Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 160342

Area Forecast Discussion
National Weather Service Medford OR
742 PM PST Fri Dec 15 2017

.DISCUSSION...A cold front has moved through the forecast area, and
snow levels have lowered to around 4000 feet. Precipitation
amounts thus far have been meager, with only a few hundredths of
an inch being reported by observation sites across the forecast
area. Not much more precipitation is expected behind the front,
and this is supported by infrared satellite imagery showing very
little in the way of developed showers behind the front. In fact,
so little precipitation has fallen that air quality hasn`t
improved that much in the Rogue Valley or elsewhere for that
matter. Still, sufficient mixing is expected tomorrow and Sunday,
so another Air Stagnation Advisory has not been issued. We`ll keep
an eye on air quality observations and see how much mixing
improves tomorrow.

Tonight clouds will fill up most valleys, but the air mass should be
sufficiently unstable to prevent widespread dense fog as occurred
some over the past week under the very stable air mass.

Tomorrow, cool temperatures and partly to mostly cloudy skies with
dry conditions are expected in most places. An upper ridge of high
pressure will dominate through Monday. Please see the previous
discussion below for details on the next precipitation event,
expected Tuesday.


.AVIATION...16/00Z TAF CYCLE...The cold front is moving through the
forecast area, and there is a mix of MVFR and IFR conditions along
and behind the front. Ahead of the front, VFR conditions continue.
Freezing levels will drop several thousand feet behind the front.
Late this evening, snow levels will drop to 4000 to 4500 feet east
of the Cascades, which could mean some snow at KLMT...with any
accumulation will be less than half an inch. However snow showers
will diminish visibility. A ridge of high pressure will begin
building early Saturday morning, and ample available moisture should
cause conditions in west side valleys to drop to IFR... possibly
LIFR. Dense fog is less likely than previous nights however, because
the air mass will be more unstable than previous nights. -MSC/SK


.MARINE...Updated 330 PM PST Friday 15 Dec 2017...Moderate to
strong north winds will continue into this evening, strongest winds
and steepest seas south of Cape Blanco. Winds will weaken slightly
overnight. High pressure offshore and a coastal thermal trough will
develop Saturday, bringing gusty north winds south of Cape Blanco.
Winds will be strongest south of Gold Beach into Sunday morning. A
stronger low pressure system will approach the marine area
Tuesday... ramping up south winds and wind waves, with heavy west
swell. -MSC


.PREV DISCUSSION... /Issued 405 PM PST Fri Dec 15 2017/

SHORT TERM...Tonight through Monday night...Current observations
show a band of light precipitation running across the Umpqua
Basin and diving southward. This will work to lower temperatures
and snow levels through evaporative cooling. That being said, snow
levels will stay above the valley floors, but elevations above
2500 feet may see a few snow flakes in post-frontal showers
tonight. All of the passes will pick up some snow, but it appears
as if the snow will be pretty light. In addition, the west-side
valleys reached above freezing before the rain began, so the
threat of freezing rain which was advertised yesterday has

The post frontal showers will continue through tonight before a
weak ridge of high pressure sets back up. A very weak inversion
may set up, but it`s more likely that west side valleys will see
low clouds instead of any fog tonight. For the rest of the
weekend, however, the high pressure will continue to build and the
inversions may become somewhat stronger, particularly south of the
Umpqua Basin. Fog and low clouds will be possible for both Sunday
and Monday morning. Despite the building high pressure system, the
frontal system today will preclude the necessity for another air
stagnation advisory for the weekend.

A more active weather pattern appears as if it will stay to the
north of our forecast area this weekend. This means that the
Umpqua Basin may see cloudiness not associated with valley
inversions, but overall rain chances will remain minimal. -Schaaf

LONG TERM...Tue (12/19) through Fri (12/22)...
Model guidance has come into better agreement on a major shortwave
trough moving in from the northwest Tuesday through Wednesday. As is
usually true for a forecast in the 5-6 day forecast window,
individual models still differ on the details of timing, speed, and
magnitude. It does appear that this frontal system will be stronger
and more moist than today`s and will bring snow levels down lower as
it moves through. The 12Z GFS and ECMWF models from today indicated
significant cold air advection on the back side of the main frontal
band, with a secondary mid-upper level impulse catching up to the
front west of the Cascades Tuesday night into Wednesday morning.
This will be something we`ll be keeping a close eye on because the
850mb and 700mb temperatures in the -4C to -6C and -14C to -16C
range, respectively, that those model runs forecast, have brought
snow levels down to 1000 to 2000 feet in the Rogue Valley in
December snowfall events of the past 5 years. Today we increased
precipitation probabilities with this frontal system some and also
lowered snow levels a bit to better capture model trends and the
colder side of consensus. This does bring a threat of snow showers
down to the 1300 to 2500 foot range for many west side locations
Wednesday morning, though the bulk of snow accumulation is currently
above 3kft. The one exception is in eastern Douglas County, where
snow levels are likely to fall faster with higher amounts possible.

After some lingering showers and snow showers Wednesday night into
Thursday morning we end up under the influence of another high
amplitude ridge of high pressure that is likely to extend from west
of California to western Canada Friday through next weekend. BTL




Pacific Coastal Waters...Small Craft Advisory until 4 AM PST Sunday for PZZ356-376.


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