Winter Storm Warning 10, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
949 PM PST Wed Dec 23 2020

.DISCUSSION...An update has been issued to thicken low cloud
cover this evening over the Umpqua Valley, and to nudge the snow
level lower on Friday night. This was sufficient to warrant
updating the Winter Storm Watch /PDXWSWMFR/ for Siskiyou County
to include elevations above 3500 feet. This adjustment most
notably adds the vicinity of Mt. Shasta City into the Watch, with
a lesser accumulation likely into southern portions of Weed, and as
low as 3000 feet along Interstate 5 between Mt. Shasta City and
Dunsmuir. The Watch is in effect between 10 AM Friday and 10 AM
Saturday, with the majority of the 10 to 15 inches of snowfall
likely to occur between 4 PM Friday and 4 AM Saturday.

Also in effect is a High Wind Watch /PDXNPWMFR/ for the Shasta
Valley from 10 AM Friday morning through 10 PM Friday evening.
South winds will be strongest near Weed with gusts up to 60 mph.

With this system to be focused on northern and central California,
there is more model variability regarding the impact on Oregon. At
least light precipitation is expected across our area, with
periods of moderate intensity likely into southwest Oregon and
northern Klamath County. Snow levels away from the most intense
precipitation in Siskiyou County will be slightly
higher...generally falling from 5000 feet to 3500 feet with 6 to
12 inches of snow forecast for the southern Oregon Cascades and
Siskiyou Mountains, except about 5 inches for Siskiyou Summit on
Interstate 5, and mainly from an inch to as much as 5 inches on
the east side. Southerly winds will be windy, most notably
including gusts up to 45 mph forecast along Highway 31 near Summer
Lake and Silver Lake.

Please see the previous discussion below for further details
regarding the 7 day forecast.

&&

.AVIATION...For the 24/00Z TAFs...VFR conditions will persist
into this evening with moderate to strong easterly winds aloft.
These winds will continue into Thursday. Satellite shows a lot of
dry air lingering around California and that should persist through
the TAF period. However, we'll see another round of LIFR
visibilities with freezing fog in valleys west of the Cascades
Thursday morning. As for valleys East of the Cascades, it's looking
like fog is unlikely Thursday morning. VFR conditions are expected
elsewhere through the TAF period. -CC

&&

.MARINE...Updated 800 PM PST Wednesday 23 Dec 2020...Easterly winds
will become southerly overnight. Winds and seas will remain below
advisory criteria into Thursday morning.

A strong front moves into the region Thursday afternoon and seas
will become very steep and hazardous Thursday afternoon. Southerly
winds will strengthen Thursday night into Friday, reaching gale
force by Thursday night. Very steep and hazardous wind driven seas
will build further, peaking Friday morning and afternoon at around
15 to 20 ft. The gale watch has been upgraded to a warning with
gales expected to persist into Friday evening. There is high
confidence in widespread gales across the waters, but it should be
noted that there is the possibility of isolated storm force gusts
Friday afternoon. Winds will ease late Friday evening and overnight,
but seas will remain very high and steep into Saturday morning.

A very high and long period swell is still possible Saturday
morning. However, guidance has trended lower with this swell over
recent runs (16-18 ft today vs 20-22 from yesterday's run) and has
also decreased the period down to 14 seconds. Considering this is a
significant departure from previous runs, we will continue to
monitor trends over the next few runs. Even with this downward
trend, seas will be very high and very steep, likely resulting in
hazardous conditions along with dangerous bar crossings. Seas peak
again Saturday afternoon and evening, then gradually diminish
Saturday night into Sunday. There will be a brief break in the
weather Sunday, but seas are expected to remain high into Monday
with active weather expected next week. -CC/BR-y

&&

.PREV DISCUSSION... /Issued 253 PM PST Wed Dec 23 2020/

SHORT TERM...Through Friday Night...Clear skies this afternoon
is due to high pressure over the area, which is resulting in east
winds and drying over the ridges with less drying in the valleys
due to the inversion. Without clouds, the inversion should weaken
during the afternoon and some meager mixing to occur. Oregon DEQ
air quality monitors are reporting good air quality in the region
and a front is 24-36 hours out, so will not be matching Portland
with a stagnant air advisory.

West side valleys will fill back in with freezing fog tonight,
except for valley locations closer to the coast where fog in above
freezing temperatures will exist. Like this morning we are expecting
only localized dense fog, with majority of the fog being less
impactful. The dry conditions created from the east winds also
raises the low possibility that some places don't observe any fog.

Thursday will be a great day for travel with generally clear skies
expected. There could be some breezy winds in the Shasta valley near
weed in the afternoon as a front approaches.

Friday, Christmas day, will be less favorable for travel and could
even become hazardous for a few locations. Winds will begin to
increase along the coast and in the Shasta valley that morning and
peak in the afternoon. Have issued a High Wind Watch for the
Shasta valley with moderate-to-high confidence that Warning level
winds will occur; that is gusts over 58 mph. Winds at the coast,
along highway 101, are about 5 mph under watch criteria. The
coastal winds match our most reliable guidance and confidence is
fairly high that they represent the most likely scenario. SREF
probability of sustained sfc winds > 29 mph along the 101 corridor
is about 45%; our forecast is generally in the 17 to 23 mph
range. Gusts could still near 50 mph, which even below warning
criteria, will create hazardous driving conditions. Winds will
also be quite strong at peaks and ridges, for example at Mt.
Ashland gusts are expected to be in the 50 to 60 mph gust range.
Winds across the east side, near Summer Lake and along route 31,
are currently under advisory criteria, and appear to be too
southerly to increase to criteria threshold (southwest winds are
more favorable for this location).

Rain will begin along the coast in the early morning, heaviest along
the Curry County coast. Rain and snow will begin further inland
later in the morning, around 10 AM. The south/southwest flow favors
the Siskiyous and surrounding mountains for larger snow
accumulations. The most impactful snowfall will begin Friday
afternoon and continue through the night. A Winter Storm Watch is in
effect above 4000 ft in southern Siskiyou County, with details on
snow totals found in the extended section below. The snow in the
Cascades will start around the same time, but larger accumulations
won't start until Friday night and is expected to be less impactful
than the snow in Siskiyou County. -Miles

LONG TERM...Saturday morning until Thursday morning...

As the low moves onshore Saturday morning, snow will continue to
fall above 4000 feet over the region.  Storm total snowfall from
this winter weather event will be around 10 to 16 inches above 4000
feet with localized peaks and mountains seeing up to 2 feet of fresh
snow in northern California.  In southern Oregon and the Cascades,
we'll see less snow as the predominate flow will be mostly southerly
through the whole event. In general, we'll see around 8 to 10 inches
with 12 inches possible over the higher peaks in the Cascades, such
as Crater Lake, Mt. Thielsen, Mt. Ashland and other higher locations.

Interstate 5 will also see some snow from this winter weather event.
We're expecting around 2 to 4 inches over Siskiyou Pass through
Saturday morning with about 4 to 6 inches around Mount Shasta City.
With the snow level around 4000 feet, the low I-5 passes will avoid
snow. This snow should only bring moderate impacts as the plows
should be able to keep up with the snowfall.

After this low move through the region on Saturday, we'll see a
brief period of high pressure before the next low passes by.
Ensemble guidance suggests there is a very high probability the next
low will miss us and move into central California late Sunday into
Sunday night.  The probability of precipitation is about 20 percent
for southern Siskiyou County and southern Curry County as this low
moves through.  Drier air will likely move in behind this low as
1000-500 mb RH is about 30%.

This dry theme will continue from Sunday through Tuesday.  Then,
models begin to diverge on the overall weather pattern.  About 55
percent of the models suggest the next trough will be approaching
the Pacific Northwest.  However, the rest keep a ridge and high
pressure over the region.  Therefore, it seems more probable than
not that we'll see some rain and snow move through the region around
Wednesday.

-Smith

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...
CA...High Wind Watch from Friday morning through Friday evening for
     CAZ081.
     Winter Storm Watch from Friday morning through Saturday morning
     above 3500 feet in the for CAZ080-082-083.

Pacific Coastal Waters...Gale Warning from 10 PM Thursday to 7 PM PST
     Friday for PZZ350-356-370-376.
     Hazardous Seas Warning from 1 PM to 10 PM PST Thursday for
     PZZ350-356-370-376.

DW/CC