Small Craft Advisory 118, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
1040 PM PST Sun Nov 20 2016

.DISCUSSION...Relatively quiet weather is expected through the
next 36 hours or so compared with the cold and wet weather
pattern that is expected late Tuesday through the weekend.

Some snow showers are expected in the mountains tonight and Monday
morning as an upper trough axis swings through, but accumulations
should generally be a few inches or less. Temperatures tomorrow
should be near normal under partly to mostly cloudy skies. Any
showers will diminish by the late afternoon.

No significant updates were needed this evening. We are now focusing
on holiday travel concerns on Wednesday, and moreso from Thursday
through Sunday due to a series of cold and wet storms affecting
Northern California and Southern Oregon. Please see the previous
discussion below for more details on the upcoming active weather
pattern.

&&

.AVIATION...21/06Z TAF cycle...Showers will continue to affect west
side terminals through the morning. Early Monday morning will
feature the most widespread shower coverage west of the Cascades.
Conditions will be generally VFR, but brief MVFR to IFR conditions
will be possible in showers. Similar conditions are expected east of
the Cascades, however, shower coverage will be much less. Moist,
westerly, upslope flow will keep the mountains obscured through
Monday morning. Showers will diminish with mostly VFR conditions
expected area-wide by Monday afternoon. -Wright/SK

Note: The ceilometer at FAA site KOTH is out of service, so there
will be no ceiling observations available generally between the
hours of 04Z and 14Z.

&&

.MARINE...Updated 800 PM PST Sunday 20 November 2016... Winds will
remain light and seas continue to diminish tonight through Monday.
Another front will bring south gales and very steep seas Tuesday.
The strength of the front and occurrence of gales is fairly certain,
but there remains some question as to the exact timing of the
strongest winds. While widespread gales are possible, it is likely
that the strongest winds occur prior to the frontal passage,
especially along the coast as a coastal jet forms. The stormy
pattern continues for the foreseeable future with cold fronts
roughly every other day all the way through next weekend. In
addition, this active storm track will produce large, powerful
swells at times, especially Wednesday through Friday. -Wright

&&

.PREV DISCUSSION... /Issued 304 PM PST Sun Nov 20 2016/

SHORT TERM...An upper level trough is centered over the Pacific
Northwest coast bringing a showery pattern to the area. A
shortwave located about 250 miles west of the Southern Oregon
coast will track inland tonight bringing additional showers to the
area. On Monday, the upper trough will moves inland and gradually
east of the area. A shortwave ridge will bring a brief break in
the showers Monday evening into Monday night. Then another Pacific
Low will move into the southern Gulf of Alaska and bring a front
into area on Tuesday. A strong upper level trough will bring
numerous showers to the area Tuesday night and Wednesday as well
as a chance for thunderstorms to the coastal waters and coast.

This evening expect continued numerous to scattered showers over
the area. Precipitation in showers is expected to mainly be light,
although some moderate showers are possible near and just inland
of the coast. Weak instability will also bring a continued slight
chance for thunderstorms near and just inland of the coast through
early this evening.

Tonight, expect additional showers to move into the area as a
shortwave moves around the trough and inland. These showers will
mainly affect areas from the Cascades west with only isolated to
scattered activity east of the Cascades. Snow levels around 5000
feet today will lower slightly to 4000 to 4500 feet tonight. So
some light snow showers are possible on the higher passes.

Monday, as the upper trough moves overhead, expect lingering
showers with cool and cloudy conditions. Showers will taper off in
the evening as the trough moves east. Light winds and stable
conditions will result in areas of valley fog Monday night into
Tuesday morning across inland valleys, mainly west of the
Cascades.

On Tuesday, another strong front is expected to reach the coast
and move inland in the afternoon over western areas then east of
the Cascades by Tuesday evening. This front will bring moderate
precipitation to the area. Snow levels are forecast near 5000 feet
elevation on Tuesday but will lower Tuesday night behind the front
in showers. Additionally, as very cold -32 deg C air aloft at 500
mb, steep lapse rates, and instability move into the coastal
waters and coast, expect a chance for thunderstorms Tuesday night
into Wednesday morning. Have added thunderstorms to the forecast
for this period. Will need to monitor model trends for areas just
inland of the coast and may need a mention for thunderstorm
chances into the coastal mountains and possible portions of
Douglas, Josephine and Western Siskiyou County.

Wednesday, a deep upper trough will bring showers and cool
conditions to the area with snow levels down to 3000 to 4000 feet
elevation through the day. This will bring light snow to mountain
passes, including in the Cascades and over Siskiyou Summit. Also
showers may be moderate to heavy at times, especially near the
coast.

Tuesday evening through Wednesday evening, travel impacts ahead
of Thanksgiving holiday will be a concern, especially over the
mountain passes in the Cascades and over Siskiyou Summit. With
the frontal passage Tuesday evening, models support around 2 to 5
inches of snow accumulation above 4500 feet elevation in the
Cascades with the highest amounts near Crater lake and Diamond
Lake. An additional 2 to 5 inches in snow showers is expected in
the Cascades Tuesday night through Wednesday evening with 1 to 3
inches possible near Siskiyou Summit. Light snow showers are also
possible in valleys east of the Cascades and in the Mount Shasta
areas Tuesday night and Wednesday morning. Snow will bring the
potential for slippery roads and hazardous conditions. Travelers
should be prepared to take extra time and care while driving
during this period.

LONG TERM...Thursday through Sunday night.
There is growing confidence that a couple of strong fronts will
have an impact on holiday travel on Thursday and Friday. A few
weaker but possibly colder systems may follow Friday night through
the weekend.

The probability of moderate to locally heavy precipitation has
increased for Thursday as the GFS and ECMWF are in good agreement
that a cold front will reach the coast late Wednesday night then
reach the Cascades around noon on Thanksgiving Day with a snow
level around 4000 feet. Although the models are not in complete
agreement, there is better than usual agreement on the strength of
the front. It is not a stretch to call it a strong front with
gales likely in the coastal waters, 1.50 to 3 inches of rain at
the coast, around 3 to 6 inches of snow in the Cascades and
Siskiyous, 1 to 4 inches of afternoon northern Klamath County
snow, and gusty southwest winds that may approach warning strength
in the southern Shasta Valley and in Lake County.

Beyond Thursday afternoon, there is general agreement on a continued
active pattern. But, model differences quickly develop and become
significant. The GFS solution more substantially weakens the front
on Thursday evening and does not generate nearly as many showers
overnight. However, it is wetter and windier with the next cold
front on Friday. The ECMWF solution is more of a split flow and a
bit closer to westerly aloft while the GFS carves out a deeper
trough and tracks it northeast toward Vancouver Island. These model
differences will affect, wind speeds, snow levels, and precipitation
amounts. We have gone with a model blended solution with a continued
high probability of precipitation Thursday night through Friday and
snow levels around 3500 to 4500 feet.

A broad longwave upper level trough is likely to persist Friday
night through the weekend. Lighter precipitation is indicated for
Friday night then it is the ECMWF which has the stronger solution
for another cold front Saturday into Saturday night. Model
differences finally become extreme on Day 7/Sunday with the GFS with
another relatively strong front while the ECMWF has a colder pattern
of light shortwave created rain and snow showers.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...None.

CA...None.

Pacific Coastal Waters...Gale Watch from late Monday night through
Tuesday afternoon for      PZZ350-356-370-376.

NSK/DW/TRW