Winter Weather Advisory 1, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
827 AM PST Fri Jan 1 2021

.DISCUSSION...Satellite imagery shows cloud cover draping across
the entirety of our area ahead of a frontal system that is coming
in. Although radar is showing limited returns near Drain/Elkton,
it is likely not nothing is reaching the ground yet. This will
change as the front moves in and falls apart today. The current
forecast looks well enough on track, so am not going to make any
changes.

The main challenge today will be deciding any winter hazards
along with the other hazards ongoing for the weekend systems,
which appears to be a longer duration event. More information is
detailed nicely in the previous discussion below, and I encourage
you to read it. -Schaaf

&&

.PREV DISCUSSION... /Issued 437 AM PST Fri Jan 1 2021/

DISCUSSION...A series of storms will impact the forecast area through
the next 7+ days, bringing periods of heavy rain, windy
conditions, and mountain snowfall. Much of the rain and snowfall
will be beneficial given the ongoing drought conditions, but there
will be some hydrological hazards and impacts to mountain pass
travel. We've observed over the past few days that the collective
of model data has trended warmer and wetter for our forecast
area. This means higher snow levels, impacting primarily
elevations 4500 feet and higher, and heavier rainfall and more
runoff that will bring significant rises to area rivers and
streams.

Drilling down into some details, a front of moderate strength will
push into the forecast area this afternoon, bringing gusty winds and
moderate to heavy rainfall (heavy rainfall focused at the coast and
especially south of Cape Blanco) and mountain snowfall. Snow levels
will bump up to about 7000 feet this afternoon, and this will likely
be as high as snow levels get for the next week or so. A brief
round of impactful snowfall is expected in the mountains,
generally above 4500 feet, late this afternoon into the evening as
the cold front passes through. Additionally, enough cold air
aloft behind the front will bring Lifted Index values to near zero
late this afternoon and evening, generally sufficient for a few
thunderstorms over the waters or just inland.

Not much of a break is expected in precipitation Saturday morning,
especially in the high terrain, before the next front arrives
Saturday afternoon. This front will feature strong south to north
pressure gradients and 50kt+ southwest winds at 10000ft, and this
means a windy period Saturday afternoon through Sunday night at
least. Both the Canadian/American and European ensemble solutions
suggest Integrated Vapor Transport (good measurement of moisture
pointed at forecast area) values at 500+, and this is top 10
percent for this time of year. These high values linger over the
forecast area from late Saturday through Sunday night as multiple
low pressure centers form and move along the front. There will
likely be two periods of high winds at the coast and portions of
the East Side-- Saturday night and again Sunday night. These will
be timed with the approach of both low pressure centers. Snow
levels through these heavier periods of precipitation will
generally be 5000+ feet, so significant snow impacts should be
above Siskiyou Summit...again, different than the models looked a
few days ago. Lastly, these types of patterns, with deep and
strong southwesterly flow, generally feature breaks in
precipitation and warmer conditions in valleys like the Rogue. We
won't be surprised to see temperatures bump into the 50s in the
Medford area.

Long-term from previous discussion...Mon Jan 4 through Thu Jan 6,
2021...Models and ensembles continue to show that an active and
wet pattern is likely through at least mid next week. A strong and
wet front will move across the area on Monday with rain and
mountain snow expected. Snow levels initially near 5000 to 5500
feet are expected to lower to 4000 to 4500 feet during the day
Monday. Model and ensemble solutions show good agreement and
continue to support the potential for moderate snow accumulations
across higher passes in the Cascades on Monday. Heaviest snow
accumulations are expected above 4500 feet in the Cascades.
Strong, gusty south winds are forecast to continue over the
mountains Sunday night into Monday, gradually lowering during the
afternoon and evening as the front shifts east. Gusty winds may
also continue on Monday for the Summer Lake area and southern
Shasta Valley, lowering in the afternoon and evening.

After a brief break on Tuesday, the active pattern continues Tuesday
night and Wednesday. Models and ensembles show good agreement that
another strong front will move gradually into the area Tuesday night
followed by a surface low on Wednesday. The parent low associated
with the front is expected to move inland into British Columbia but
the GFS, ECMWF and many of the ensemble solutions show a second low
developing to the west and moving into the southern end of the
front, before moving inland into the southern Oregon and northern
California coast on Wednesday. There is low confidence on the exact
track and strength of this low which will be a significant factor
into how much precipitation the area receives and how strong winds
will be. The main concern with this system is that will bring
another round of moderate (and possibly locally heavy)
precipitation. The stronger solutions for the low would also
indicate the potential for strong gusty winds. Snow levels are
forecast to be in the 3500 to 4000 ft range, so with moderate
precipitation there is a chance for significant snow accumulations
to affect most passes in the Cascades and Siskiyous as well as the
mountains in western Siskiyou County.  Areas east of the Cascades
could also see some light to moderate snow with this event. Given
the lower confidence in the track and strength of the low, the
exact details are not certain. We will continue to monitor and
evaluate the potential for impacts with this storm.

In addition to individual storm impact, the long period of moderate
to occasionally heavy precipitation from a train of storms this
weekend  into next week will bring significant precipitation,
especially to coastal areas and the Umpqua Basin. This is forecast
to bring significant rises on rivers,  especially in Curry and Coos
Counties, next week. We have issued a hydrologic outlook for the
potential of significant rises on mainstem rivers as well as the
potential for small stream flooding with these storms. Please see
the ESFMFR for details. -CC

HYDROLOGY...An active and wet pattern containing a strong
atmospheric river will bring widespread rainfall to the region
this weekend into early next week. Small streams and creeks will
likely rise significantly during this active weather pattern. More
rain is expected in the middle of next week, but confidence is
low in the details for where the heaviest rain will fall. This
extended period of wet weather through mid week may result in some
main stem rivers nearing bankfull, particularly the coastal
rivers, including the Chetco and Coquille Rivers, and some of our
more flashy creeks such as Deer Creek near Roseburg.

AVIATION...For the 01/12Z TAFs...A warm front will brush by
this morning, with some light rain and drizzle mainly over the
coastal waters. This warm front is likely to result in a variety of
conditions across the area due to lingering moisture, clouds from
the warm front, and breaks in the cloud cover. There is low
confidence for how much IFR clouds and fog form in the Oregon
western valleys this morning because of competing factors of lots of
low level moisture but with a VFR/MVFR ceilings above. The focus for
any IFR conditions will be between 14z and 18z.

A fairly strong and fast moving cold front is likely to reach KOTH
between 1 and 2 pm, then KRBG around 3 pm, KMFR near or just after 4
pm, and KLMT between 5 and 6 pm. Some wind shear is expected along
and near the coast late this morning into the early afternoon
followed by a mix of MVFR and IFR in moderate to heavy rain as the
front moves through. Mainly VFR conditions inland this afternoon and
evening except for brief MVFR conditions near the frontal passage.
Sandler

MARINE...Updated 420 AM PST Friday, 1 Jan 2020...A new very long
20 to 25 second period swell will build today. Light showers in
southerly flow will increase to moderate to heavy rain this
afternoon as a cold front moves through. Gale force wind gusts are
expected from about Pistol River northward. Westerly swell is likely
to follow the front, peaking around 20 feet at 19 seconds Saturday
morning.

Saturday into Saturday night another strong frontal system will move
through the waters with gales likely and even storm force winds
possible from the Cape Blanco area northward. A gale watch remains
in effect for this front.

Large, hazardous seas in the 15 to 25 foot range are likely from the
3rd into the 6th of January as two to three more strong frontal
systems move through. Model guidance has trended further north and
west with the strengthening low Sunday night into Monday. Strong
gales to low end storm force winds currently appear most likely with
that frontal system.
Sandler/BTL

BEACH HAZARDS...Updated 845 PM PST Thursday, 31 Dec 2020... High
surf is expected to continue for much of the next week as
multiple storm systems move through the region. Breaking wave
heights will subside into today to around 15 feet before a new
long period swell builds them back into the 15 to 25 foot range
this evening through Saturday morning. Breaking wave heights for
this next round of very long period swell are then likely to peak
in the 25 to 30 foot range on the morning of the 2nd.

Additional strong frontal system activity across the northeastern
Pacific from the 3rd through the 7th will bring at least 2 more
large surf events to the area with breaking wave heights peaking at
about 30 feet with each of these events. Currently, the timing of
the peak of those two large breaking wave events is the night of the
3rd and the late afternoon to evening of the 6th of January.

Use extra caution along and near the immediate coast, especially
along and near beaches, rocks, jetties, and bar entrances during
this period of significant waves, as the Pacific is both cold and
powerful. These large waves can easily sweep people and large
objects into the water. ~BTL

HYDROLOGY...An active and wet pattern containing a strong
atmospheric river will bring widespread rainfall to the region
this weekend into early next week. Small streams and creeks will
likely rise significantly during this active weather pattern. More
rain is expected in the middle of next week, but confidence is
low in the details for where the heaviest rain will fall. This
extended period of wet weather through mid week may result in some
main stem rivers nearing bankfull, particularly the coastal
rivers, including the Chetco and Coquille Rivers, and some of our
more flashy creeks such as Deer Creek near Roseburg.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Surf Advisory from 4 PM this afternoon to 4 PM PST Sunday
     for ORZ021-022.
     High Wind Watch from Saturday afternoon through late Sunday
     night for ORZ021-022.
     Wind Advisory from 4 PM this afternoon to 1 AM PST Saturday for
     ORZ031.
     High Wind Watch from Saturday evening through late Sunday night
     for ORZ031.

CA...Wind Advisory from 1 PM this afternoon to 10 PM PST this evening
     for CAZ081.

Pacific Coastal Waters...Gale Watch from Saturday afternoon through late Saturday night
     for PZZ350-356-370-376.
     Small Craft Advisory until 4 PM PST this afternoon for
     PZZ350-356-370-376.
     Gale Warning until 4 PM PST this afternoon for
     PZZ350-356-370-376.
     Hazardous Seas Warning from 4 PM this afternoon to 1 PM PST
     Saturday for PZZ350-356-370-376.