Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 230420
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
820 PM PST Thu Feb 22 2018

.UPDATE...Snow showers are continuing across portions of southern
Oregon and northern California, mainly south of the Umpqua Divide.
These showers will continue for the next few hours as they
continue to move south and dissipate. This will allow skies to
clear out tonight and allow temperatures to be very cold. This
means that anywhere that had snow melt during the day will see
the roads refreeze and become slippery once again. Please use
caution while traveling tonight, and allow extra space between you
and the vehicle in front of you.

The only change made to the immediate term is to add a little bit
more cloud cover over the Umpqua basin, thus increasing
temperatures just a bit. The freeze warning still continues across
the coast. See the NPWMFR for more details.

Otherwise, the main forecasting challenge will be regarding the
details of the next system that passes through southern Oregon and
northern California. A winter storm warning for Highway 138
between Steamboat and Toketee Falls is in effect, with several
winter weather advisories across southern Oregon. Will need to
evaluate if western Siskiyou County is going to get enough snow,
particularly on Highway 3 to warrant an advisory. See the WSWMFR
for more details. -Schaaf

&&

.AVIATION...23/06Z TAF CYCLE...Some snow showers persist this
evening, mostly from the Cascades est. Local IFR cigs/vsbys may
accompany them. Overall, VFR conditions will prevail through
Friday evening. The exception, other than the above mentioned snow
showers, will be late night through morning low clouds and fog in
some of the west side valleys, including KRBG.

&&

.MARINE...Updated 800 PM PST Thursday 22 Feb 2018...North winds
and choppy seas will diminish tonight as high pressure builds
offshore. Wave heights will be lower Friday, but may still be
hazardous to small craft. A weak front will move onshore Friday
evening. High pressure will briefly return Saturday. A stronger
front will move onshore Sunday, and it will bring high and steep
northwest swell with it. Even heavier swell will move in Monday.
Yet another weak front will approach the area Tuesday.

&&

.PREV DISCUSSION... /Issued 259 PM PST Thu Feb 22 2018/

DISCUSSION...

..Short Term...Rest of today through Sunday night...What a busy
week it has been, but we`re not finished yet with the active
winter pattern. Snow showers continue across the area this
afternoon, with some reports of graupel and rain mixing in here
and there. These will gradually diminish and move off to the south
by this evening, with dry air on the back side of the departing
wave likely to bring us some clearing overnight. This clearing, if
and when it happens, is the first concern on the docket.

Where we see significant clearing tonight, temperatures are
likely to drop quickly to below freezing, and inland from the
coast, much cooler than that. This will be helped by any residual
snow on the ground and the cold air mass in place overhead. Any
snowmelt on road or walking surfaces is likely to freeze, and this
could lead to patches of black ice, a significant concern to
anyone driving or walking on said surfaces. Also, these cold
temperatures will drain down to the coast and valley bottoms,
leading to cold overnight lows that could have a detrimental
impact on local agriculture. At the coast, where freezes are
uncommon, a freeze warning has been issued for tonight. Elsewhere,
since the growing season has yet to begin, there are no
headlines. However, the warmer than average temperatures of the
past month have lead to some early growth, and these plants and
trees could see some damage or stunted development unless the
proper precautions are taken.

The second concern today has been the next system arriving Friday
night into Saturday. While seeming a bit more robust in the model
solutions today, it is also trending a bit warmer. As
the system enters the area Friday night, snow levels will begin
very low, but as precipitation continues into the morning, snow
levels are expected to rise. to above the valley floors of the
West Side. The predominant flow of the system will be west to
east, and this will lead to upslope lift for westerly slopes,
especially along the Cascades, where the heaviest precipitation is
likely to occur. Have issued a winter weather advisory for the
Cascades and the higher elevations and passes of the West Side,
and a Winter Storm Warning for eastern Douglas County, where the
heaviest snow amounts are expected. For more details on snow
amounts and timing, see the winter weather message at PDXWSWMFR.
There is a very slight chance that some localized areas,
especially in the smaller valleys of the coastal ranges, could
see a brief period of freezing rain as precipitation begins or
changes over, but confidence is very low.

Showers will taper off Saturday afternoon and evening, but never
quite come to an end before the next wave arrives on Sunday. Snow
levels will begin this event on the lower side once again, then
rise throughout the day Sunday. It appears the bulk of
precipitation will arrive after snow levels rise above the valley
floors of the West side Sunday afternoon, but significant snow is
expected in the mountains Sunday evening and night, with around a
foot of new snow possible above 5000 feet. Snow levels will begin
to fall heading into the late night, but precipitation is
expected to lighten and transition to showers heading into Monday
morning.

..Long term...Monday through next Thursday...Operational 12Z
models start to diverge Monday, but the 18Z GFS has come more in
line with the 12Z operational ECMWF. The ensemble mean for the GFS
and EC ware very close however, and leaned towards whatever
operational run was closest to the ensemble mean for that period.
Winter continues to make up for being absent in our lives for
December by doting upon us with cold and low level snow through
most of next week.

On Monday the tail end of a system moves through Monday morning,
and unlike today there should be enough clearing and with nearly due
north flow at 700MB, there should be enough clearing to allow
temperature to rise into the mid 40s and melt the previous snow on
sun exposed road and sidewalk surfaces. The upper low digs south
into central CA by Tuesday morning, with the upper level ridge
nudging into the region with weak offshore flow indicating clearing
before early Tuesday morning allowing temperature to drop below
freezing in the west side valleys, and well below freezing on the
east side. The ridge retrogrades and the next, wetter system moves
in Late Tuesday afternoon to evening. Although the entire forecast
for the next week looks interesting, Wednesday could be very
interesting with 850 MB temps indicating below -5 C during the warm
conveyor time. However, confidence is not high this far out and will
not stress this period until we get closer. -Sven


&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 10 PM Friday to 4 PM PST Saturday
     for ORZ027-028.
     Freeze Warning until 10 AM PST Friday for ORZ021-022.
     Winter Weather Advisory from 10 PM Friday to 4 PM PST Saturday
     for ORZ021-023>026.
     Winter Storm Warning from 10 PM Friday to 4 PM PST Saturday for
     ORZ025.

CA...None.

Pacific Coastal Waters...Small Craft Advisory for hazardous seas until 10 PM PST this
     evening for PZZ350-356-370.
     Hazardous Seas Warning until 10 PM PST this evening for
     PZZ350-356-370-376.

$$

BMS/JRS/BPN


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