Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 180328

Area Forecast Discussion
National Weather Service Medford OR
728 PM PST Sun Dec 17 2017

.DISCUSSION...Another night with areas of fog and near normal low
temperatures is expected. The upper ridge in place is shunting
weak disturbances up and over our forecast area. The only change
we made to the forecast was to remove fog from some California
areas, after looking at current observations and trends over the
past week or so. On the whole though, humidity has trended higher
compared to 24 hours ago, and stability is significantly higher in
the low levels compared to yesterday, per the afternoon balloon
sounding in Medford. All this points to more fog coverage in West
Side valleys than last night.

The main weather impacts this week will occur from late Tuesday
through Wednesday when a strong cold front brings snow to all
mountain passes, and potentially to some West Side valley floors.
Wind will also accompany the front, primarily at the coast, in the
mountains and east of the Cascades. Travel will become difficult
over mountain passes due to snowy roads and lower visibility. Those
with travel plans should take this into account. We maintain a
Winter Storm Watch (WSWMFR) and Special Weather Statement (SPSMFR)
to highlight these weather impacts.

Please see the previous discussion below which provides some more
detail on the upcoming wintry conditions.


.AVIATION...18/00Z TAF CYCLE... VFR conditions at all locations this
afternoon are again expected to deteriorate across many of the west
side valleys from about Medford westward this evening through Monday
morning. However, the situation will be a bit different than the
consistent pattern than we`ve been seeing, as a weak warm front will
push toward the Coos and Douglas County coastlines late tonight
through Monday morning. MVFR to IFR low clouds are expected with
this warm front north of Cape Blanco along and near the coast during
that time period. Further inland, from Roseburg to Medford to Grants
Pass MVFR to LIFR fog and low clouds are expected, but confidence is
only low to moderate in the details because of the influence of high
clouds and temp/dewpoints supportive of frost formation. -BTL/BPN


.MARINE...Updated 300 PM PST Sunday, 17 Dec 2017...
High pressure centered offshore and a coastal thermal trough will
continue to support somewhat gusty north winds and choppy seas south
of Cape Ferrelo into this evening. A weak warm front is expected to
turn winds southerly north of Cape Blanco this evening. Winds will
become southerly across all areas Monday evening and then increase
ahead of a strong cold front. The front will sweep through Tuesday
into Tuesday evening with high seas and advisory to gale force winds
likely. High and steep swell will follow Tuesday night into
Wednesday night. A prolonged period of north winds likely, but it
now appears it will take until the weekend for these north winds to
reach advisory levels. BTL


.PREV DISCUSSION... /Issued 325 PM PST Sun Dec 17 2017/

SHORT TERM...Tonight through Wednesday night...Current satellite
observations are showing high clouds across most of the Pacific
Northwest. Radar and surface observations indicate that we are
still under the influence of high pressure, meaning that the air
stagnation advisory will continue through tomorrow before the next
front comes through on Tuesday.

A few important changes have occurred in this forecast cycle
regarding the arrival of the frontal system Tuesday. The biggest
of those changes are that models are now indicating that this
front will arrive a few hours earlier than previously expected,
and that this will spread the timing of the bulk of the
precipitation. That being said, temperatures on Tuesday will start
out a bit warmer before the front moves through. Then the
temperatures will plummet quickly behind the front. The
precipitation will transition to showers quickly after the front
passes, and all of the mountain passes within the area will see
snow fall. Have issued a Winter Storm Watch (WSWMFR) for areas
above 2000 feet where confidence is higher that impactful snow
will occur. Have issued a special weather statement (SPSMFR) for
most of the other passes above 1000 feet. See these products for
details, or our Weather Story on the front page for details and
possible snow amounts.

This system is even poised to bring snow to the valley floor with
less than a half an inch forecast for Medford. This is due to a
secondary shortwave that ejects through the area while moisture is
high and snow levels are already below the Rogue Valley floor.
Confidence is not as high as pass snow, but still high enough to
add to the forecast. Other than impactful low elevation snow,
this storm is typical for this time of year. Beyond that,
temperatures remain cold, especially with snow on the ground and
as high pressure sets up yet again (even if brief). -Schaaf

LONG TERM...Thursday through Sunday Night...Big changes have
cropped up in the models since yesterday. Where most guidance
showed a dominating ridge and offshore flow just 24 hours ago,
some ensemble members from the GFS suggested this ridge would
shift a bit to the west, and swift northerly flow would set up
over the forecast area instead. Today, not only has the
operational GFS and the bulk of its ensemble members trended
towards this solution, but the Canadian, the ECMWF, and the bulk
of the EC ensemble members have done so as well.

This slight shift in the ridge results in a significant change to
the forecast. Now a shortwave within the northerly flow is expected
to drop south over our area, producing a chance of light
precipitation Friday into Saturday morning. Cold air already in
place will produce snow levels of roughly 2000 to 3000 feet, so some
light snow is not out of the question for most of the passes and all
but the coast and West Side valleys. But behind this wave, a shot of
very cold air will quickly bring temperatures down to some of the
coldest of the season. Snow on the ground and clear skies will only
make it colder, especially on the East Side where low temperatures
Saturday morning could easily drop to well below zero. High
temperatures on Saturday may not even make it above freezing for all
but the coast.

Of course, all of this hinges on the location of the ridge. As of
this writing, confidence on the cold forecast solution is good given
the shear bulk of the guidance showing the influx of arctic air. A
late look at some additional ensemble and long term guidance has
reinforced this confidence, and quite frankly, these new colder
forecast temperatures are likely too high for any location that
still has snow on the ground when the cold air arrives. -BPN


OR...Air Stagnation Advisory until 8 AM PST Tuesday for ORZ029>031.
     Winter Storm Watch from Tuesday afternoon through Wednesday
     morning for ORZ029-030.
     Winter Storm Watch from Tuesday afternoon through Wednesday
     morning for ORZ023>026.
     Air Stagnation Advisory until 8 AM PST Tuesday for

CA...Winter Storm Watch from Tuesday afternoon through Wednesday
     morning for CAZ080>083.

Pacific Coastal Waters...Gale Watch from Tuesday morning through Tuesday afternoon for


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