZ021-022.
     Winter Storm Warning until 10 AM PST Monday above 2000 feet in
     the for ORZ023-024.
     Winter Weather Advisory from 6 AM this morning to 10 AM PST
     Sunday above 1500 feet in the for ORZ025-026.

CA...Winter Weather Advisory until 4 AM PST early this morning for
     CAZ085.
     Winter Storm Warning from 6 AM this morning to 10 AM PST Monday
     above 2000 feet in the for CAZ080.
     Winter Weather Advisory from 6 AM this morning to 10 AM PST
     Sunday above 1500 feet in the for CAZ080.
     Winter Weather Advisory until 4 AM PST early this morning above
     4000 feet in the for CAZ080-082-083.
     Winter Storm Warning from 1 PM this afternoon to 10 AM PST
     Monday above 2000 feet in the for CAZ082-083.
     Winter Weather Advisory from 1 PM this afternoon to 10 AM PST
     Monday for CAZ081.
     Winter Weather Advisory from 1 PM this afternoon to 10 AM PST
     Sunday for CAZ084-085.

Pacific Coastal Waters...Small Craft Advisory from 1 AM Saturday to 4 AM PST Monday for
     PZZ350-356-370-376.
     Hazardous Seas Warning until 1 AM PST Saturday for
     PZZ350-356-370-376.