Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 102301

Area Forecast Discussion
National Weather Service Medford OR
301 PM PST Sat Dec 10 2016

.DISCUSSION...The latest IR image shows the front that brought
steady precipitation last night and this morning is just south of
the area. Meanwhile a shortwave is swinging through Vancouver Island
and a surface trough is just offshore. This feature will move inland
in the next couple of hours and places like Coos County and
northwest Douglas County will have an uptick in shower activity.
Elsewhere at the moment showers are far and few in between. A look
at the high res short range HRRR model shows showers increasing
along the coast late this afternoon, then moving inland early this
evening west of the Cascades. Snow levels this evening will be
around 3500 feet along the Cascades, so accumulating snow is most
likely to occur at Crater Lake and Diamond Lake where showers are
expected to be most frequent. Could not rule out a inch or two near
Lake of the Woods and an inch or less at Siskiyou Summit tonight.

A strong westerly 100+ kt jet at 300mb will remain over the region
through early next week. This will keep our weather active most of
the time. The flow at the mid levels will also remain westerly with
disturbances riding along it. Therefore we can expect showers to
continue mainly west of the Cascades Sunday into Monday morning.
Snow levels are expected to start out around 3000 feet Sunday the
morning, then rising to around 3500 feet Sunday afternoon. Same for
Monday morning and Monday afternoon. This type of flow at the mid
levels typically results in higher precipitation amounts along and
west facing slopes of the Cascades. However its not expected to be
excessive, even for areas like Crater Lake and Diamond Lake.

We`ll catch a relative break in the action Monday afternoon into
Monday night with weak ridging building into the area, then a good
slug of moisture (IVT related) could impact Northern Cal Tuesday
morning. The models vary with respect to the tining and northward
extent of the heavier precipitation. However they do agree that the
heaviest precipitation should remain south of our area. If the core
of the heavier precip ends up further north, say into southern Modoc
and Siskiyou county, then we could be dealing with significant snow
accumulations. As previously mentioned, it`s still a ways out so
we`ll have to keep a close eye on this in the days to come.

.Long Term...Wed, Dec 14th - Saturday, Dec 17th...
The long term period will bring with it the possibility of
significant precipitation amounts across Northern California and
portions of the East Side, followed or accompanied by the intrusion
of an Arctic airmass into the region. This will set the stage for
the possibility of low elevation snowfall west of the Cascades late
int the week. However, it should be noted that there has been a
tremendous amount of run to variability in the forecast models on
timing and amount related details for this entire extended period.
What there is fairly high confidence in is Arctic air spilling onto
the East Side late week and moderate confidence in this also
affecting the West Side.

More specifically, on Wednesday, ESRL reforecast analog data from
the 00Z GEFS indicates a 90th percentile (of climatology for this
time of the year) precipitation event for the Northern Sierras from
Monday night through Thursday afternoon, while, for our area, it
indicates a 50-70 percent chance of a 90th percentile event in
portions of eastern Siskiyou and Modoc Counties. Meanwhile, for
Wednesday, the operational GFS and ECMWF 12Z runs indicated an
atmospheric river affecting the Northern Sierras with precipitation
pushing northward into our area into at least Eastern Siskiyou and
Modoc Counties. During that time period snow levels appear they will
be in the 4.5-5.5kft range. How much precipitation falls in our area
certainly depends on the details and, while the operational models
and reforecast analogs look promising for significant amounts,
guidance has wavered on whether or not this will be a significant
event. The 18Z GFS has just come in, and indicates 0.50" to 1" of
water late Wed afternoon through late Thu afternoon for the
mentioned areas in northeastern California. I would not be surprised
if the guidance continues to waiver here for a couple more days.

On Thursday and Arctic airmass intrudes from the north, spilling
mostly out of the Columbia Basin, but also in modified form on the
West Side. Models have periodically indicated (every other run or
more for the day) a shortwave trough riding through Thursday night
into Friday with 850mb temperatures falling from -4C to -8C. This
brings a possibility of another round of West Side snowfall,
possibly lower than was observed in the past week. It should be
noted that some guidance simply shows the cold air working in on the
West Side and not much else.

A very cold air mass will then settle in for the weekend, with the
next frontal system expected to dive in from the northwest early
next week. It appears that temperatures on the East Side are very
likely to fall into the single digits or lower for lows this
upcoming weekend, with low in the 10s to lower 20s expected for
most of the West Side. BTL


.AVIATION...10/18Z TAF CYCLE...VFR along the coast and in the Umpqua
Basin is expected to lower to MVFR mid-afternoon as another round
of showers move in, likely falling to IFR overnight into Sunday
morning. Across the remainder of the West Side IFR to MVFR is
expected to improve to VFR this afternoon and then IFR to LIFR is
expected to develop overnight into Sunday morning. All high
terrain areas west of the Cascades will experience at least
partial terrain obscurations. East of the Cascades VFR and partial
terrain obscurations will generally prevail, though areas of MVFR
area expected this morning. The freezing level, between 5,000 and
6,000 feet, late this morning, is expected to lower to 4,000 to
5,000 feet through this evening. BTL


.MARINE...Updated 905 AM PST Saturday 9 December 2016...Periods of
enhanced west to southwest winds are expected through Sunday
evening. Winds will remain at or just below small craft advisory
levels throughout this time. Meanwhile, west swell will combine with
the resulting wind waves to produce steep seas in nearly all of the
coastal waters. Have maintained a small craft advisory for both
winds and seas throughout this time period, as conditions will be
hazardous to small craft, but the confidence in the exact location
and timing of winds is low.

Beyond Sunday, winds and seas will become relatively calm for much
of the week under high pressure and east winds, with a stronger
system forecast to arrive by the weekend. BTL/BPN




Pacific Coastal Waters...Small Craft Advisory until 10 AM PST Monday for


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