Winter Weather Advisory 34, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
1037 PM PST Mon Dec 20 2021

Updated Aviation and Marine Discussions

.DISCUSSION...A surface low is offshore of Southern Oregon with a
warm front north of Douglas County, perpendicular to the central
Willamate Valley, and a cold front roughly 120 NM from shore. The
main flow over the region is from the south and this is causing
rain showers to parallel the coast, limiting inland precipitation.

Where rain has fallen along the northern coast range and lowlands
of Coos and (coastal) Douglas County, amounts have been fairly
steady. In the the past six hours most stations are reporting 0.1"
of rain at least, with those at higher elevations reporting over
0.25". These showers will begin to taper off, focusing more
offshore, as higher pressure ridging noses north over central
Oregon and halting the progression of the surface low and cold
front.

The building of high pressure is also opening Tuesday (Dec 21) as
a more reasonable day to travel. This is because the front will be
slower to progress onshore and hold off on snowfall around 4 PM
PST; there is likely to be a wintery mix between rain, snow, and
sleet in the hours leading up to this. After that time, the
southerly flow will target snow in the mountains of Siskiyou
County. Snow on passes (Siskiyou & Black Butte) should be
relatively light with the chance for greater than 1" of snow at
55% & 30% and for greater than 2" at 30% & 5% at Siskiyou & Black
respectively. So considering these amounts will be spread over
about 12 hours, we are not expecting only minor travel impacts if
any.  -Miles

&&

.AVIATION...21/06Z TAFs...Expect mainly VFR conditions along the
coast tonight into Tuesday. Then as a front just offshore moves into
the area, expect rain, MVFR cigs/vis and local IFR cigs to develop
Tuesday afternoon and evening along the coast. Inland, areas of
IFR/LIFR conditions in fog/low clouds will develop in west side
valleys tonight and Tuesday morning then improving to VFR late
Tuesday morning into early Tuesday afternoon. Areas of precipitation
will spread inland late Tuesday afternoon and Tuesday evening
resulting in local MVFR/IFR cigs/vis and terrain obscurations.
-CC

&&

.MARINE...Updated 1000 PM PST Monday, 20 December, 2021...Winds will
gradually lower this evening into tonight, but fresh southerly swell
will maintain steep to very steep, short period and hazardous seas
across all the waters through Tuesday morning. Then, steep to very
steep seas are expected to linger across outer portions of the
waters through Tuesday evening. The next front is expected Wednesday
with very steep seas possible and southerly winds likely approaching
gale force. Another strong front is likely Thursday night into
Friday. Then active weather will continue into the weekend. -CC

&&

.PREV DISCUSSION... /Issued 308 PM PST Mon Dec 20 2021/

SHORT TERM...

Rain showers are still persisting along the coast this afternoon,
although there are sections of clearing near Brookings and the
southern portions of the coast. Tonight into Tuesday will be a
quiet period for our region as high pressure pushes into our area
as a low spins off the coast. This will result in some clearing
and some pretty cold temperatures overnight east of the Cascades.
Last night Lakeview dropped to 18 degrees and we would expect
another night in the mid teens as high clouds partially clear out.
However for tonight, don't be surprised if some locations east of
the Cascades are able to slip into the single digits.

By Tuesday, things become a little more interesting as the low to
our west finally makes an attempt to move onshore. One could also
analyze an occluded front also pushing onshore during this time.
This will result in the first of many precipitation events over
the next week. Sometimes these fronts can overproduce on the
precipitation. At this time however, we're expecting minor snow
impacts in California and the higher terrain in California. If
there is one place to watch out for, it'll be Black Butte Summit
on I-5 as it does have a 30 to 50% chance to see 2 inches rather
quickly Tuesday night according to the NBM.

LONG TERM...Wednesday until Sunday. The pattern becomes more
active and more "wintry" as we approach the weekend. The first
round Wednesday morning into Thursday morning continues on track
with warning amounts still indicated above 4000 ft elevation from
the Mt Shasta region to western Sikiyou county. The next front
after that, Thursday into Friday morning, is not as strong with
advisory amounts possible above 2500 ft elevation from the
Cascades to the coastal ranges into Friday morning.

The upper trough that brings these successive fronts through shifts
east with northwest cool flow behind it ahead of the next front that
moves in Friday morning into Christmas day. With the upper trough
again deepening offshore as the next round of even colder air moves
in this weekend. This system currently is showing heavy snow above
2500 ft in western Siskiyou county and in The Kalmiopsis through
Christmas morning, with snow looking to reach the valley floors west
of the Cascades, but with light amounts indicated at this time.

As we head into Christmas night the really cold air mass moves in.
Current operational model runs area showing a high probability of
snow to the valley floors in the Rogue valley and Siskiyou county,
with lighter amounts in the Umpqua valley. The ensemble
probabilities of greater than 2 inches per 12 hour periods are
showing likely probabilities Sunday morning through Tuesday night,
with a 73% chance of greater than 2 inches of snowfall at the
Medford airport indicated Sunday. -Sven

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...
CA...Winter Storm Watch from late Tuesday night through Thursday
     afternoon above 4000 feet in the for CAZ080-081.
     Winter Storm Watch from late Tuesday night through Thursday
     afternoon for CAZ082-083.

Pacific Coastal Waters...
     Small Craft Advisory until 10 AM PST Tuesday for PZZ350-356.
     Hazardous Seas Warning until 10 PM PST this evening for
     PZZ350-356.
     Small Craft Advisory from 10 AM to 10 PM PST Tuesday for
     PZZ370-376.
     Hazardous Seas Warning until 10 PM PST Tuesday for PZZ370-376.
     Gale Warning until 10 PM PST this evening for PZZ376.