Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 221114
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
314 AM PST Mon Jan 22 2018

.Short Term...through Wednesday...Snow showers continue to produce
accumulations in portions of Siskiyou County this morning. In
fact, from webcams it looks pretty snowy around Snowman Summit,
and while it doesn`t look like it`s sticking to I-5 that much,
it`s snowing from Dunsmuir up through Mt. Shasta. High resolution
guidance shows this continuing through mid morning but gradually
tapering off. Another inch or two of snow is possible through
dawn, then impacts will diminish. As a result, the winter weather
advisory was extended through 10 AM for these locations. It is
still also snowing occasionally around Mt. Ashland and in the
Cascades. Accumulations should generally be light and diminish as
the airmass dries and upper forcing moves off to our east today.
The rest of us in the lower elevations will see a couple of
showers roaming around this morning, but those will decrease as
well. The exception will be along the coast where onshore flow
will keep shower potential going through today.

We`ll see a short break tonight into early Tuesday, but the next
front will already be knocking on the door by daylight Tuesday.
This front doesn`t look quite as strong as the last one, but it
does look more "traditional" in that the flow aloft is more
southerly and thus is more supportive of strong winds in the usual
areas like Shasta Valley and east side. This one isn`t a slam
dunk for winds, but there is certainly enough potential to warrant
a high wind watch in the usual spots, and one has already been
issued.

The front will be much more slow moving than its predecessor, and
thus it will be much wetter. This is the first front in a while
that looks like a true "atmospheric river." Current forecast
rainfall amounts are 3-6 inches along the south coast and coastal
mountains, 1-2 inches in the Cascades and Siskiyous, up to an
inch in interior west side valleys, and 0.25 to 0.50 inch on the
east side. Snow levels will rise to around 5000 feet as warm
southerly flow develops, but above this elevation, these copious
precipitation amounts will be snow. As such, a winter storm watch
was issued for the Cascades and Siskiyous. We could easily see a
foot of snow from Crater Lake down to Mt. Ashland, which is not
great news for travelers, but it is to would-be skiers. This setup
also looks to favor the Mt. Shasta area this time. Flow aloft is
much more southerly, precipitation should be heavy, and
thermodynamics are favorable. We held off on issuing a winter
storm watch for Siskiyou County to avoid confusion with the
current advisory, but one could be forthcoming when the advisory
expires later this morning. -Wright

.Long Term...Thu, Jan 25th through Sunday night, Jan 29th, 2018...
The long term period begins with an anomalously cold trough of low
pressure extending from southern Alaska southward to southern
California along the West Coast. GEFS 500MB geopotential heights
begin the period at 537 decameters, which equates to a normalized
anomaly of -1.75 to -2 standard deviations. Thus, it will be colder
than normal across the forecast area with showers and low snow
levels expected. Model guidance is well agreed on showers from along
and near the Cascades westward during the day on Thursday,
diminishing Thursday night into Friday morning. Snow levels are
expected to generally be between 1500 and 2500 feet, +/- 500 feet.
Therefore, we`ll be on the lookout for snowfall on the lower passes
such as Jacksonville Hill and the Sexton Summits. Snow will also be
possible on or near the valley floors of the Rogue and Illinois
valleys. Have adjusted snow levels Thursday morning through Friday
morning to account for some expectation of the local effect of lower
snow levels in convective SW flow over and near the Illinois Valley
versus the Rogue Valley. Given the showery pattern, current
expectations are that accumulations in the valleys will be generally
light, but could be locally significant in locations such as Hayes
Hill in the Illinois Valley. For the relatively parched downhill ski
areas, models are indicating 0.25" to 0.75" of water equivalent at
Mount Ashland and 0.50" to 1.0" at Mount Shasta Wednesday night
through Friday morning. Due to the cold air mass snow amounts are
expected to be a fluffy 5-10 additional inches at Mount Ashland and
6-12 additional inches on Mount Shasta.

Friday the trough begins to lift out to the northeast as high
pressure begins to build in from the southwest. However, due to
replacement of a colder air mass by a warmer one, we anticipate
precipitation could linger Friday into Saturday, especially over NW
sections of the forecast area, with snow levels remaining below 5500
feet.

Anomalously high amplitude high pressure is then expected to move
through Saturday night through Monday, followed by another fairly
strong trough of low pressure arriving from the Gulf of Alaska
Tuesday into Wednesday. This trough is not as deep as recent ones on
the GEFS, but should be enough to bring us a bit more rain and some
freshening of the mountain snowpack. It is notable that both the 12Z
ECMWF and 18Z GFS do indicate the trough to be deeper/colder than
does the GEFS mean, so this means there is uncertainty related to
the amplitude and resultant southern periphery of this trough. BTL

&&

.AVIATION...22/06Z TAF CYCLE...The front has moved through the
region and we`re now watching showers on radar.  CIGS are quite high
in the majority of areas around 3500 feet, so mountain obscuration
is likely over the Cascades during the overnight hours.  There
appears to be some clearing to the west, so the valleys are expected
to see some low level MVFR stratus at times and perhaps some
visibility reductions.

Also added snow showers near Klamath falls and those can reduce
visibilities down to 2-3 miles when the move over head.  Those
should be pretty isolated through the next 6 hours.

Not expecting fog to force IFR rules because the low level
instability increases during the overnight hours with the colder air
moving in aloft.

As for Monday,  look for VFR conditions to prevail with just a few
cumulus clouds forming in the cold and unstable airmass.

-Smith

&&

.MARINE...Updated 200 AM PST Monday 22 Jan 2018...A break
between systems is expected today. Winds will be relatively calm,
but seas will remain steep and hazardous to small craft through
tonight. The next system will affect the waters early Tuesday
morning. This front is not as strong as the recent one on Sunday, so
only gales are expected. However, this front may stall and the
duration of strong winds may be longer than the Sunday system.
Currently it looks like winds will be just below gale force after
Wednesday morning, but are expected to remain increased through
Thursday morning.

Seas will build again late Tuesday morning becoming very steep and
hazardous 14 to 17 feet at 10 to 11 seconds through Wednesday
morning. Seas will remain elevated through much of the week, peaking
again at 19 to 20 feet on Thursday. Conditions may briefly improve
Friday before the next front moves through the waters Friday
evening. /BR-y

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Watch from late Tuesday night through Wednesday
     evening for ORZ030-031.
     Winter Storm Watch from Tuesday evening through Wednesday
     evening above 5000 feet in the for ORZ027-028.

CA...High Wind Watch from Tuesday afternoon through Wednesday morning
     for CAZ081.
     Winter Weather Advisory until 10 AM PST this morning for
     CAZ082-083.

Pacific Coastal Waters...Small Craft Advisory until 4 AM PST Tuesday for
     PZZ350-356-370-376.
     Gale Warning from 4 AM Tuesday to 4 AM PST Wednesday for
     PZZ350-356-370-376.
     Hazardous Seas Warning from 4 AM Tuesday to 4 AM PST Wednesday
     for PZZ350-356.

$$


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