Small Craft Advisory 116, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
941 AM PST Thu Nov 17 2016

.UPDATE...
A few updates were made to the forecast for this morning through
Friday morning. These adjustments were mostly for cloud cover
tonight and to raise forecast lows tonight to the same to several
degrees warmer than were observed last night into this morning.
BTL

&&

.DISCUSSION...
Freezing temperatures were observed across the Scott Valley and
portions of the Illinois and Applegate Valleys this morning.
Additionally, patchy frost likely occured in portions of Douglas
County. Altogether, temperatures this morning across the interior
valleys of the West Side were 2-6 degrees warmer than forecast
due to unexpectedly thick mid-high level clouds that developed and
moved overhead between 4 AM and 8 AM PST from a shortwave riding
the tail end of yesterday's exiting upper level trough. This
prevented frost and/or freeze conditions in some areas. For the
Medford Airport, we are currently experiencing our longest stretch
of days without a low temperature of 36F (frost threshold) or
lower and 32F (freeze threshold) or lower on record going back to
1911. More on this in the afternoon discussion.

Today we'll catch a break in the wet weather over inland areas
ahead of the next frontal system that will arrive late tomorrow
through Saturday. Model guidance is indicating tonight through
Friday morning to be the same to several degrees warmer than was
observed this morning, so no freeze warnings are anticipated for
tonight. Temperatures have been adjusted to reflect this
expectation. BTL

&&

.AVIATION...17/12Z TAF cycle...A stable air mass with plenty of
moisture in the low levels is bringing IFR conditions to some of the
West Side valleys this morning. There are some VFR ceilings east of
the Cascades. All areas should clear to VFR by this afternoon.
Tonight will likely feature more in the way of fog instead of low
clouds, at least for West Side valleys. 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 230 AM PST Thursday 17 November 2016...Moderate
west swell will keep seas steep through today, but there won't be
much in the way of wind. Seas will subside below 10 feet tonight,
but they won't stay down very long because a strong front will move
onshore Friday night. At this point, gale force southerlies and very
steep high seas look likely on Friday, mostly beyond 10NM from shore
except closer to shore near Cape Blanco. More fronts will follow
through the weekend and into next week, and gale force winds may
accompany some of these fronts as well. SK

&&

.PREV DISCUSSION... /Issued 313 AM PST Thu Nov 17 2016/

SHORT TERM...through Saturday...Some showers continue to affect
the waters and immediate coast this morning, and there have even
been a couple of lightning strikes offshore as the tail end of an
upper trough swings through. A weak upper ridge will temporarily
build in behind this departing trough today with showers
diminishing. Freeze warnings and frost advisories remain in effect
for west side valley locations this morning. It looks like these
will verify pretty well as outlying locations such as Cave
Junction have already reached freezing, but cloud cover has
limited the cooling in places such as Medford proper.

As the upper ridge axis slides through tonight, low clouds and fog
are expected again in the valleys. This may be limited somewhat by
high cloud from an approaching system, but satellite shows these
clouds currently thinning as they move into the ridge, so we'll
stick with the going forecast for tonight.

A large trough currently approaching 140W will be our weather
maker for the weekend. It will bring gales to the waters Friday
with rain spreading inland Friday night and Saturday. Breezy to
windy conditions are also expected over the higher terrain and the
east side both Friday and Saturday. Winds in these areas are
generally below high wind criteria right now, but we may need wind
advisories eventually.

The bigger impact may come in the form of snow, especially over
portions of Siskiyou County. Moist, southerly flow develops over
the Sacramento valley Friday night and drives strong upslope into
south central Siskiyou County around Mt. Shasta into Saturday
morning. This often lowers snow levels in the area much lower than
they would be elsewhere, and this is the case this time as well.
Local studies have shown that 1000-700 mb thicknesses at or below
2884 meters, 850 mb temperatures at or below 2C, and strong
southerly flow (700 mb south winds in excess of 50 KT) to be good
indicators of heavy snow down to I-5 around Mt. Shasta City. All
of these parameters are exceeded by a fair amount Friday night
into Saturday morning, and models show upwards of an inch of
liquid falling at the same time. The main question is: will road
surfaces be cold enough to produce a major travel impact? Metro
forecasts don't go out that far yet, but antecedent conditions are
cold, and precipitation rates may be quite high (1-2 inch per hour
snowfall rates) at the peak of the event Saturday morning. All of
this adds up to a decent chance for heavy snow, so we issued a
winter storm watch for the area Friday night and Saturday morning.

We will also see some snow in the Cascades, but not nearly as
heavy as further south. Precipitation chances are also lower in
our Oregon zones Friday and Saturday as strong south flow tends to
dry out a bit by the time it reaches us. We will all probably see
some precipitation from the system eventually, but it doesn't
look like a big deal for northern areas. The exception may be
along the Curry Coast where they could see fairly steady rainfall
adding up to a couple of inches of rain Friday night through
Saturday night. But even this is hardly out of the ordinary for
November.

With the focus on short term concerns, nothing was changed in the
extended forecast, and the previous long term discussion follows
for reference. -Wright

Long Term....Sunday through Wednesday Night...The upper level
trough that was centered off the coast at the end of the short
term will become elongated as it begins to move forward during the
beginning of the extended. The models have trended weaker with the
trough as it moves onshore and therefore have trimmed the winds
back a bit for Sunday. However, it will still be breezy to gusty
in the typical areas such as Summer Lake and higher elevations.
Expect ongoing precipitation to continue into Monday morning as
the trough axis moves overhead.

There will be a lull in precipitation Monday afternoon as a brief
period of ridging moves over the area. This break will be short
lived as the models are showing the next front and upper trough
moving in right on the heels of the weak ridge. There is fairly
good agreement among the models in having rain arrive at the
coast by Tuesday morning. However, confidence in the progression
of this upper trough and associated front after this is low at
this time given the differing solutions beyond this point.

The GFS is slower to move the trough overhead and actually
elongates it offshore, remaining there through Wednesday. The EC
on the other hand, keeps the upper trough trucking along and moves
it over the area by Wednesday. The difference in solutions would
result in differing snow levels and temperatures for Wednesday,
with the EC being the cooler solution of the two. If the EC pans
out, snow levels could be as low as 3500 feet during the day
Wednesday. Compare this to the GFS which is advertising snow
levels around 5000 feet. The cooler solution could result in
travel impacts for the day before Thanksgiving. /BR-y

&&

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

CA...Winter Storm Watch from Friday evening through Saturday morning
     ABOVE 3500 FEET for for CAZ080>083.

Pacific Coastal Waters...
- Gale Watch from Friday morning through late Friday night for
  PZZ350-356-370-376.
- Small Craft Advisory for hazardous seas until 7 PM PST this
  evening for PZZ350-356-370-376.

BTL/NSK/TRW