Small Craft Advisory 120, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
930 PM PST Tue Nov 22 2016

.DISCUSSION...An update to the forecast is not necessary. The wind
advisory for central Siskiyou County has expired while the wind
advisory for the higher terrain of the east side and portions of
highway 31 in Lake County continues for another 4 hours. That is
also roughly the expected duration of the moderate intensity snow
over the Cascades and Siskiyou Mountains. The winter weather
advisory above 4000 feet remains in effect through 10 AM PST with
snow tapering to frequent light snow showers overnight into the
morning. While significant accumulation is expected above 4000
feet, an accumulation of snow is possible as low as 3000 feet.
After 10 AM On Wednesday, light showers and snow showers will
diminish in number into the afternoon with not much more than a
dusting of additional snow above 3500 to 4000 feet.

There remains uncertainty in the forecast for Thanksgiving Day
through the weekend into early next week. The 00Z GFS has arrived
and continues to have significant differences with the 12Z ECMWF
and 00Z NAM. The biggest concerns are for high winds and heavy
rain at the coast with each model showing a strong front near or
at the coast. The GFS and NAM paint a much more favorable picture
than the ECMWF for travel in the vicinity of Lake of the Woods,
Diamond Lake, and Crater Lake with the front remaining stalled at
the coast.

The early Thursday into early Saturday forecast will be a focus
for the night shift with heavy coastal rain, strong winds, and
mountain and east side snow possibly making a significant impact.
Please see the previous discussion below for further information.

&&

.AVIATION...23/00Z TAF cycle...Areas of MVFR/IFR cigs/vis will move
into the Umpqua Basin tonight as rain increases. With light wind
near the ground, low level wind shear will be an issue at all 4 of
our TAF locations as short term guidance shows 30-35 knot winds
around 2000 feet AGL. Rain will spread into the Rogue Valley this
evening but MVFR cigs are not expected at KMFR until around 06Z and
at KLMT until around midnight local time. /FB

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

&&

.MARINE...Updated 200 PM PST Tuesday, 22 November 2016...Very stormy
conditions will continue for the PacNW coastal waters, with several
strong frontal systems passing through the area one after another.
There appears to be no significant time period in the next 7 days in
which conditions would decrease below small craft advisory levels
either from wind, wind waves, swell, or some combination thereof.

Widespread gales and very steep seas will continue through this
afternoon then weakens overnight and Wednesday. The front will also
bring with it the possibility of thunderstorms and a 3 to 6 hour
period of heavy rainfall. Winds will diminish as the become west-
southwesterly tonight, but incoming fresh swell will keep seas
elevated through Wednesday.

The next front arrives Wednesday night into Thursday morning, and
appears as if it will be stronger than Tuesday's front. We have
issued a gale watch from Wednesday evening through Thursday night
with storm force winds certainly possible, especially in the coastal
jet near Cape Blanco. In addition to the winds and seas, a
background swell around 20 feet may push combined seas up to 25
feet.

Another very strong front will arrive later Thursday into Friday.
Models are indicating the potential for deepening low pressure along
this front as it approaches the coast, which is an ingredient for
very strong wind storms around here. Confidence in the low pressure
(and intense winds) is still low, but this is something to keep an
eye on. There will likely be a day or so break after this front,
then another system is due on Saturday. /FB

&&

.PREV DISCUSSION... /Issued 312 PM PST Tue Nov 22 2016/

SHORT TERM...Precipitation will continue to spread inland this
afternoon and evening. Snow levels ahead of the front around
4500-5500 feet will drop significantly as the front moves through
this evening, reaching as low as 3000 feet by Wednesday morning.
This will bring snow to the major passes near Lake of the Woods,
Siskiyou Summit and Black Butte/Snowman's Summit near Mount Shasta
City. The best snowfall rates (up to 1"/hr) will likely accompany
the frontal passage this evening in the mountains from about 5 to
10 pm, then snow showers will continue behind it. Winter weather
advisories are out at WSWMFR. Wind advisories also continue
through late this evening for the Shasta Valley and East Side as
gradients tighten with the front moving in. Details can be viewed
at NPWMFR.

Thanksgiving Day through Friday night, Nov 24-25th, 2016...

WINDS, RAIN AND FLOOD POTENTIAL...
Models waver a bit with the details regarding the next storm
system expected to arrive at the coast late Wednesday night, then
move through Thursday into Friday. Models continue to show parent
low pressure offshore of the Queen Charlotte Islands Thursday with
another low developing and deepening rapidly offshore of the
Washington coast by Thursday evening. This would slow the front
down and keep much of the precipitation confined to areas west of
the Cascades through Thursday night. Then, another wave is
expected to develop that will ultimately push the front onshore
Thursday night and then inland Friday. We have favored the 12z
ECMWF/SREF and their depictions of this scenario. This will bring
quite a bit of rain to coastal areas through Friday, which will
result in, at the very least, rapid rises on area creeks, streams
and rivers. We have issued a Hydrologic Outlook at ESFMFR to
highlight rain amounts, the quick rises and the potential for
flooding. Areal flood watches may be necessary soon, but there is
enough uncertainty in the guidance to hold off for now. Winds are
also expected to increase significantly ahead of this system, so
we have issued a high wind watch along the coast late Wednesday
through Thursday night. Local guidance suggests potential wind
gusts to 60 mph. Additional wind headlines may be necessary inland
as well Thursday into Friday. -Spilde

FLOOD POTENTIAL, SNOW...
Confidence remains high for a heavy round of rainfall along and
near the coast during this time period that could result in minor
flooding. Confidence is moderate to high for a period of moderate
to heavy mountain snowfall during this time period for elevations
above 4.5kft that could result in moderate travel impacts.
Confidence is currently moderate on timing of the heaviest
precipitation for inland areas, though it continues to be most
likely to be on Friday.

In summary, we expect that some flooding, even on the main stems
of the Coquille and Chetco Rivers, will be possible Friday into
Sunday in the coastal counties. Current river forecasts, as of
this afternoon, indicate moderate flooding on the South Fork of
the Coquille late Friday and minor flooding on the Coquille at
Coquille on Saturday. We also expect that either a winter storm
watch or winter weather advisories will be needed for elevations
above 4.5kft, and are currently leaning toward a winter storm
watch for that time period (see next paragraph for more info).
Advisories might be needed for elevations between 3.5kft and
4.5kft, but it is at least equally as likely that amounts won't
reach thresholds at these lower elevations.

More specifically, a cold front in the coastal waters early
Thanksgiving Day morning will be accompanied by large swells around
20 feet. The usually favored ECMWF and some SREF members push this
front inland across the West Side and into the Oregon Cascades, but
less aggressively than they did 24 hours ago. Light snowfall in
the 3-6 inch range is, therefore, expected in the Crater and
Diamond Lake areas along portions of highways 62 and 138, as well
as along highway 140 west of Klamath Falls with snow levels
3500-4500 feet. Meanwhile, the NAM12 and GFS20 models remain
drier, further northwest with the precipitation along the frontal
boundary. These solutions show much less precipitation in our
Oregon Cascades as southeast winds increase. Based on fairly good
agreement among the GFS Ensemble members and the ECMWF Ensembles
from earlier model runs, the going forecast for Thanksgiving leans
more toward the ECMWF and SREF solutions.

Thursday night through Friday night model guidance is well agreed on
the frontal boundary and its associated atmospheric river
progressing eastward across the area with snow levels generally
between 3500 and 4500 feet. There is better agreement among the
models and ensemble members for this time period, and, therefore,
significant snow appears likely above 4500 feet and for some lower
areas. Impacting snow is expected during this time period over the
mountains passes, to include Interstate 5 at Siskiyou Summit and
near Mount Shasta City, as well as along a portions of Highway 140
west of Klamath Falls and east of Medford, plus the Crater and
Diamond Lake areas, so be advised. If you have to travel that day,
stay tuned for more details as we figure them out. Above 5kft, we
currently expect 10-20 inches during this time period, and 3-8
inches appears possible on the mentioned mountain passes. BTL

LONG TERM...Extended 12z Saturday until 00z Tuesday...

The active weather pattern continues after Saturday as a few
smaller scale systems move through the region. For the Sunday
system, a colder air mass will push into southern Oregon resulting
in snow levels around 3000 feet, so almost low enough for the
lower passes along I-5.

Heading into Monday, A warm front will come down from the north west
resulting in a high probability of precipitation in addition to
raising the snow levels up to 4500 to 5000 feet.  This is by no
means a major system, but it's look like a high probability of
precipitation with some decent rain accumulations inland and on the
east side.

Finally, we get a brief break on Tuesday before a strong and well
formed cold front pushes inland on the 30th. CZS

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory until 1 AM PST Wednesday for ORZ030-031.
     High Wind Watch from late Wednesday night through late Thursday
     night for ORZ021-022.
     Winter Weather Advisory until 10 AM PST Wednesday ABOVE 4500
     FEET for for ORZ027-028.

CA...Wind Advisory until 1 AM PST Wednesday for CAZ085.
     Winter Weather Advisory until 10 AM PST Wednesday ABOVE 4000
     FEET for for CAZ082-083.
     Wind Advisory until 9 PM PST this evening for CAZ081.

Pacific Coastal Waters...Gale Watch from Wednesday evening through
     late Thursday night for PZZ350-356-376.
     Small Craft Advisory for hazardous seas until 7 PM PST Wednesday
     for PZZ350-356-370-376.
     Gale Watch from Wednesday evening through late Thursday night
     for PZZ370.

DW/MAS/FJB

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE MEDFORD OR
1108 PM PST TUE NOV 22 2016

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1100 PM     HEAVY RAIN       2 SSW OBRIEN            42.05N 123.71W
11/22/2016  M1.00 INCH       JOSEPHINE          OR   TRAINED SPOTTER

            . THE DURATION OF THE HEAVY RAIN EVENT WAS 9
            HOURS.

&&

DW