Heavy Snow in Oregon, December 13 to 14, 2016

Area Forecast Discussion
National Weather Service Medford OR
800 PM PST Tue Dec 13 2016

.UPDATE...A slight change in timing this evening has required a
very modest change to the wind forecast tonight, but all other
parameters appear to be on track. Numerous headlines remain in
place for the upcoming storm system tonight and tomorrow. for more
information and detail, see the previous discussion below. -BPN

&&

.AVIATION...14/00Z TAF CYCLE...A strong warm front will bring light
to moderate rain to the west side terminals including the coast and
offshore from south to north starting late this evening and
continuing through the remainder of the TAF period. Freezing levels
will rise on the coast from 2500 to at least 5000 feet. Cigs/Vis
are expected to be generally MVFR although conditions may worsen
tomorrow morning as a cold front begins to push in as well. On the
east side, snow will spread in with similar timing. Flight
conditions are likely to be much worse as a result with IFR to LIFR
Cigs/Vis likely to develop after midnight, including at KLMT, and
significant snow accumulations by dawn Wednesday. Snow will change
to rain from south to north late morning tomorrow. -Wright/Schaaf

&&

.MARINE...Updated 800 PM PST Tuesday 13 December 2016...Although
winds are expected to stay below advisory criteria for most of the
area, wind waves will oppose the swell and things could become quite
choppy over the outer waters where this occurs. A small craft
advisory for hazardous seas continues to address this issue. A
frontal boundary is expected to move towards the area late this
evening bringing steady precipitation. The models are in pretty good
agreement showing a surface low developing along the frontal
boundary late tonight. The models have come into better agreement
regarding the track of the low and now indicate that it will come
ashore over brookings which will keep winds below gales in our
waters. Have cancelled the gale watch as confidence has increased in
this solution which will show borderline small craft advisory winds
from Brookings south. Any south winds will quickly shift to
northerly winds as the surface low moves inland Wednesday afternoon
and night, but should remain below small craft. Wind waves and fresh
swell are expected to sneak into the southern waters on Wednesday
morning and create small craft advisory seas. -Schaaf/BPN

&&

.PREV DISCUSSION... /Issued 352 PM PST Tue Dec 13 2016/

.DISCUSSION...About 48 hours of high-impact weather will affect the
forecast area from this evening through Thursday night. A frontal
system well offshore, with ample moisture of tropical origin
streaming along it. This storm will bring heavy snow, heavy rain,
and strong winds to portions of Northern California and Southern
Oregon.

Tonight, the approaching warm front will overrun cold air over
portions of Northern California, mountainous areas, and east of the
Cascades, resulting in a period of moderate to heavy snow. In the
Mount Shasta area, snow levels should generally be above 4500 feet
during the significant precipitation after an initial start at about
3500 feet briefly this evening. There are a few colder pockets of
air in the Shasta and Scott Valleys this afternoon, but enough
warming should occur to minimize any winter impacts. The same can
be said for areas near the Umpqua Basin to include Highway 38 near
Elkton. On Interstate 5, Siskiyou Summit should see a period of
moderate snowfall and several inches this evening before a
changeover to rain late tonight.

The highest impacts tonight are expected in the high Cascades and
east of the Cascades, where moderate to heavy snow is expected
before any changeover to rain Wednesday morning. The transition from
rain to snow is expected last in areas further north like Chemult.
Models differ on whether snow levels will jump above Crater Lake,
with more than half the solutions showing snow levels remaining
below 7000 feet in the Cascades. Therefore, we have no periods of
rain and about 2 feet of snow forecast for the Crater Lake Rim at
7000 feet, and the highest amounts east of the Cascades are
expected in Chemult, with about 17 inches forecast there from this
evening through Wednesday. Lower mountain passes in the Cascades
are expected to see about a foot of snow before changing to rain
Wednesday. While on the subject of passes/highways, very cold air
will move in behind the cold front and change the rain back over
to snow Wednesday night, so the combination of snow, then rain,
then freezing temperatures and more snow is of concern for travel
and road crews. We have issued a group of Winter Storm Warnings
and Advisories which can be found at WSWMFR.

Another high-impact element from this storm will be heavy rain.
Integrated vapor transport values are impressive, and both this
data and ESRL reforecast analogs for precipitation suggest this
will be a top 5-10% precipitation event for portions of Northern
California. There is still only moderate confidence on
precipitation amounts and locations because a lot will depend on
whether this front stalls over Northern California or just moves
through on Thursday. Any stalling will likely bring an inch of
precipitation in 24 hours to Alturas which is very uncommon.
Snowmelt from heavy rainfall on early season snowpack could
contribute to flooding in this area. We have issued an Areal Flood
Watch at FFAMFR to account for potential small stream and urban
flooding, as well as sharp river rises.

Last but not least, the frontal system will also bring strong
winds to the mountains, Shasta Valley, and much of the east side
tomorrow. Pressure gradients between Medford and Redding, CA peak
at somewhere between 8 and 9 millibars Wednesday afternoon, and
this coupled with strong winds aloft are usually good indicators
of high winds through the Shasta Valley. In addition, models all
show 700 mb winds of 80+ knots and strong boundary layer winds
which are the indicators for high winds on the east side and in
the mountains. One unusual aspect of this high wind event is that
it will be preceded by heavy snow in many locations on the east
side. The strong winds kick in just as the snow changes to rain,
and this is expected to create a period of very slick roadways due
to the combination of ice and water. This may make it easier than
usual for high winds to force vehicles off roadways. Motorists
should use extreme caution if traveling on the east side or
through the mountains tomorrow. A variety of high wind warnings
and advisories have already been issued, so please see PDXNPWMFR
for details. -Wright

Behind this storm very cold and dry air will move into the forecast
area, lasting Friday into early next week. Near-zero overnight
low temperatures are expected Friday night east of the Cascades.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning from 1 PM to 10 PM PST Wednesday for
     ORZ030-031.
     Winter Weather Advisory until 10 AM PST Wednesday for
     ORZ029>031.
     Winter Storm Warning until 4 PM PST Wednesday for ORZ029>031.
     Wind Advisory from 1 PM to 10 PM PST Wednesday for ORZ030-031.
     Flood Watch from 1 AM PST Wednesday through Wednesday evening
     for ORZ021-022-024-028.
     Winter Storm Warning until 4 PM PST Wednesday ABOVE 5000 FEET
     for for ORZ027-028.
     Wind Advisory from 1 PM to 10 PM PST Wednesday for ORZ029.
     Winter Weather Advisory until 10 AM PST Wednesday for
     ORZ027-028.

CA...High Wind Warning from 1 PM to 10 PM PST Wednesday for CAZ085.
     Wind Advisory from 1 PM to 10 PM PST Wednesday for CAZ085.
     Wind Advisory from 1 PM to 10 PM PST Wednesday for CAZ084.
     Flood Watch from 4 AM PST Wednesday through Thursday afternoon
     for CAZ080>083.
     Flash Flood Watch from 4 AM PST Wednesday through Wednesday
     evening for CAZ080.
     Winter Weather Advisory until 4 AM PST Wednesday ABOVE 4000 FEET
     for for CAZ082-083.
     Winter Storm Warning until 10 AM PST Wednesday ABOVE 5000 FEET
     for for CAZ082-083.
     Winter Weather Advisory until 4 AM PST Wednesday ABOVE 4500 FEET
     for for CAZ080.
     High Wind Warning from 10 AM to 7 PM PST Wednesday for CAZ081.
     Flood Watch from Wednesday evening through Thursday evening for
     CAZ084-085.
     Winter Weather Advisory until 4 AM PST Wednesday for CAZ084-085.

Pacific Coastal Waters...Flood Watch from 1 AM PST Wednesday through Wednesday evening
     for PZZ356.
     Small Craft Advisory from 10 AM to 4 PM PST Wednesday for
     PZZ356-376.
     Small Craft Advisory for hazardous seas from 10 AM to 10 PM PST
     Wednesday for PZZ356.
     Small Craft Advisory for hazardous seas until 10 PM PST
     Wednesday for PZZ376.
     Small Craft Advisory for hazardous seas until 4 PM PST Wednesday
     for PZZ370.

BPN/NSK/TRW/BMS