Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 100452
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
852 PM PST Fri Dec 9 2016

.DISCUSSION...The forecast is on track this evening. A frontal
boundary draped across SW Oregon and northern California is being
supported by a strong 120-kt west to east jet. This is bringing in
widespread light to moderate precipitation across the area with
snow levels in the Cascades north of Highway 140 as low as 4700
feet (recent web cams showed rain changing over to snow along
Highway 97 in Chemult). Snow will be heavy at times in the
Cascades overnight into early Saturday morning with rates of
1"/hour or even slightly higher than that at times...and a winter
storm warning is in effect above 5000 feet. Winter weather
advisories are also out above 4000 feet in the Cascades and over
portions of the East Side/Warner Mountains. Check out WSWMFR for
the specifics. Snow levels will lower down to around 4000 feet
Saturday morning, but snow impacts should not be as significant
over Siskiyou Summit, since it will take most of the night for the
snow level to get that low. There may be an inch or so there by
morning.

Meanwhile, highest reported rainfall rates have been around
0.20-0.30"/hour in portions of western Siskiyou County as well as
southern Curry County, where 2-3 inches of rain is expected. The
front will slowly sag southeastward through the area, but most of
the steady precipitation will end Saturday morning (except in
Modoc where it will continue until early afternoon). Showers will
persist Saturday afternoon, focused over north and west-facing
terrain and also along the coast/Umpqua Basin. -Spilde

&&

.AVIATION...10/00Z TAF CYCLE...Precipitation will be
heaviest this evening but precipitation and mountain obscurations
will remain widespread overnight with a freezing level around 6000
feet MSL falling to 4500 feet MSL. This will result in mainly MVFR
ceilings and visibilities with local IFR/LIFR and local VFR
conditions. Conditions will deteriorate somewhat in the early
morning with an increase in valley IFR. But, there will be a brief
break between systems from mid-morning into Saturday afternoon with
gradual improvement as most of the area becomes VFR. The next system
will spread rain with MVFR ceilings to the Douglas and Coos County
coast including KOTH toward 00Z/the end of the afternoon. Showers
with MVFR and mountain obscuration will be most common in southwest
Oregon on Saturday evening. -DW

&&

.MARINE...Updated 500 PM PST Friday 9 December 2016...Advisory level
southerly winds will be strongest north of Gold Beach this evening
then veer to northwest and decrease from northwest to southeast
behind a front. Wind waves, fresh southwest swell, and a longer
period west swell are producing high and very steep hazardous seas.

As winds diminish later tonight into early Saturday, seas will
become swell dominated and steep but remain high. The brief break
between systems will end with the return of advisory level southwest
to west winds late Saturday morning. These winds will persist
through Sunday and will be strongest north of Gold Beach. High and
steep seas are expected to reach a peak during Sunday morning then
remain high and steep into Sunday evening.

Relatively calmer weather is expected for Monday but there is low
confidence in the details of will likely remain an active pattern.
-DW

&&

.PREV DISCUSSION... /Issued 535 PM PST Fri Dec 9 2016/

SHORT TERM...IR imagery is showing a nice moist plume streaming
across the eastern Pacific into southwest Oregon and northern
California late this afternoon and evening. Mid-level forcing is
sufficient for widespread precipitation along a frontal boundary
that is draped across the area. The front has support from a
west-east 120-kt 300mb jet streak. A surface wave will develop
along the front this evening and move onshore near Cape Mendocino
overnight into Saturday morning. Snow levels this evening are up
around 5000-6000 feet, but these will gradually lower overnight as
the front settles southward. Expect them to be down around 3500
feet by Saturday morning. Significant snow is expected in the
Cascades above 5000 feet, where a winter storm warning is in
effect. With snow levels lowering overnight, expect winter
conditions to impact the higher Cascade passes and a winter
weather advisory is also out for elevations above 4000 feet there.
Snow amounts over Siskiyou Summit though will be limited since it
will take all night for the snow level to fall down that low. Some
travel impacts may also be felt over the higher terrain of the
East Side, including near Chemult, and the Warner Mountains. The
surface low and front will push south and east of the area by
late Saturday morning and precipitation will become showery or
just end completely. Showers will be most numerous along the coast
north of Cape Blanco Saturday afternoon.

Onshore flow and short wave disturbances moving by from time to
time will keep showers going from the Cascades and Siskiyous north
and west into Sunday night, while the East Side and northern
California are comparatively drier. Snow levels will range from
2500-3500 feet during this time frame, so there could still be
some winter impacts over the mountains. However, snow amounts at
this time don`t appear to be too substantial but could be a minor
inconvenience for travelers. We should catch at least a bit of a
break Monday. -Spilde

LONG TERM...Tuesday through Friday...Models continue to show
considerable variability from run to run and between solutions next
week. An upper level jet is forecast over the Northern California
region on Tuesday and may shift north into southern Oregon.
Meanwhile, a low pressure system will bring a very moist front to
areas in Northern California. Models are showing low confidence
on the track of this low and whether this moist frontal system will
be aimed at the San Francisco bay area or further north near the
CA/OR border. If this moist front remains south of the area on
Tuesday, expect a cold showery pattern with a trough over the CWA.
Otherwise, if the front is aimed over the CA/OR border, expect
moderate to locally heavy precipitation with increasing snow levels.
On Wednesday, models show general agreement with a weak shortwave
ridge and with cool conditions and decreased chances for
precipitation. Of concern, is that if the GFS solutions is correct,
much colder temperatures are possible, especially for the low
temperatures Wednesday morning and Wednesday night. East of the
Cascades,  temperatures in the single digits to teens would be
possible if the drier and cold pattern materializes. Late in the
week, Thursday and Friday, confidence remains low in the forecast
details with models showing considerable variability in the strength
and track of a low pressure system and trough  expected to move
towards the area and inland over the Oregon and California region by
Friday. Expect this system will bring a chance for colder and wet
conditions late next week. Due to the model variability and
potential for weather impacts, will need to monitor model forecasts
closely and add details to the Tuesday through Friday forecasts as
confidence increases. /CC

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 10 AM PST Saturday for ORZ029>031.
     Winter Weather Advisory until 10 AM PST Saturday ABOVE 4000 FEET
     for for ORZ027-028.
     Winter Storm Warning until 10 AM PST Saturday ABOVE 5000 FEET
     for for ORZ028.
     Winter Storm Warning until 10 AM PST Saturday ABOVE 5000 FEET
     for for ORZ027.

CA...Winter Weather Advisory until 10 AM PST Saturday ABOVE 5500 FEET
     for for CAZ085.

Pacific Coastal Waters...Small Craft Advisory for hazardous seas until
     10 AM PST Saturday for PZZ350-370.
     Small Craft Advisory for hazardous seas from 4 AM to 7 PM PST
     Sunday for PZZ350-370.
     Small Craft Advisory from 10 AM Saturday to 4 AM PST Sunday for
     PZZ350-356-376.
     Hazardous Seas Warning until 10 PM PST this evening for
     PZZ350-356-370-376.
     Small Craft Advisory for hazardous seas until 7 PM PST Sunday
     for PZZ356-376.
     Small Craft Advisory from 10 AM Saturday to 7 PM PST Sunday for
     PZZ370.

$$

MAS/CC/DW


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