High Wind in Oregon, January 4, 2021

Area Forecast Discussion
National Weather Service Medford OR
951 PM PST Sun Jan 3 2021

Updated AVIATION Section

.DISCUSSION...04/00Z NAM in. 04/00Z GFS in through 132hr.

A warm front is now lifting north through the area. Precipitation
is widespread from the Cascades west, but spotty on the east
side. Winds are also increasing as a cold front approaches the
coast.

The rain, wind, and mountain and east side snow will be the main
issues now through Monday.

First, wind. As the cold front approaches, most ensemble members
are showing surface lows spinning on the frontal boundary.
However, unlike previous runs, none of these lows look to be
particularly strong. Ensembles suggest that the most likely
scenario will have the strongest low moving onshore between
Reedsport and Florence.

The 04/00Z deterministic runs show that the maximum Coos Bay to
Arcata gradient is now -6.8 (NAM at 04/12Z and GFS at 04/09Z).
This gradient will still support a significant storm, but is a
far cry from the much stronger gradients of previous runs.

Gradients are still high between Medford and Redding, with the
04/00Z deterministic runs showing a maximum gradient of -8.7 (NAM
at 04/15Z) or -8.4 (GFS at 04/09Z and 04/15Z) which supports
strong winds in the Shasta Valley, Mt. Ashland, and the Rogue
Valley.

Overall, this wind event looks to be widespread with the
strongest winds continuing to be at the coast and east of the
Cascades with advisory level winds in many other places. Winds
will diminish quickly behind the front. High Wind Warnings and
Wind Advisories are in effect. Details can be found in NPWMFR.

Second, snow. There will be periods of moderate to heavy mountain
snow Monday. Snow levels are currently between 5000 and 6000 feet
across southern Oregon and northern California. Once the front
comes through, the snow levels will quickly drop to around 4000
feet Monday. In addition, because this system is a deepening
trough moving through our area, the southerly flow will further
increase precipitation amounts across the Mt. Shasta area. Heavy
precipitation will also bring down colder air aloft, which can
further lower snow levels in localized areas. With snow levels
being so close to major roads (Highway 89 at Snowmans Summit,
Highway 97, and Interstate 5 at Black Butte Summit), it is
possible that our current snowfall forecast is too low and that
these areas could see 4-8 inches of snow (worst case scenario). It
is also possible that this same effect could occur at Siskiyou
Summit providing a possible 3-6 inches of snow (worst case
scenario) Monday. More likely, the snow levels will remain above
those roadways, except possibly the top of Siskiyou Summit.
Winter Storm Warning and Winter Weather Advisories are in effect.
Details can be found in WSWMFR.

Third, rain. There will be periods of moderate to heavy rainfall
across the southern Oregon coast and Coast Range as well as in
western Siskiyou County in northern California. Rain rates look to
be up to a quarter of an inch an hour, approaching 0.35 inches
per hour. For brief periods of time, rates may approach a half an
inch an hour particularly in the coast range, but only for short
time periods. Overall, there is a 5-10% chance for additional
rock falls, debris flows, or slides across this area, which the
Weather Prediction Center highlighted as a marginal risk for flash
flooding. Despite all the rain, flooding of main stem rivers is
not expected, although some smaller streams may approach bankfull.
However, given the string of incoming storms, flooding may become
an issue eventually. An ESFMFR has been issued to discuss this
aspect of the forecast.

Weather conditions will improve late Monday and Tuesday looks to
be relatively dry with much lighter winds. It will be a short
break, as the next front will approach the coast Tuesday night.

Long term discussion from the Sunday afternoon AFD...Wed 06 Jan
through Sun 10 Jan 2021. Models and ensembles continue to show
that an active and wet pattern is likely through most of the
extended period, although not as wet or windy as the days heading
up to this timeframe. Operational models start in very good
agreement with a weaker front moving across the area on Wednesday
with rain and mountain snow expected. Snow levels will be around
5000 feet with very cold temperatures bringing levels down to
valley floors east of the Cascades, rising above 5000 ft during
the day.

Operational models, as well as the bulk of the ensembles, are
showing a lull in the wet pattern, albeit brief, with  an upper
ridge over the area Thursday. Another front moves in Thursday night
with the EC and Friday morning with the GFS.

The operational runs remain in very good agreement into Saturday,
building an upper ridge over the region again, before shifting east
ahead of the next front that moves in Saturday night. An somewhat
continuous active pattern sets in after the extended period with IVT
models showing a weak to moderate AR event though much of the
beginning of next week.

In addition to individual storm impact, the long period of moderate
to occasionally heavy precipitation from a train of storms heading
into the extended will bring lingering hydrological concerns to
coastal areas and the Umpqua Basin, but no river points are forecast
to go into Acton stage at this time. However, the Illinois river at
Kirby is continuing to show action stage that lingers above action
stage through the 7th. -Sven

&&

.AVIATION...For the 04/06Z TAFs...A warm front is pushing north
of the area late this evening. A cold front will follow and move
across the area late tonight and Monday morning.

There have been some returns on radar this evening, but most
precipitation that's reaching the ground is light. Cigs/vis
continue to vary from VFR to MVFR in most areas, though some
IFR/LIFR is out there, especially at the coast.

Low level wind shear (LLWS) presents an aviation hazard tonight at
the coast, especially north of Cape Blanco and at North Bend where
terrain to the south and a slight offshore component tends to block
the surface winds. Wind just off the surface at 2,000 ft are
around 45-55kt. LLWS could also impact the terminals at Roseburg
tonight, then Medford after 09z due to strong south-southwest winds
aloft.

Late tonight through Monday morning, as the front moves onshore,
MVFR cigs/vis will increase west of the Cascades with areas of IFR
and terrain obscurations. Strong and gusty south winds will surface
at the coast overnight with a brief period of gusty SSE winds also
pre-front at Medford and Klamath Falls Monday morning. Wind gusts of
30-35 kt are expected.

Periods of rain and snow will accompany the front with freezing
levels falling quickly behind it late Monday morning into Monday
afternoon, though precipitation and winds will diminish. -Spilde

&&

.MARINE...Updated 800 PM PST Sunday, 3 Jan 2021...Storm warnings
are up for the entire area tonight into Monday morning. A warm front
is pushing north of the area this evening. Southerly boundary layer
flow is increasing over the coastal waters as indicated by a jump up
in the winds to gales (30-40kt) at Buoy 46015 earlier. Model
guidance is steadfast in showing the BL flow peaking near 55kt ahead
of a very strong cold front that will move across the waters late
tonight and early Monday morning. Strong gales are expected along
with storm force wind gusts. Near term ensemble (HREF) depictions of
simulated radar have been showing a narrow cold frontal rainband
moving through the waters between 3 and 7 am. These rain bands,
albeit short in duration, are driven by an embedded area of strong
convergence within the cold front and are often intense with heavy
rainfall and strong wind gusts. Indeed, peak wind gusts associated
with this feature could be in the 55-60kt range during this time
frame. It pretty much goes without saying that seas will be
dangerously high and very steep, in excess of 20 feet in most areas.
The front will move onshore around 7 am and winds will become
westerly and drop off quickly.

Even so, very steep and hazardous seas will persist Monday and
Monday night, becoming dominated by building westerly swell at 18
seconds. Of note, seas may briefly lower below 18 ft across portions
of the waters (mainly the inner waters) Monday afternoon before
building again Monday evening and night.

Two additional strong fronts will move across the waters Tuesday
through Wednesday morning. This is expected to result in an extended
period of gales and very steep and very high seas. Seas of 16 to 24
ft are possible Tuesday into Wednesday due to a mix of west swell
and very steep wind seas.

Periods of moderate to heavy rain will accompany each of these
fronts tonight through Wednesday.

High to very high seas are expected through late this week due to
west swell and additional fronts. -Spilde

&&

.BEACH HAZARDS...Updated 200 PM PST Sunday 03 Jan 2021...Very
active weather with multiple rounds of heavy, long period swells
will lead to dangerous surf conditions early this week. Breaking
wave heights in the 22 to 28 foot range will persist into Tuesday.
Additional strong frontal system activity across the northeastern
Pacific through the end of the week will bring more large surf
events to area beaches, with breaking wave heights of around 20 to
25 feet possible toward the end of the week.

Use extreme caution along and near the beaches and stay away from
the ocean. Jetties, rocks, logs and bar entrances are not safe
places to be during this period of significant waves. The Pacific
Ocean is both cold and powerful. These large waves can easily sweep
people and large objects into the water. NEVER turn your back on the
ocean!

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning until 7 PM PST Monday for ORZ030-031.
     Winter Weather Advisory from 1 PM to 10 PM PST Monday for
        ORZ029-030.
     Wind Advisory from 7 AM to 7 PM PST Monday for ORZ029>031.
     High Wind Warning until 1 PM PST Monday for ORZ021-022.
     High Surf Advisory until 4 PM PST Tuesday for ORZ021-022.
     Wind Advisory until 1 PM PST Monday for ORZ023-024.
     Winter Weather Advisory from 7 AM Monday to 1 AM PST Tuesday
        above 4000 feet in the for ORZ027-028.
     Wind Advisory from 1 AM to 4 PM PST Monday for ORZ026.

CA...High Wind Warning until 7 PM PST Monday for CAZ085.
     Wind Advisory from 7 AM to 7 PM PST Monday for CAZ084-085.
     Winter Storm Warning from 4 AM Monday to 4 AM PST Tuesday above
        5000 feet in the for CAZ080-082-083.
     High Wind Warning from 1 AM to 1 PM PST Monday for CAZ081.

Pacific Coastal Waters...Storm Warning from 10 PM this evening to
        7 AM PST Monday for PZZ350-356-370-376.
     Gale Warning until 10 PM PST this evening for
        PZZ350-356-370-376.
     Hazardous Seas Warning from 7 AM Monday to 4 AM PST Tuesday for
        PZZ350-356-370-376.

15/15/18