Winter Storm Watch 8, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
237 PM PST Tue Dec 6 2016

.DISCUSSION...The shortwave trough that brought lower elevation snow
is moving east of the area and showers will be on the decrease the
rest of the afternoon. It will be dry, but cold tonight with the
coldest temperatures so far this year over most locations. Despite
the dry conditions icing will be a concern on any wet roads later
this evening and tonight. Any we roads on elevated surfaces;
bridges and sun sheltered areas will especially be at risk for
ice. Therefore anyone traveling later this evening and tonight
need to be mindful of this.

Clouds will dissipate shortly after sunset with clear skies for most
of the night. High and mid level clouds ahead of a warm front will
start to move into southwest oregon later tonight, but will probably
have minimal or no impact with respect to cooling temperatures. In
fact low temperatures in the single digits or low teens will be
common east of the Cascades; low to mid 20s west of the Cascades and
upper 20s to lower 30s near and at the coast. Therefore the freeze
warning for the Umpqua Basin and Coos County and frost advisory for
the coast will remain in effect. -Petrucelli

Our next time period of concern will be late Wednesday afternoon or
evening through Thursday afternoon. The models show a warm front
approaching the area Wednesday evening and night. Models continue to
show the threat of freezing rain and/or sleet as the warm front
passes through the area and snow changes over to rain. There have
been some changes in the timing of this progression, pushing back
the onset of precipitation by about three hours or so, but the
models have come to good agreement, finally, with the nature of the
pattern, as well as the timing of the frontal passage. Generally
speaking, some of the coldest air of the season will still be in
place as the warm front approaches, and the warm front will overrun
the area, producing light precipitation and a narrow belt of above
freezing temperatures above the surface. Therefore, there will be
areas where any precipitation that does fall will melt, then drop to
the surface where temperatures remain below freezing.

Precipitation type forecasting is notoriously tricky, and relies on
differences of only a degree or two over periods of only an hour or
two. This places a large degree of uncertainty into the forecast.
However, after checking model soundings and temperature profiles
across the area, there is moderate to high confidence that icy
precipitation will fall in our area. There is much less confidence
in the exact timing or locations. Given the high threat of dangerous
travel due to ice, have opted to issue a Winter Storm Watch for the
areas that appear most likely to receive frozen precipitation. This
area includes many heavily travel routes, including Interstate 5,
highways 97, 140 and 138, and many other area roads. For more
details on the event, see the winter weather message PDXWSWMFR.
-BPN

Snow levels are expected to be between 1500 and 2000 feet as the
bulk of the overrunning precipitation arrives (late Wednesday
afternoon) Therefore a rain/wet snow mix is possible near the valley
floors in the Rogue and Illinois Valley and portions of the Umpqua
Basin Wednesday evening, then will rise to between 3500 and 4000
feet Wednesday night as the warmer air aloft moves overhead. Despite
the lower snow levels early Wednesday evening, confidence remains
low on snow accumulating on roads. This is because there may be
enough of an opportunity for roads in the lower elevations to warm
up enough to prevent or limit accumulating snow.

East of the Cascades snow levels will be down to the valley floors
and it will take longer for the warmer air mass to move in (probably
not until late Thursday morning or afternoon depending on the timing
of the front). Therefor there is a greater opportunity for higher
snow accumulations. Meanwhile were expecting all snow along the
Cascades and this is where were expecting most of the accumulating
snow to be at. For example Crater Lake could get up to a foot;
Diamond Lake: 5-10 inches and Lake of the Woods: 4-8 inches from
Wednesday evening through Thursday morning. -Petrucelli

.Winds...Strong gusty south to southeast winds are possible in
the Southern Shasta Valley late Wednesday night through Thursday
afternoon as a frontal system brings strong winds aloft with an
increasing surface pressure gradients. Winds aloft have a west-
southwest direction which is less favorable than a southerly
direction for strong winds. However, the surface pressure gradient
is forecast to increase to a round 9 mb between Medford and Redding
and with the winds aloft, supports the potential for strong winds in
the southern Shasta Valley. Strong southwest winds are also possible
in the Summer Lake area and over the higher mountains near Summer
Rim as well as in the Warner Mountains on Thursday. Winds will
increase for these eastern areas Thursday morning and may become
strong Thursday afternoon through Thursday night. Models show good
agreement with a 55 to 65 kt southwesterly 700 mb jet moving into
this area Thursday afternoon and evening. Of note, the NAM keeps
stronger winds in place Thursday night into Friday morning. High
wind watches are in effect for the Shasta Valley, Summer Lake areas
and the eastern mountains. Please see the NPWMFR for details. Gusty
winds are also likely to develop over the mountains late Wednesday
night into Thursday. Also may see some gusty southeast winds develop
in the southern Rogue Valley during this period, mainly in the
Talent to Ashland area with winds gusting to around 35 to 40 mph. /CC

An active weather pattern is in the cards from Friday through at
least the early part of next week with a strong 100+ mph jet over
our region. Friday into Friday night, we can expect valley rain and
mountain snow. We may catch a relative break in the action this
weekend, but a moist fast west flow aloft remains, so it's a good
bet precipitation will be highest west of the Cascades.
-Petrucelli

&&

.AVIATION...06/18Z TAF CYCLE...Widespread mvfr ceilings are expected
this morning into early this afternoon with terrain obscured. Snow
could lower ceilings and visibilities down to ifr at times this
morning at kmfr and klmt. local ifr may also occur at krbg. ceilings
will improve to vfr all sites this afternoon and evening, but areas
of ifr low clouds and fog/freezing fog are likely to develop in many
of the valleys overnight into wednesday morning, including at krbg
and kmfr. klmt may also have some freezing fog patches around
sunrise wednesday. -spilde/bs

&&

.MARINE...Updated 200 PM PST Tuesday 6 December 2016...Steep, choppy
seas consisting mostly of a 10 to 12 foot northwest swell will
continue into the evening then diminish. The small craft advisory
for seas has been extended through this evening. Winds will be
relatively light and offshore through Wednesday as weak high
pressure slides by over land. Late Wednesday, a warm front moves
through and winds shift to the south and increase. Low pressure then
moves by and a cold front approaches Wednesday night and Thursday.
Model consistency remains high that gales will affect a large
portion of the waters, and the gale watches have thus been upgraded
to gale warnings for the period.

Winds become west and diminish Thursday night but are likely to
remain moderate through Friday then lighter over the weekend. A long
period west swell of 15 to 18 feet is expected to arrive Friday then
diminish Saturday. Relatively calm conditions from both wind and sea
are expected Sunday. However, the storm track is likely to become
active again next week. -Wright

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Watch from Thursday afternoon through late Thursday
     night for ORZ030-031.
     Winter Storm Watch from late Wednesday night through Thursday
     afternoon for ORZ029>031.
     Frost Advisory from midnight tonight to 8 AM PST Wednesday for
     ORZ021-022-024.
     Winter Storm Watch from Wednesday evening through Thursday
     morning for ORZ023>026.
     Freeze Warning from 7 PM this evening to 9 AM PST Wednesday for
     ORZ023.
     Winter Storm Watch from Wednesday evening through Thursday
     afternoon for ORZ027-028.
     Freeze Warning from 1 AM to 7 AM PST Wednesday for ORZ021.

CA...High Wind Watch from Thursday afternoon through late Thursday
     night for CAZ085.
     Winter Storm Watch from late Wednesday night through Thursday
     afternoon for CAZ084.
     Winter Storm Watch from Wednesday evening through Thursday
     morning for CAZ080-081.
     Winter Storm Watch from Wednesday evening through Thursday
     afternoon for CAZ083.
     High Wind Watch from late Wednesday night through Thursday
     afternoon for CAZ081.

Pacific Coastal Waters...Small Craft Advisory from 10 PM Wednesday to 4 PM PST Thursday
     for PZZ350-356-370-376.
     Gale Warning from 4 AM to 4 PM PST Thursday for
     PZZ350-356-370-376.
     Small Craft Advisory for hazardous seas until 10 PM PST this
     evening for PZZ350-356-370-376.