High Wind in Oregon, December 18 to 19, 2020

Area Forecast Discussion
National Weather Service Medford OR
820 PM PST Fri Dec 18 2020

.DISCUSSION...A weakening front, depicted by warming cloud tops
in the latest infrared satellite imagery, will move through the
forecast area tonight. The front will bring generally light
precipitation focused from the Cascades westward. Snow levels will
be 4000-5000 feet tonight, and a few inches of snow is expected
in the high terrain. Winds are gusty ahead of this front, mainly
east of the Cascades, gusting to around 50 mph near Summer Lake
along Highway 31. Precipitation will diminish Saturday for much of
the forecast area. We're not expecting widespread fog concerns
because of some high clouds tonight and suboptimal radiational
cooling. That said, we're in the peak of valley low cloud/fog
season, and with recent rainfall, we expect some patchy fog in
valleys tonight.

Please see the previous forecast discussion on rain/mountain
snow chances through the weekend and into next week. Keene

&&

.AVIATION...For the 19/00Z TAFs...At the coast and just offshore.
MVFR conditions are expected to lower to occasional IFR as a
front approaches. Gusty south winds are expected at North Bend,
but were not expecting low level wind shear. There's a chance the
lower conditions could improve to occasional VFR after the front
moves onshore later tonight, but confidence is no high enough to
include improving conditions for the North Bend TAF.

Inland westside, VFR conditions are likely to start, followed by
MVFR, along with mountain obscurations overnight into Saturday
morning. However it could be delayed at Medford until around 12z.

VFR will prevail east of the Cascades as increasing clouds, cooling
temperatures above the boundary layer, and increasing winds greatly
diminish fog and low cloud potential there. -Petrucelli

&&

.MARINE...Updated 200 PM PST Friday 18 Dec 2020...An active
period with elevated seas and periods of gusty winds will continue
into early next week.

Winds and seas will increase as another cold front moves across the
coastal waters this afternoon and evening.  South winds will
increase with the winds gusting to gale force north of  Cape Blanco
and a mix of steep to very steep, choppy seas. To the south, small
craft advisory conditions will continue with steep seas and winds of
20 to 30 kt. Tonight, winds will lower but westerly  swell will
build across the waters resulting in continued steep seas.

A  front will move into the waters Saturday and Sunday. Models are
in better agreement, showing the front move into northern portions
of the waters Saturday and Saturday night then lifting northward on
Sunday. South winds are expected to increase across the waters north
of Cape Blanco on Saturday to at least small craft advisory levels.
Otherwise, the mix of swell and wind waves will result in continued
steep seas across the waters Saturday into Sunday. Of note, in the
area of the strongest winds from Bandon northward may produce very
steep wind-driven seas mixed with the increased swell on Saturday
night. Confidence in this is beginning to increase, but would like
to wait for the current gales to end before issuing any watches.
Please note that a hazardous seas watch may be issued later this
evening with the next set of eyes looking at, but we will otherwise
continue to monitor the situation.

Sunday, winds are expected to lower as the front gradually moves
north. However. steep west swell is expected to continue to affect
the waters. Monday, another front arrives and is expected to bring
additional small craft advisory level winds and seas to the waters.
Winds are expected to change to northerly Monday night and Tuesday.
Steep, west swell-dominated seas may linger into Tuesday before
gradually lowering. However, gusty northerly winds may develop
across the waters south of Cape Blanco late Tuesday through
Wednesday resulting in the potential for steep, choppy seas
affecting these waters. -Schaaf/CC

&&

.PREV DISCUSSION... /Issued 237 PM PST Fri Dec 18 2020/

DISCUSSION...

Short Term...Tonight through Monday Night...Mid and high
level clouds are streaming in overhead this afternoon and
evening, the first signs of an approaching frontal system that
lies just offshore. The front will move onshore later this evening
and tonight, although it will be one of our weaker systems, and
other than some increased winds and a chance for light rain along
and west of the Cascades, there will be little to show for its
passage. One issue of note, however, will be winds on the East
Side, particularly near the Winter Rim/Summer Lake area. Model
guidance is showing 700mb winds of 50 to 60 kts, which typically
signify the potential for strong winds along Highway 31 between
Paisley and Summer Lake. With this in mind, and with some of our
trusted guidance suggesting wind gusts up to 50 mph, a Wind
Advisory has been issued for that area for this evening and
tonight. See the weather message at PDXNPWMFR for more
information.

Most of the precipitation will end by Saturday morning, but moist
onshore flow will keep a chance for showers lingering along the
coast and along the Cascades north of Crater Lake through
Saturday afternoon. By Saturday evening, precipitation chances
will increase again as a system begins pushing moisture into the
Pacific Northwest. This Atmospheric River system, containing very
moisture rich air flowing directly into the coastline, will set
up to our north, with the main axis of rain wavering north to
south between roughly Astoria and Florence. This will keep the
bulk of the rain to the north of the forecast area, but that's
not to say that we won't see plenty of precipitation chances in
southern Oregon (although northern California does appear to be
mostly left out of this event). Most rain will be centered along
the coast, the coastal ranges, the Umpqua Basin, and up into the
Cascades and Cascade foothills. There does remain some uncertainty
regarding how far south the moisture plume will waver, and should
it make a run farther south, we could see a significant increase
in rainfall rates and amounts, but this seems a more unlikely
scenario. This northerly track will also produce warmer
temperatures and should push snow levels much higher, perhaps to
around 9000 feet or more, thus all precipitation should fall as
rain, with snow expected only over the highest peaks.

The moisture plume will waver to our north through Monday
morning, until a trough arrives from the Pacific and pushes it
through the area Monday night as a cold front. This is when the
entire forecast area can expect some precipitation, but the front
will move through quickly, so amounts will still remain on the
lighter side. Meanwhile, snow levels will drop from around 6000 ft
Monday afternoon to as low as 3000 feet by the time precipitation
ends Monday night, which could lead to some winter impacts over
the area passes in the evening and overnight. -BPN

Long Term...Tuesday through Friday Night... Overall, the
ensemble collective appears to have trended a bit cooler for this
time period in the last 24 hours, more toward the retention and
building of mountain snowpack above 6kft. More specifically, the
GEFS has trended more toward the European Ensemble mean.

One weather system will be exiting the area Tuesday with some
lingering shower activity expected in the morning. Snow levels are
expected to be in the 2500 to 3500 foot range with precipitation
lingering longest over northwestern portions of the forecast area.

We'll then catch a break in the weather until sometime on Christmas
Day per the guidance. While the majority of the model guidance
suggests precipitation won't arrive over inland portions of the
forecast area until the afternoon hours, some suggest that
precipitation could come in earlier, resulting in the potential for
a rain/snow mix into some of the west side valleys down to around
the 1,000 to 1,500 foot range. Once the inversions mix out in the
afternoon, snow levels are expected to be in the 4kft to 5kft range
before they fall back to around 3500 feet on the morning of the
26th. Altogether, this weather system looks moderate to somewhat
strong, in that rain amounts are looking like they'll be over an
inch along the coast and in the coastal mountains and inland
mountains will likely pick up several inches to possibly up around a
foot of snow. As usual, the highest amounts are likely to be in the
Crater Lake NP area. Therefore, some advisories may be necessary.

Ensemble models then suggest near to slightly cooler than
normal temperatures with generally lighter frontal system potential
over the area, though the GEFS does have a modest system in SW flow
around the 27th. ~BTL

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory until 4 AM PST Saturday for ORZ031.

CA...None.

Pacific Coastal Waters...Small Craft Advisory until 10 PM PST Sunday for
     PZZ350-356-370-376.
     Gale Warning until 9 PM PST this evening for PZZ350-370.