Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 080630
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
1030 PM PST Wed Dec 7 2016

.DISCUSSION...Snow has been accumulating since about 4pm in much
of Siskiyou County, and we have reports of 1-4 inches so far. This
should continue for the next several hours, so expect 3-6 inches
in general for Western Siskiyou County before snow changes to
freezing rain (mainly in Scott Valley) and then rain. Latest RAP13
output is showing a changeover to rain for many of Western
Siskiyou County valleys around midnight. Additionally, heavier
snow along I-5 in Northern California will likely taper off
shortly per radar trends and latest HRRR output. Areas south of
Weed will likely maintain the snow the longest.

Further north, we are monitoring the situation in Southwestern
Oregon for the potential for snow and freezing rain. In general, the
higher elevations like Siskiyou Summit along I-5 and narrower
valleys like the Illinois, are expected to see the greatest impact,
with snow and potentially a brief period of freezing rain. In the
wider valleys like Medford, a period of snow is possible but a quick
changeover to rain is expected. Road temperatures are well above
freezing in the Rogue Valley, so expect little to no impact on I-5
in the Rogue Valley and other city roads. Further south in Ashland,
windy conditions exist and have increased winds in the forecast for
that. But it is not a warm wind from the east, with temperatures 35
and dew point well below freezing. Again, not expecting major
impacts here, but we will be monitoring closely.

Further east in Klamath Falls, a quick shot of several inches of
snow is expected later tonight with the potential for a period of
freezing rain from sunrise through about 10am.  This is the latest
model consensus, factoring in the latest RAP13 forecast sounding for
that area.

There`s plenty more weather impacts to speak about, and these are
covered nicely in the previous discussion below. Be sure to read the
Winter Hazard Product at WSWMFR for additional details as well.

&&

.AVIATION...08/06Z TAF CYCLE...A warm front is pushing from south to
north across the forecast area tonight. The IFR and MVFR conditions
currently over Northern California will spread into Oregon
overnight. Some areas will see lower visibility due to snow, like
the mountains and portions of the East Side like LMT, while other
West Side valley locations like MFR will see conditions lower to
MVFR by Thursday morning. Periods of snow will likely result in
several inches of snow at LMT through Thursday morning, at which
point precipitation may briefly change to freezing rain before
changing to rain and tapering off later in the morning. Models
suggest widespread sub-VFR conditions continue for much of the day
on Thursday and likely continuing into Thursday night.

Additional hazards include terrain becoming obscured from south to
north tonight and increasing south winds in the Shasta Valley
tonight into tomorrow and east of the Cascades tomorrow. SK

&&

.MARINE...Updated 300 PM PST Wednesday 7 December 2016...Broad
surface low pressure and an associated occluded front stretches
along 40N this afternoon. This front will lift north as a warm front
tonight which will switch winds to the south. As low pressure rides
along that front Thursday morning and is then followed by a cold
front Thursday evening, south winds will increase to gales, and seas
will become very steep. This system has been delayed approximately 6
hours from previous timing, and all the hazard products will be
updated and pushed back accordingly.

A secondary strong low pressure system will move by to the north on
Friday, and winds will be southwesterly to westerly but remain
strong and gusty just below gale force through Friday. While winds
remain just below gales, seas will continue to be very steep,
especially across the north, so the hazardous seas watch will be
upgraded to a warning across the north. The delay in the system also
means a delay in the large swell that follows, and a new hazardous
seas watch will be issued for this swell Friday night and Saturday.

Calmer conditions are expected Sunday and Monday, but the storm
track may become active again later next week. -Wright

&&

.PREV DISCUSSION... /Issued 621 PM PST Wed Dec 7 2016/

Updated aviation discussion.

DISCUSSION...Cold air is in place today and a warm front will
gradually move into the area from the west-southwest this evening
through Thursday morning followed by a cold front later Thursday. As
a warm layer at 6000-7500 ft moves over the area, expect
precipitation to fall into a colder layer below. This will result in
areas of freezing rain possible tonight and Thursday morning. Areas
that start out as snow may change over to freezing rain then to
rain. The best chance for freezing rain is in Western and Central
Siskiyou County northward into the Siskiyous and over portions of
Southwest Oregon, including the Cascades, western mountains and
portions of the Illinois and Applegate valleys. Rain is expected in
the Rogue and Umpqua valleys and along the coast. East of the
Cascades, freezing rain or sleet is possible Thursday morning in
eastern Siskiyou County, into the Klamath Basin, northward to the
Chemult area.

Accumulations of ice are expected to be mainly light but enough to
cover walking surfaces or roads. Freezing rain accumulation may be
more isolated in the Illinois Valley and Applegate where ground
temperatures are warmer and freezing rain is more likely to be
brief. These valleys may see impacts more limited in extent and
bridges will be the main concern. Elsewhere, the timing and exact
strength of the warm layer are a concern and will determine whether
freezing rain occurs for a brief or longer durations. This includes
the mountains passes on I-5 near Siskiyou and Sexton summits, the
passes in the Cascades, into Siskiyou County, as well as into
eastern areas such as along Highway 97 in Siskiyou and Klamath
counties. The area with the greatest threat for impacts and
sustained freezing rain is the Scott Valley in Northern California
where cold temperatures, a warm layer aloft and precipitation are
expected to bring around a tenth of an inch of ice to that area.

Snow will also be a concern with areas west of the Cascades expected
to receive 1 to 4 inches of snow this evening into tonight. Snow
levels will start out low around 2000 feet and then rise overnight
to closer to 4000 feet. Snow levels west of the Cascades will rise
to around 6000-7000 ft Thursday morning. In the Cascade Mountains,
expect around 4-8 inches of snow accumulation before snow levels
rise by late Thursday morning. East of the Cascades, snow levels
will start out near valley floors then rise during the late morning
and early afternoon. Snow accumulations of 2 to 5 inches are
expected through Thursday morning for eastern areas.

Winter weather Advisories are out for many locations and an Ice
Storm Warning is out for the Scott Valley. See the WSWMFR for
details.

Southerly winds will increase late tonight and Thursday over the
higher terrain and in the southern Shasta Valley as winds aloft
increase and the surface pressure gradient tightens. Gusts to 50 mph
are possible in the southern Shasta Valley. Southeast winds may also
increase into the southern Rogue Valley south of Medford to near
Ashland with gusts to around 40 mph. East of the Cascades, strong
gusty winds are possible Thursday afternoon into Thursday night, for
the higher terrain of the Fremont Winema forest, the Warner
mountains and the Summer Lake areas. Gusts to 60 mph are possible
for these eastern areas as a strong 700 mb jet moves over the area
late Thursday. A high wind watch is out for the east side. See the
NPWMFR for details.

Friday through Sunday...Confidence is low in the timing, but high in
the scenario that a cold front will move across the area on Friday
then be followed by a surface trough with numerous showers and snow
showers later Friday into early Saturday. The GFS has its usual bias
of being the most progressive and is about 6 hours faster than the
ECMWF. The GFS timing was not completely discounted as the NAM is
more similar to it. Given the uncertain timing, there is low
confidence in how far snow levels will fall by the end of the day.
The snow level will begin the day around 5000 to 5500 feet then fall
to around 3000 to 3500 feet behind the front. Snow amounts are
forecast to be around 1 to 4 inches with the majority falling on the
Oregon Cascades. As happens quite often in our area, a more
impactful burst of showers and snow showers will occur late Friday
into Friday night (or solely on Friday night) with the surface
trough as the axis of the 500 MB trough swings through. Snowfall
above 3500 feet on Friday night is forecast at 3 to 7 inches in the
Oregon Cascades and 1 to 3 inches elsewhere.

A moist onshore flow is expected for Saturday into Saturday night
with a slow gradual tapering off of showers and snow showers as the
upper trough lingers over the Pacific Northwest and shortwave
disturbances move onshore. Snow levels are forecast to remain at
around 3000 to 3500 feet but they will be reliant on the
track/southward extent of the upper trough as it makes its move
eastward. The highest probability for widespread shower and snow
shower activity will be in the morning but there may not be much
diminishing through the day into the evening. Another 1 to 5 inches
of snow is currently forecast for Saturday through Saturday night
with highest amounts in the northern portion of the southern Oregon
Cascades near Diamond Lake and Crater Lake.

At the start of the new week, confidence in the details of the
forecast drops another notch or two. But, there is general model
agreement on weak shortwave ridging over central and southern
California on Sunday for a relative break between systems.
Additional disturbances will likely be producing precipitation north
of our area on Sunday and there is a chance of light showers in our
area, with the highest probability near the coast and across Douglas
County.

Monday through Wednesday...Another batch of frontal systems in an
active westerly flow aloft are likely to affect the area from
northern California northward into Canada. The impact of these
systems is expected to become clearer with future model runs. For
now, they can be characterized as most likely at around normal snow
levels (typical higher passes affected by snow but not west side
valleys), and wetter than normal with a possibility of heavy coastal
rain. -DW

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 4 PM PST Thursday for ORZ029-030.
     High Wind Watch from Thursday afternoon through late Thursday
     night for ORZ030-031.
     Winter Weather Advisory until 9 AM PST Thursday for ORZ023>026.
     Winter Weather Advisory until 1 PM PST Thursday for ORZ027.
     Winter Weather Advisory until 1 PM PST Thursday for ORZ028.

CA...High Wind Watch from Thursday afternoon through late Thursday
     night for CAZ085.
     Winter Weather Advisory until 1 PM PST Thursday for CAZ082.
     Ice Storm Warning until 9 AM PST Thursday for CAZ080.
     Winter Weather Advisory until 1 PM PST Thursday for CAZ080-081.
     Winter Weather Advisory until 1 PM PST Thursday for CAZ083-084.
     Winter Weather Advisory until 10 AM PST Thursday for CAZ085.

Pacific Coastal Waters...Small Craft Advisory from 4 AM Thursday to
4 PM PST Friday for      PZZ350-356-370-376.      Hazardous Seas
Watch from Friday afternoon through Saturday      afternoon for
PZZ350-356-370-376.      Gale Warning from 7 AM to 4 PM PST Thursday
for      PZZ350-356-370-376.      Small Craft Advisory for hazardous
seas from 4 PM Thursday to 4      PM PST Saturday for PZZ356.
Hazardous Seas Warning from 4 PM Thursday to 4 PM PST Friday for
 PZZ350-370.

$$

NSK/DW/NSK



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