Small Craft Advisory 104, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
1057 PM PDT SAT OCT 22 2016

The Marine and Aviation sections have been updated...

.UPDATE...
Minor adjustments were made to the Coastal Waters Forecast to
increase west swell tonight through tomorrow morning, consistent
with the ongoing Small Craft Advisories across the coastal waters.

&&

.DISCUSSION...
A weakening frontal boundary approaching the coast this evening
has yet to result in measurable rain at reporting stations in the
coastal counties, except at the climatological wet spot of
Charlotte Ridge, where 0.01" has been observed in the past hour.
Infrared satellite imagery and radar do indicate more rainfall
about to move inland, so the forecast remains on track.

This first front will weaken and stall early Sunday before lifting
westward as a warm front in the coastal waters Sunday afternoon.
For much of the area it will take until late Sunday into Monday
for winds to pick up and precipitation to move in.

Today's model guidance has trended very notably lower with snow
levels in the Mount Shasta area, especially for Monday morning.
This development will be analyzed further overnight, but suffice
it to say that there could be some travel impacts from wet snow on
Highway 89 at Snowmans Summit on Monday. BTL

&&

.AVIATION...23/06Z TAF CYCLE...
Broken to overcast high clouds tonight are expected to keep inland
areas VFR. A weakening cold front moving onto the coast will lead to
some MVFR along and near the coast and in the coastal waters this
evening through Sunday morning and partial terrain obscurations in
the Coastal Mountains. The front will then drift back northwestward
as a warm front on Sunday, so conditions are expected to improve to
VFR. South and southeast winds will increase over land areas Sunday
afternoon and evening leading to low level turbulence in much of the
area. BTL

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 900 PM PDT Saturday, October 22, 2016...
High and steep seas will continue tonight as elevated west swell
continues. South winds increasing ahead of an approaching cold front
will weaken slightly overnight as this first front stalls and
weakens. A period of gale force winds and very steep seas is then
expected Sunday evening through early Tuesday as a stronger cold
front approaches and then moves through. Heavy rain and a slight
chance of thunderstorms is also expected with this strong front
Monday. BTL

&&

.PREV DISCUSSION... /Issued 246 PM PDT SAT OCT 22 2016/

DISCUSSION...

Short Term...Tonight through Tuesday Night...A denser overcast of
mid to high level clouds has kept much of the area out of the sun
today, and this has resulted in a cooler day than forecast, but
not by a significant amount. However, these same clouds could also
keep the region slightly warmer tonight, should they stick around
long enough. Models do show the high clouds thinning and moving to
the northeast through this evening, but satellite imagery shows
only a hint of this upstream, so suspect that most clouds will
remain, and temperatures should not be as cool tonight as last
night.

Tonight, the very edge of a weak and dissipating front will skirt
the coastline, with some light showers possible along the
immediate coast and into the coastal ranges, but not very far
inland. The boundary is then expected to slowly move back offshore
Sunday morning into the afternoon. This is a significant change
from the past few runs, which stalled the front over the West Side
for a day or so. Now, with the model solutions trending west, the
bulk of the area should remain dry through Sunday, aside from the
aforementioned coast.

The upper level trough, up until this time happy to meander just
offshore of British Columbia, will finally dig south Sunday
evening and overnight, reinforcing the front and pushing it
onshore Monday morning. Precipitation will spread from the coast
Monday morning, through to the Cascades by the afternoon, then
across the East Side during the evening and overnight. Rainfall
amounts with this front will be typical of our usual wet season
fronts, while gusty winds will be likely at the usual trouble
spots, including along Interstate 5 near Weed in the Shasta
Valley, along Highway 31 near Paisley on the East Side, and
perhaps even into the valley near Ashland. Wind Advisories may be
needed for some or all of these areas.

While generally, snow levels will remain well above our higher
pass levels, there is some concern for the Mount Shasta area,
where upslope dynamics will likely force snow levels down much
further than the rest of the forecast area. The usual signals for
a heavy wet snow event in this area are marginal at best, and do
not suggest snow levels down into Mount Shasta City, they do show
a healthy mix at Black Butte Summit on Interstate 5 and a full
transition to snow at Snowman's Summit along Highway 89. If snow
does make it to these levels, it should only be for a few hours at
most on Monday morning, but this could occur during the morning
commute. While confidence is not high, and snow should melt
quickly due to warm road surfaces, travelers in the Mount Shasta
area should be ready to deal with some wintry weather along the
passes, just in case.

Showers will then linger Monday night into Tuesday, before the
next front arrives midweek. Temperatures through the week should
hover near to slightly below normal for this time of year. -BPN

LONG TERM...Wednesday through Saturday...A long-wave upper
trough remains between roughly 130W and 150W throughout the
extended period. A significant spoke of energy rotates around the
base of the trough and up along our coast on Wednesday. The
associated surface cyclone is expected to track south to north
along or just west of 130W. While much depends on the track of
this low, models are in really good agreement that we'll have at
least gales over our outer waters, so winds in our forecast were
raised. It is a little too far out to have enough confidence to
issue any wind headlines for it, so we'll just keep an eye on it.

From Thursday on, there has been a shift in the ECMWF which
complicates the forecast. Energy feeding into the long wave trough
is expected to form a closed low off California, but the 12Z ECMWF
is now much more progressive with it. The GFS holds the low off
California with rain likely confined to our coast Thursday. The
ECMWF now moves it northeast much more quickly and would have
widespread rain for us Thursday. The Canadian has a weaker wave as
opposed to a closed low, but its rainfall pattern is much more
like the GFS. Given these complications, we've chosen not make an
major changes from Thursday on. Any solution is likely wet for the
coast, but solutions vary from wet to dry inland. We've kept pops
at or above climo with the highest at the coast. -Wright

&&

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

CA...None.

Pacific Coastal Waters...
- Gale Watch from Sunday evening through Tuesday morning for
  PZZ350-356-370-376.
- Small Craft Advisory until 5 AM PDT Sunday for PZZ350-356-370-376.
- Small Craft Advisory for hazardous seas until 5 AM PDT Sunday
  for PZZ350-356-370-376.

BTL/BPN/Sven