High Wind in Oregon, December 21, 2020

Area Forecast Discussion
National Weather Service Medford OR
901 AM PST Mon Dec 21 2020

.UPDATE...Satellite imagery this morning shows plenty of valley
fog and stratus in the area, as well as the broad shield of clouds
offshore that will be moving inland later this afternoon. This
cold front will feature a very sharp drop in snow levels as it
passes, and although several inches of snow are expected in the
Cascades, the bulk of the precipitation will come to an end before
snow levels drop far enough to create significant impacts
elsewhere. Forecast efforts today will focus on the snow tonight,
and the potential for another storm with more mountain snow
during the holiday weekend ahead.

Only a few updates were necessary this morning to bring sky and
fog conditions and trend into line with the current observations.
See the previous discussion below for more forecast information.
-BPN

&&

.AVIATION...For the 21/12Z TAFs...Along the coast and just offshore,
a mix of IFR and MVFR cigs are expected this morning before they
lower to IFR again late in the morning as a strong cold front moves
into the area. The lower ceilings are expected to last into the
evening, then improve to MVFR as the cold front moves on shore with
a broken sky cover.

Inland west of the Cascades expect areas of valley IFR/LIFR cigs and
vis in low clouds and fog through this morning. Expect gradual
improvement late this morning into the early afternoon. Then
later this afternoon and into this evening, a strong front
arrives and ceilings lower as the front moves onshore affecting
Roseburg by late afternoon, then Medford by the evening. Moderate to
heavy precipitation could also result in reduced visibility. One
thing to note, there's a chance low clouds and patchy fog could
develop towards daybreak Monday at Roseburg and Medford and the
lower conditions are included in the TAF's. However, it's possible
cloud cover could be sufficient enough to keep them from forming.

A strong, sharp cold front will move through the area Monday, this
will result in a abrupt and quick shift in the winds for all TAF
sites west of the Cascades. The details were added into the TAF'S
hence the 5 to 6 line TAF'S. However the timing of when the wind
shift could vary, so stay tuned for the latest updates.

East of the Cascades, expect mainly VFR conditions through this
afternoon except for local IFR/LIFR cigs/vis in the valleys
this morning. There will be a period of increasing winds late in the
afternoon. Local MVFR cigs are possible this evening and tonight
with showers. -CC

&&

.MARINE...Updated 800 AM PST Monday, 21 Dec 2020...A strong and
quick-moving cold front will move through the region late this
morning through this afternoon. Ahead of this front, expect south
winds to increase to gales, especially for the waters from Gold
Beach northward. Behind the front, expect an abrupt wind shift to
strong northwest winds, also approaching gales at times, late this
afternoon and this evening. This will result in a mix of steep to
very steep and hazardous, chaotic seas. There is moderate confidence
in the wind forecast with north winds expected to reach 25 to 30 kt
with occasional gusts to 35 kt. However, some model solutions favor
slightly lower winds.

Conditions will improve tonight as winds diminish and seas begin to
subside. Steep seas will persist tonight into Tuesday morning. Winds
will become northerly Tuesday into Wednesday and may reach advisory
levels by Tuesday evening, especially south of the Cape Blanco.
Winds ease Wednesday and a period of sub-advisory conditions are
expected into Thursday. The improvement will be short lived,
however, as more fronts are expected over the holiday weekend with
the first arriving as early as Christmas Eve night. -CC

&&

.PREV DISCUSSION... /Issued 620 AM PST Mon Dec 21 2020/

DISCUSSION...It seems appropriate that the winter season began at
202 AM this morning, with some portions of our area (roughly
between 2000 and 3000 feet of elevation) expected to soon receive
their first measureable snowfall since last winter. Although,
barely measureable in most cases.

Thickening mid-level clouds ahead of a fast moving cold front in
the afternoon near the coast or in early evening inland will make
for poor star-gazing tonight. The front will move across our area
this afternoon through early Tuesday morning with gusty southwest
winds expected to gust up to 55 mph at the coastal headlands,
over the higher terrain, and east of the Cascades (where a Wind
Advisory has been issued for portions of Lake and Modoc counties
from 1 PM this afternoon through 1 AM Tuesday). Snow levels will
lower from around 7000 feet to 2000 feet with precipitation
focused on southwest Oregon during the evening. High pressure will
build in Tuesday through Thursday with colder and drier weather.
The pattern will turn active again on Friday into next week.

First, there is currently a thinning of mid-level clouds that are
exiting to the east. But, thick stratus in valleys (mainly west of
the Cascades) that is likely to linger through mid-morning with a
slow dissipation to follow into early afternoon. The latest model
data supports a timing and strength of the approaching cold front
that was largely consistent with the inherited forecast. The front
will weaken as it moves inland with rain likely at the coast by
afternoon. By 4 PM, rain is likely to reach central Douglas County
(near Roseburg) and far western Josephine County. Precipitation
intensity will be moderate at times as the front crosses southwest
Oregon this evening, with light precipitation as the front spreads into
Klamath and northern Lake counties...as well as with the slightly
weaker southern portion of the front moving into northern
California. The challenging aspect of this frontal passage has
been determining the extent of impact with a sizable portion of
the area, including mountain passes to experience rain changing
over to snow. A Winter Weather Advisory remains in effect from 4
PM this afternoon through 2 AM Tuesday for the south central
Oregon Cascades, including the vicinities of Diamond Lake and
Crater Lake. That area is forecast to quickly change to snow and
receive an accumulation of 4 to 8 inches. The Siskiyou Mountains
and southern Oregon Cascades, including Siskiyou Summit and Lake
of the Woods are expected to receive 1 to 4 inches of snow. In
general terms, the highest intensity burst of snow will be around
7 PM to Midnight with snow levels falling from 4500 feet to 2500
feet. The further drop to a snow level of 2000 feet will occur
with much lighter, post-frontal, lingering late night
flurries...tapering off around 4 AM Tuesday. The Mount Shasta
region and much of the east side will receive up to an inch of
snow, with a couple of inches for the higher terrain of western
Siskiyou County. The city of Mount Shasta is likely to see
precipitation begin as rain around 8 PM and change over to snow
around 10 or 11 PM before tapering off around 1 AM.

Our weather will be dry Tuesday through Thursday, with
particularly cold mornings on Wednesday and Thursday...mainly in
the upper teens to lower 30s on the west side and in the single
digits on the east side.

The pattern will become active again late in the week. Confidence
has increased that a cold front of uncertain strength will
reach our area on Christmas day or Christmas night, with a focus
for the highest probability of precipitation from the Mt. Shasta
region to the San Francisco Bay area. A slight majority of GEFS
and ECMWF ensemble members are noticeably weaker than their
respective operational model solutions. By the end of Friday
afternoon, there is a 7 out of 10 chance of rain reaching the
coast and 3 out of 10 for reaching Mt. Shasta. Snow levels on
Christmas are most likely to be in a range of 4000 to 5000 feet.
The highest probability of inland precipitation is during Friday
evening with a continued chance of precipitation with a secondary
front on Saturday. There is about an even chance of sub-advisory
or advisory level amounts of a few to several inches of snow over
our most traveled high mountain passes (above 4000 feet). Further
refinement of the holiday forecast as we draw nearer to Christmas
is a certainty. At least a short break between systems is likely
to occur during the weekend with a very wide variety of solutions
at this time.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory from 1 PM this afternoon to 1 AM PST Tuesday for
     ORZ030-031.
     Winter Weather Advisory from 4 PM this afternoon to 2 AM PST
     Tuesday for ORZ027.

CA...Wind Advisory from 1 PM this afternoon to 1 AM PST Tuesday for
     CAZ085.

Pacific Coastal Waters...Small Craft Advisory until 4 PM PST this afternoon for
     PZZ350-356-370-376.
     Hazardous Seas Warning from 4 PM this afternoon to 10 PM PST
     this evening for PZZ356-376.
     Gale Warning until 7 PM PST this evening for PZZ350-370.