Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 032356

Area Forecast Discussion
National Weather Service Medford OR
356 PM PST Sat Dec 3 2016

.DISCUSSION...Interesting and active pattern moves in Sunday. A
system moves in early in the morning in the less favorable right
exit region of a 150 kt jet. However, there will be upper level
divergence over the region that will combine with 50 kt 700 mb
flow bringing broad lift the the region from the Cascades
westward. Snow levels will start around 5500 feet in the morning.
lowering to below 4000 feet in the Cascades on the trailing end of
the system.

The system in line moves in in Northwest flow in this cooler air
mass with 850 MB temperatures around -5 C. Although the flow will
not be as moist as a southerly flow from the bay area, this system
moves in in a more favorable left exit region of a 130 kt jet and
broad upper level divergence and westerly flow of 30-40 kt to
bring broad lift that will not confine the moisture mainly to the
west side. Snow levels start low, around 2500 to 3000 feet in
Oregon Sunday afternoon, dropping to near or below most of the
inland west side valley floors by Tuesday morning. The EC is more
in line with the NAM, bringing snow to to Rogue valley floor
Tuesday morning, with the GFS showing a mix of rain and snow. At
this time I am leaning more towards the NAM and valley floor snow
in the Rogue Valley, with rain in other west side valleys.
However, a strong vort axis moves through Monday night into
Tuesday morning and snow levels may drop to the valley floors
other than the Rogue briefly at times with higher precipitation
rates. Sven

...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...03/18Z TAF cycle...IFR conditions are affecting coastal
areas from Cape Blanco northward and into the Umpqua Basin. Further
inland, MVFR ceilings, along with mountain obscuration, will
continue to affect the area through the late morning. All areas
should lift to VFR by afternoon. This evening and tonight, ceilings
will again lower to MVFR at the coast as a cold front and associated
precipitation approaches the coast. Lower flight conditions and
precipitation will gradually spread west to the Cascades by
daybreak, but will generally hold off on the East Side until after
the current TAF period. -BPN


.MARINE...Updated 100 PM PST Saturday 3 December 2016...Very long
period swell cause waves to break further from shore than normal
today with a larger than normal surf zone. A cold front will move
through Sunday with increasing winds and a well defined wind shift
behind the front.  Winds should be strongest north of Cape Blanco
Sunday morning. Additional weak fronts will follow Monday into
Tuesday. High pressure will briefly build Wednesday, then a stronger
front will move through late Wednesday night into Thursday.  Gale
force winds are possible with this front.


OR...High Surf Advisory until 6 PM PST this evening for ORZ021-022.
     High Wind Warning from 1 PM to 7 PM PST Sunday for ORZ030-031.
     Winter Weather Advisory from 4 AM to 10 PM PST Sunday for
     Wind Advisory from 1 PM to 7 PM PST Sunday for ORZ030-031.

CA...Wind Advisory from 1 PM to 7 PM PST Sunday 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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