Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 150333
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
733 PM PST Sat Dec 14 2019

.UPDATE...
.Updated Marine and Aviation sections...
The only changes to this evenings forecast was to add an additional
small, broad amount of snow coverage along the Siskiyous and Cascades
near Klamath falls. On radar, the most recent bands have been falling
apart after moving over Coos County, but a few isolated showers are
pushing further inland causing this change to be made.

The surface low that is responsible for this precipitation is visible
on IR and the nighttime RBG satellite products. The low is just offshore
of Florence and will continue to move onshore through the night with
slight southerly movement. This will shift any linger showers south,
mainly near the Oregon, California border.

Models are trending down cloud coverage for tomorrow afternoon and
more places may be likely to get some much desired sunlight. With
this, the high temps will likely need to be nudged up a few degrees.
However, this break in cloud cover will be fairly brief, as
another weak front will graze the coast Tomorrow night. More
information on this and the active weather pattern following it
can be found in this afternoons discussion below. -Miles

&&

.PREV DISCUSSION... /Issued 159 PM PST Sat Dec 14 2019/

DISCUSSION...The latest visible image clearly shows an upper
through moving in from the northwest. Precipitation ahead of it is
already reaching the coast with Brookings and Gold beach reporting
rain. Precipitation will continue to push inland early this evening
with most centered along the coast, coastal mountains and northwest
California. A weak surface low will follow the front late this
evening and tonight. The models are in good agreement tracking the
low through southwest Oregon and northwest California.

Snow levels this afternoon are expected to be around 3000 feet, then
lowering to around 2500 feet tonight, but road snow accumulations
are not expected to be a concern because were not expecting a
significant amount of precipitation. For example up to an inch is
expected over Siskiyou Summit, Lake of the Woods and Diamond Lake
area from late this afternoon into early Sunday morning.


Sunday, there`s some hint as warm frontal precipitation over the
northwest part of the forecast area in the afternoon through Sunday
night. However at the same time 500 mb heights will be on the rise
as upper ridging builds in. For now kept in a slight chance to
chance pops of the above mentioned area.

Monday will be dry as shortwave ridging builds into the area. If
there`s enough clearing, we could be dealing with low clouds and
patchy to areas of fog later in the evening through the morning
hours Sunday night through Tuesday.

The upper ridge axis shifts east Monday night into Tuesday as an
upper trough approaches from the west. Models vary on the evolution
and details, but the general consensus is for the upper trough to
split which will limit the amount of precipitation. Most of Tuesday
will be dry, but could not rule out a chance of rain along the
coast late in the afternoon. -Petrucelli

EXTENDED DISCUSSION...12Z Wednesday through 12Z Sunday.

Heading into Wednesday, the models are starting to come into
consensus with a low strengthening in the Pacific and with high
pressure building off the southern California coastline.  This will
set the stage for a strong atmospheric river (AR) hitting southern
Oregon from roughly Thursday into perhaps Saturday.  Given how low
the rivers will be before the AR arrives, this AR will be more of a
beneficial AR for building snow pack and avoiding serious flooding.
At this time, some points on the Coquille river are projected to hit
action stage, yet avoid flood. However, we are still quite far out
from this event, so flooding still remains possible, especially on
the Coquille river. Many of the rivers will likely experience some
rapid rises between the 20th and 21st.

As for building on the snow pack, we`re expecting snow levels around
5000  to 6000 feet for most of this event.  This will result in some
travel concerns over the Cascade passes, specifically around Diamond
Lake, Crater Lake and elevations above 6000 feet.

Eventually, this low will probably become more progressive and start
to deepen and shift eastwards, this would be a good thing.  The AR
would then probably take aim towards Canada and Washington with the
heavy precipitation shifting northwards.  The other scenario is we
stay at the receiving end of this AR and still see some significant
precipitation well into Saturday.  It`s still too uncertain to say
at this point what exactly will happen Saturday and beyond.

Overall, the potential for minor flooding along the Coquille remains
a concern along with travel above 5000 to 6000 feet from Thursday
into Saturday.

-Smith

AVIATION...For the 15/00Z TAFs...Rain will start to valley at most
terminals this evening.  Look for patchy fog and perhaps some LIFR
visibilities later tonight in valleys west of the Cascades.  East of
the Cascades,  we`ll see some snow reduce visibilities to IFR
thresholds.  Eventually skies should clear up near the end of the
TAF period on Sunday afternoon.

-Smith

MARINE...Updated 730 PM PST Saturday, 14 December 2019...Heavy
and steep long period swell will remain high into Sunday, with
winds diminishing this evening and overnight. Seas will continue
to diminish tonight into Sunday, finally subsiding below advisory
criteria late Sunday into Monday. This will be short-lived,
however, as another swell train moves into the waters Tuesday
ahead of the next frontal system. This front will bring increasing
southerly winds Tuesday night and Wednesday, and with building
seas, conditions will likely become hazardous to small craft. BR-y

&&

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

CA...None.

Pacific Coastal Waters...Small Craft Advisory until 10 PM PST Sunday for
     PZZ350-356-370-376.

$$

MB/MB/MNF


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