Winter Storm and Heavy Snow in Oregon, January 1 to 4, 2017

Area Forecast Discussion
National Weather Service Medford OR
925 PM PST Sat Dec 31 2016

.DISCUSSION...Evening update. With temperatures near to or below
freezing, and dew points lower indicating wet bulbing precip down as
snow have lowered the snow levels to bring snow down the valley
floors this evening. BUFKIT soundings in Medford and Roseburg are
also indicating this event to start as snow tonight. This obviously
brings up the snow amounts in the Roseburg area, and 1 to3 inches
are possible by late Monday morning. Sven

.SHORT TERM...A winter storm is about to move into our forecast
area which will bring widespread impacts from snow over the next
few days, even in the lower elevations. A short wave trough and
associated cold front is moving down the B.C. coast this afternoon
and is currently rolling across Vancouver Island and approaching
Washington State. The cold front will sweep through our area later
tonight, and then the upper low will close off over the PacNW
tomorrow. The upper low then gradually elongates and stretches off
shore Monday into Tuesday.

Precipitation associated with the front will spread into the
north coast and Umpqua from the north this evening and then across
the rest of the west side overnight. Snow levels will be high
enough that it will start as rain over the coast and Umpqua, but
model soundings and cross sections show the airmass cooling enough
that it will likely start as snow over areas above 1000 feet.
Models are not showing a lot of QPF over the inland west side
valleys early on, but as we get into Sunday and the strongest
upper dynamics settles in over us, rates will increase. We expect
snow levels to remain at around 1000 feet or so through the day
tomorrow, which means it will likely be all snow in Medford, but
there is the possibility it mixes with rain at times during the
afternoon. Precipitation will be heaviest in the mountains, but we
are still looking at a few inches of snow over the lower
elevations, which will certainly produce impacts. As the main part
of the closed upper low shifts offshore Monday, snow is expected
to decrease over the valleys. However, snow will continue to pile
up in the mountains, especially the coastal mountains and in
western Siskiyou County. All of this is well covered by our going
winter storm warnings and advisories, which will, of course, be
continued.

By Tuesday, the upper low elongates and shifts further offshore,
while the upper ridge over Canada becomes positively tilted and
much higher heights move in to our north. The net result of this
is that very cold arctic air that was expected to come here at
full force is bumped just a little further east than expected. It
will still be cold over our area, just not as bitterly cold as
previously thought.

Of more concern is what the models do next, which is to produce a
surface low pressure along the southern periphery of the upper
low, and bring it ashore Tuesday night into Wednesday. This brings
significant precipitation back into our area, and with cold air
already in place, this could be quite a mess. While all the models
show this, it is a relatively new development. We did warm
temperatures a bit and raised QPF a lot, but we will hold off on
any additional winter weather headlines to see if this pattern
change sticks. Please note that if this happens as the models
currently suggest, we could have quite a lot of snow and/or
freezing rain in the valleys as warm Pacific moisture overruns
cold air trapped in the valleys. Confidence in this is currently
low, but it will be watched very closely as such a scenario could
be a huge impact to travel in our area. -Wright

.LONG TERM...Confidence is low in the details of the long term
forecast. But, there has been a shift in the models toward
weakening/modifying the previously indicated mid-week arctic
blast. The long term forecast was updated to reflect a blend of
the models. Rather than a strong northerly flow aloft with a ridge
centered offshore, the favored upper level pattern now stretches
and retrogrades the early week trough...from the Pacific Northwest
out over the eastern Pacific. Rather than a fire hose of frigid
air it looks to be more like a leak. This shift in the pattern,
besides modifying temperatures, is likely to allow a developing
disturbance to get caught in the westerly flow far offshore from
northern California then track northeast into northern California
and far southern Oregon late Tuesday night into Wednesday. The
probability of precipitation has been increased at mid-week. The
timing of this disturbance will be very important as cold air
could be trapped in valleys...resulting in a mixed bag of rain,
sleet, freezing rain, and snow across our area. The highest
probability of precipitation and highest amounts are likely in
Siskiyou County...especially near Mount Shasta.

As big as the mid-week changes are, forecast agreement is worse
beyond Wednesday. The 12Z GFS ensemble bears some resemblance to
the 12Z ECMWF and the forecast leans in that direction. The idea
is for weak westerly flow Thursday into Friday with a series of
weak embedded shortwaves that will bring the highest probability
of showers and snow showers south of area though a higher
probability (a chance) for the coast and Cascades than other
portions of our area (a slight chance). It does bear mentioning
that the recently arrived 18Z GFS is less progressive and would
result in a less cold but wetter solution. Toward the end of the
week...the ECMWF and GFS ensembles agree on a deep upper low
developing in the Gulf of Alaska which could open the door to a
bit warmer but much wetter pattern shift. -DW

&&

.AVIATION...01/00Z TAF CYCLE...A cold front will sweep across the
area tonight with freezing levels lowering to around 2000 feet MSL.
The front will bring MVFR conditions with initially light
precipitation to the coast around 02Z. Conditions will further
deteriorate at the coast tonight then widespread MVFR with
occasional IFR, mountain obscuration, and light to moderate
precipitation (mainly snow away from the coast) will spread south
and east across the area. The lower ceilings are expected to develop
at the Umpqua Divide and the Cascades around 07Z and spread north to
south across most of the area by around 15Z Sunday morning...then
persist through Sunday. Snow won't be as widespread east of the
Cascades compared to west of the Cascades, but lower visibility and
mountain obscuration is still expected. Additionally, the cold and
unstable atmosphere will bring a chance for thunderstorms along the
coast north of Cape Blanco Sunday afternoon.
-DW/SK

&&

.MARINE...Updated 230 PM PST Saturday 31 December 2016...Very steep
seas will continue this evening over portions of the waters south of
Cape Blanco beyond 10 NM beyond shore. Northwest winds are expected
to increase late this evening reaching small craft advisory levels
over the outer waters and some of the inner waters. These winds will
remain gusty through Sunday. Confidence remains high that a long
period northwest swell will become high tonight then high to very
high and very steep at its peak of around 15 to 20 feet Sunday
afternoon and evening. Northwest winds and high seas will persist
late Sunday night into Monday. Seas are expected to gradually
diminish Sunday night into Monday morning then will subside below 10
feet Monday night and should remain low through much of the week
although with moderate northerly winds possible Tuesday night into
Wednesday night.

A very cold and unstable air mass is still expected to move into the
region Sunday with thunderstorms possible Sunday afternoon. The cold
air mass brings the chance for snow all the way down to the beaches
late Sunday night into Monday. -Petrucelli/SK

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 10 PM this evening to 10 AM PST
     Monday for ORZ030.
     Winter Storm Warning from 4 AM Sunday to 10 AM PST Monday ABOVE
     2000 FEET for for ORZ024-026.
     Winter Storm Warning from 10 PM this evening to 10 AM PST Monday
     ABOVE 2000 FEET for for ORZ023.
     Winter Weather Advisory from 10 PM this evening to 10 AM PST
     Monday ABOVE 500 FEET for for ORZ023.
     Winter Weather Advisory from 10 AM Sunday to 10 AM PST Monday
     ABOVE 500 FEET for for ORZ021-022.
     Winter Weather Advisory from 10 PM this evening to 10 AM PST
     Monday for ORZ027-028.
     Winter Storm Warning from 10 PM this evening to 10 AM PST Monday
     for ORZ025.
     Winter Weather Advisory from 4 AM Sunday to 10 AM PST Monday
     BELOW 2000 FEET for for ORZ024-026.

CA...Winter Weather Advisory from 4 AM Sunday to 10 AM PST Monday for
     CAZ080.

Pacific Coastal Waters...Small Craft Advisory until 4 PM PST this
     afternoon for PZZ356-370-376.
     Small Craft Advisory from 10 PM this evening to 4 AM PST Sunday
     for PZZ350-356-370-376.
     Hazardous Seas Watch from late tonight through late Sunday night
     for PZZ350-356-370-376.
     Hazardous Seas Warning until 10 PM PST this evening for PZZ376.