High Wind in Oregon, January 10, 2020

Area Forecast Discussion
National Weather Service Medford OR
253 PM PST Fri Jan 10 2020

.DISCUSSION...

.Short Term...Tonight through Monday Night... The first front of
out period of extended winter weather is moving onshore now. Snow
levels are starting out at around 5000 feet, but once the front
passes tonight, they will quickly drop to 3000 to 3500 feet.
Precipitation will continue into Saturday afternoon as the upper
level low passes overhead, and dynamics support several more hours
of lift. Due to the more westerly component of the flow in this
system, upslope enhancement of the precipitation will be strong,
especially in the Cascades, and snowfall amounts there will be
significant. Winds will also be a concern, with midlevel flow of
60 to 70 kts producing gusty winds across the higher terrain and
the East Side. Greatly reduced visibilities are likely to result
from blowing and drifting snow, making the situation all the more
hazardous. In some areas, near blizzard-like conditions are
possible through Saturday.

This front is merely the first of at least four disturbances that
will push through the region between now and Monday, each
reinforcing the cold air in the region and producing plenty of
precipitation. The second disturbance passes just to our north
Saturday night into Sunday, and snow levels will drop to around
3000 feet by Sunday afternoon. With very little break in between
these events, have extended the current winter weather headlines
out through Sunday evening, and updated timing and amounts though
that time. For more detail on these headlines, including snow
amounts, see the winter weather message at PDXWSWMFR.

Another impulse arrives late Sunday, bringing more cold air and
another round of rain and snow to the area. Snow levels are likely
to drop to between 800 and 1000 feet by Monday morning, which
would produce snow down to many of the West Side valley floors.
The question will be when does precipitation end, as models do
show a break in the action around that time. So while snow levels
will be low enough, precipitation may be light, and the valleys
could only receive a dusting or a few tenths of an inch of snow.
Conversely, if snow level drops quicker or precipitation lingers,
a couple of inches could result. With a wide variety of model
solutions, details are uncertain, but morning commuters should be
prepared for the good possibility of wintry travel Monday morning
on the valley floors.

The next impulse arrives Monday night, and again, there is a wide
spread in the model solutions. We are confident that plenty more
precipitation will arrive with this system, but much will depend
on the track of the surface low. If the low moves onshore a
hundred miles farther north, more warm air will filter into the
region and snow levels will remain too high for the valley floors.
A hundred miles to the south, and snow levels will plummet, and
plenty of snow is possible. We will revisit this over the next
shift or two, and hopefully will pin down the details soon.

Long story short, there will be significant snow, and in some
areas wind, across most of the forecast area. While some areas in
Cascades could receive up to 6 feet of snow by Monday night (with
more to come next week), valley floors could also see impacts
Monday and Tuesday. Anyone driving in the area should be prepared
for significant delays or road closures, and if traversing the
passes, may need to reschedule travel for a better time. The work
commute in the West Side valleys could become troublesome as well
Monday and Tuesday, so the proper precautions and preparations
should be made. -BPN

.LONG TERM...Tuesday through Friday...The bottom line: expect
copious amounts of snow if you are higher than 4000 feet and expect
at least some snow (except for the coast) if you're below 4000 feet.

We are confident that the extended forecast will remain quite
active. We are also confident that many of the storms next week will
provide some relatively significant snows, especially for the
mountains and the passes over the next week. We have low confidence
regarding the timing of the systems and what the exact snow levels
will be. That being said, we do expect snow levels to reach valley
floors on Tuesday evening and Tuesday night. Have gone with a blend
of the US National Blend of Models (NBM), the previous forecast, and
a few other models as well to help make a guess with the timing of
the systems since the NBM is ensemble based and by design will
spread out precipitation chances.

The first of the systems will have moved through our area by
Tuesday, leaving lingering showers. Snow levels are expected to be
just above the valley floors during the day (around 1400 feet or
so), but once the sun sets, the snow levels will fall once again
likely reaching the valley floors. The snow levels could fall enough
to reach Roseburg. Valley Floor snow amounts will not be very high
(less than an inch), but the passes including Jacksonville Hill,
Hayes Hill, Camas Mountain could see a few inches.

The next storm arrives on Wednesday and will provide another blast
of precipitation across southern Oregon and northern California.
Snow levels will rise to around 1500 to 2000 feet throughout the
day, and they are expected to hover around there for the rest of the
week. The main issue here will be whether or not any shortwaves
during this system could bring potentially heavy enough
precipitation to bring down snow levels another 500 to 1000 feet in
small areas.

After a brief break with lingering showers Wednesday night, the next
system will come through on Thursday and could be even stronger than
the first system. Similar snow levels and amounts are expected
during this period. Another system is poised to reach the area next
weekend as well.

In all we're expecting 2-4 feet of snow in the Cascades in the
extended forecast, with 1-2 feet over the next week at Siskiyou
Summit and some of the lower passes 2-3 feet around Mt. Shasta City,
and up to a foot of snow across the east side. Reminder, although
these amounts are high, each system will bring only a portion of
these numbers with the biggest system coming in Thursday. Please
stay tuned to the weather forecast as there will be many snow
impacts for a lot of people, and since confidence is low; expect
changes to the extended forecast. -Schaaf

&&

.AVIATION...For the 10/18Z TAF Cycle....VFR conditions will prevail
initially, until an incoming front this afternoon produces areas of
MVFR cigs/vsbys and local IFR cigs/vsbys in low clouds and rain/snow
along with obscuration of higher terrain. The lower conditions will
spread across the West Side through this afternoon, to the Cascades
by evening, and east of the Cascades tonight. Snow levels will be
around 5000 feet, lowering to 3500 feet by tonight. Low level wind
shear will also be a concern at KOTH, KRBG, and KMFR ahead of the
front in the mid-morning for speed and direction, to late afternoon
hours for mostly speed shear. Low level wind shear will effect KLMT
later in the evening. By the end of the TAF period, MVFR/IFR
conditions with precipitation and gusty winds will affect most, if
not all, of the forecast area. -BPN

&&

.MARINE...Updated 200 PM PST Friday 10 Jan 2020...A strong front
is moving onshore this afternoon. South winds have increased to gale
force along the coast. Winds will lower slightly and become westerly
this evening through Saturday. However occasional gale force gusts
are expected for the waters from Cape Blanco north. Behind the
front, northwest swell will rapidly build into the waters tonight
into Saturday. Seas are expected to peak at 22 to 30 feet at 16
seconds late tonight through Saturday afternoon. Very high and very
steep seas will continue Saturday evening then gradually lower late
Saturday night. More fronts will move in Sunday into Monday night
with high and steep to very steep seas, but winds will remain
westerly and will not reach gale force. Seas may remain elevated
Tuesday then increase again mid to late in the week as another
strong front moves into the area bringing strong gusty winds and
building west swell. -CC

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 10 PM PST Sunday for ORZ029-030.
     Winter Storm Warning until 10 PM PST Sunday for ORZ027-028-030.
     Winter Weather Advisory until 10 PM PST Sunday for ORZ031.
     Wind Advisory from 7 AM to 4 PM PST Saturday for ORZ031.
     High Surf Warning from 10 PM this evening to 10 PM PST Saturday
     for ORZ021-022.
     Winter Storm Warning until 10 PM PST Sunday above 3000 feet in
     the for ORZ025-026.

CA...Winter Weather Advisory until 10 PM PST Sunday for CAZ085.
     Winter Weather Advisory until 10 PM PST Sunday above 3500 feet
     in the for CAZ080.

Pacific Coastal Waters...Small Craft Advisory until 7 PM PST this evening for PZZ356-376.
     Gale Warning until 7 PM PST this evening for PZZ350-356-370-376.
     Hazardous Seas Warning from 7 PM this evening to 2 AM PST Sunday
     for PZZ350-356-370-376.