Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 041657

Area Forecast Discussion
National Weather Service Medford OR
855 AM PST Sun Dec 4 2016

.DISCUSSION...Morning update. Looking at the observations the front
has crossed North Bend with winds now blowing from the north. made
some minor wind edits, otherwise everything looks on track. Sven

.Short Term...A strong cold front is dropping south along the
Pacific Northwest coast this morning. Based on observations,
pressure tendencies, and radar signatures, it looks like the front
is near the mouth of the Columbia River right now. The front will
move steadily down the coast today and through our area late this
afternoon and evening. It has already started snowing in the
Cascades around Diamond and Crater Lakes and raining in the lower
elevations, and precipitation will increase from north to south
through the morning. Snow levels are around 5000 feet this morning
and will fluctuate a little through the day, but precipitation
should generally be all snow above 5000 feet. A winter weather
advisory remains in effect for the Cascades above 5000 feet. This
front will also produce gusty winds on the east side this
afternoon, and wind advisories and warnings remain in effect for
portions of the east side.

Snow levels will plummet tonight, and by tomorrow morning they
should be around 1500 feet which is very close to the valley floor
in Medford. At the same time, precipitation will be on the
decrease, so while there may be some snowflakes down to the lower
elevations, any impacts from snow should be minor.

As the upper trough and strong upper jet nose into our area Monday
afternoon and night, precipitation will increase again and will
probably last into Tuesday morning. Snow levels will likely be
just high enough (2000-2500 feet) that precipitation starts as
rain in the lower elevations. However, the passes in the Cascades
will certainly be impacted by snow Monday evening and we may also
see some impacts by evening on the lower passes on I5 north of
Grants Pass. Snow levels drop further overnight and into Tuesday
morning, and this is when west side valleys stand the best chance
of accumulating snow. It looks like some snow will fall in the
valleys and there will be lower elevation impacts by Tuesday
morning, but confidence in the exact locations and amounts of snow
is still low. However, the higher in elevation you are, the more
likely you will see snow.

I would expect to see winter weather advisories issued for many
locations once these details become apparent. Until then,
residents and those with travel plans in the lower elevations of
southern Oregon and northern California early this week are urged
to be ready for at least light snow accumulations. Those with
plans in the mountains should be prepared for possible heavy
accumulations. This includes all Cascade passes and Siskiyou
Summit. Lower passes along I5 should also see at least some snow
with moderate accumulations possible at times.

This should clear out rapidly Tuesday, and then we can expect the
coldest night of the year so far Tuesday night. -Wright

.Long Term...Wednesday through Saturday Night...The coldest air
mass so far this season will be in place over the forecast area
Wednesday morning, but with winds out of the east, portions of the
West Side may see warmer temperatures than what might otherwise be
expected due to a downsloping effect off of the Cascades. Whatever
the final effect, the cold air will play a major role in one of
the more pressing forecast concerns of the long term, which will
begin impacting the area by Thursday morning.

Despite some timing differences of about 12 hours or so among them,
the GFS and ECWMF, as well as their respective ensemble members,
are are still showing a significant system arriving Wednesday night
into Thursday. The warm front that leads the system will overrun
this colder air, and there will be a potential for light freezing
rain to fall along the western foothills of the Cascades and the
valleys of the East Side. Confidence in this scenario is low, and
much will depend on timing and temperatures swings of only a degree
or two, so as of now there is only a wide ranging period of slight
chances as the warm front passes overhead. Adding to the low
confidence, the ECMWF has weakened the thermal strength of the warm
front slightly over yesterday, while the timing of the GFS and ECMWF
have both changed by roughly 6 hours. Despite the uncertainty and
low-end chances, those traveling along roads in the areas of
concern Wednesday evening into Thursday morning should keep an eye
on the forecast for the latest updates, as only a very light amount
of ice can make for very hazardous conditions.

Once the warm front passes, the next set of major concerns arrives
with the cold front Thursday into Friday. While winds do not appear
to be overly strong, but will likely approach Warning and Advisory
criteria for many areas Thursday. Meanwhile, moisture transport
with this system has maintained a much higher than normal strength
over the past several days, so heavy precipitation is also likely.
Snow levels during the height of the precipitation should be between
4000 and 5000 feet, and the result could be heavy snow along many
area passes, as well as significant mountain and perhaps East Side
snow as well. Due to local effects, snow levels in the Mount Shasta
City area may be even lower. Again, confidence is low but improving
in the timing of the system, but confidence is much higher than
normal for this far out in regards to the likelihood of heavy rain
and snow, given the consistent signature in nearly all model suites.

once the main system passes, onshore flow and perhaps a secondary
front, will keep at least showers in the area through Friday night,
but high pressure will return by Saturday morning, and should keep
the area mostly clear of precipitation until the next system
arrives late in the weekend. -BPN


.AVIATION...04/12Z TAF cycle...Conditions are degrading from west to
east as a front approaches the coast this morning. Moderate to heavy
rain, mountain obscuration, and periods of IFR conditions will
accompany the front, with showers and plummeting freezing levels
behind the front. As the day progresses, the cold front will move
from northwest to southeast across Oregon and into far northern
California resulting in a period of moderate precipitations and
continued IFR to MVFR in the valleys. Moderate to strong southwest
to west winds are expected east of the Cascades today, and
turbulence will increase as a result. Some improvement to MVFR and
VFR is expected this evening. Later tonight though, given plenty of
moisture in the low levels, a return to IFR conditions in many
valley locations is likely. SK


.MARINE...Updated 800 AM PST Sunday 4 December 2016...A cold
front is moving north to south across the waters this morning. A
well defined wind shift from southerly to northerly winds will
accompany the front. Seas will remain steep just about everywhere,
and winds may briefly reach small craft advisory criteria north of
Cape Blanco as the front moves through. Unsettled showery weather
will continue Monday into Tuesday. High pressure will briefly build
Wednesday...then a stronger front will move through late Wednesday
night into Thursday, with gales possible. -BPN/SK


OR...High Wind Warning from 1 PM this afternoon to 7 PM PST this
     evening for ORZ030-031.
     Winter Weather Advisory until 10 PM PST this evening for
     Wind Advisory from 1 PM this afternoon to 7 PM PST this evening
     for ORZ030-031.

CA...Wind Advisory from 1 PM this afternoon to 7 PM PST this evening
     for CAZ085.

Pacific Coastal Waters...Small Craft Advisory for hazardous seas
     until 10 PM PST Tuesday for PZZ350-356-370-376.

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