High Wind in Oregon, January 26, 2021

Area Forecast Discussion
National Weather Service Medford OR
954 AM PST Tue Jan 26 2021

.DISCUSSION...MAJOR LONG DURATION WINTER STORM moving in with no
big changes this morning. This storm will bring plenty of wind and
snow to the area later today through Thursday night with numerous
wind headlines (NPWMFR) and winter headlines (WSWMFR) in effect.
The focus of heavy precipitation will be across western/central
Siskiyou County in northern California, but especially Interstate
5 from the south end of the Shasta Valley to Mount Shasta City and
Dunsmuir. Our simple message remains the same in those areas --
please don't travel during this time frame. Dangerous conditions
are expected to develop with intense snowfall rates at times
(1-3"/hour) from Weed/Mount Shasta City southward, very limited
visibility and low wind chills. This also includes McCloud and
some spillover into Tennant. Strong winds at times (45-65 mph),
especially north of Mount Shasta City near Weed could also result
in possible whiteout conditions at times. Snow load could become
an issue as snow piles up with tree damage and power outages
possible. We are also expecting wind/snow impacts across southern
OR (Coast Ranges of Curry/Josephine County), but impacts won't be
as extreme as what's expected in Siskiyou County.

Winds are picking up along the coast currently as pressure
gradients strengthen rapidly in advance of the frontal band
offshore. There has been some lightning too with the frontal band
(well offshore), but this has waned in the last hour or so. We've
had some cold temperatures this morning across inland areas and
this is a bit disconcerting given the frontal band poised to move
onshore this afternoon. However, this is expected to be a
significant downslope event here in the Rogue Valley with winds
surfacing late this morning/afternoon (if not already in
Ashland/south Medford) and precipitation should be held in check.
Even so, snow levels will be around 1500 feet and this could
bring some snow to the higher spots on the edges of the valley.
Our biggest concern is still in the Illinois Valley, a narrower
valley where cold air can get trapped more easily. When
precipitation starts there later this afternoon, it'll probably be
snow. Amounts ranging from 1 to 4 inches are expected, but
perhaps slightly more than that near Hayes Hill and the higher
spots along Highway 199. The highest amounts with the front (over
a foot) will be in the Coast Range of eastern Curry/Josephine
County. The passes north of Medford and Siskiyou Summit are also
expected to have a period of moderate snowfall tonight. Please see
previous discussion below for more details about Wednesday and
beyond. -Spilde

&&

.AVIATION...For the 26/18Z TAFs...Over the coastal waters and along
the coast...Areas of MVFR cigs in low clouds and rain will become
widespread by this afternoon with higher terrain obscured. The lower
conditions will persist into Wednesday with partial clearing
Wednesday afternoon. Winds will be strong until a front moves inland
late Tuesday afternoon. LLWS will be a concern at KOTH.

Inland from the Cascades west...VFR conditions will prevail
initially, except for patchy IFR cigs/vsbys over the west side
valleys, which will clear to VFR by Tuesday afternoon. However,
widespread MVFR/IFR conditions will develop this evening in low
clouds, rain and snow. The lower conditions will persist into
Wednesday with partial clearing Wednesday afternoon. Winds will be
strong until a front moves through this evening. LLWS will be a
concern at KRBG and KMFR.

East of the Cascades....VFR conditions will prevail initially, but
areas of MVFR cigs/vsbys in snow and local IFR cigs/vsbys in snow
will develop late this afternoon with higher terrain becoming
obscured. The lower conditions will persist into Wednesday morning,
clearing Wednesday afternoon. Winds will be strong until a front
moves through tonight. LLWS will be a concern at KLMT.

&&

.MARINE...Updated 800 AM PST Tuesday 26 Jan 2021...Steep swell
dominated seas this morning will quickly give way to worsening
conditions later this morning and this afternoon as a very strong
cold front approaches the coast. South winds will rapidly increase
to storm force by this afternoon, with wind-driven seas becoming
very steep and hazardous. Winds will weaken after the front moves
inland this evening, but very steep fresh swell will follow
tonight into early Wednesday morning.

South winds increase again Wednesday after a warm front moves
through, followed by a cold front Wednesday evening. Winds will
be much weaker than the Tuesday system, and are only expected to
reach advisory levels at 20 to 30 kt. But, very steep seas build
again Wednesday into Wednesday evening, peaking around 14 to 18 ft.

Weak low pressure will remain over the coastal waters Thursday
into Friday, then additional fronts will move onshore Friday night
and Saturday evening.

&&

.PREV DISCUSSION... /Issued 409 AM PST Tue Jan 26 2021/

DISCUSSION...A dangerous winter storm with historically significant
snowfall is expected to impact parts of Northern California,
including Interstate 5, late today into Wednesday morning, with
snow impacts continuing into Thursday night before tapering off.
Our message to those considering traveling this evening into early
Wednesday morning around Mount Shasta City area including on
Interstate 5 is simple--please don't travel during this timeframe.
Conditions will be very dangerous with heavy snowfall, very low
visibility, and dangerously cold wind chills as low as 10 degrees.
Snow and wind impacts will reach into Southern Oregon as well,
though not expected to wield the type of impact that's expected in
Siskiyou County. More to follow on that.

Satellite imagery this morning shows a well developed low pressure
system over the offshore waters moving towards the coast.
Precipitable water values are significant, about an inch along the
offshore front. As the associated front moves towards the coast, the
surface pressure gradient will tighten and southerly winds will
increase across the forecast area. These winds, in combination with
the approaching front and moisture, will fuel heavy precipitation
into favored upslope areas like the Coast Range, the Siskiyous,
and heaviest in the vicinity of Mount Shasta and Mount Shasta
City. The front will run into a cold air mass, so much of this
heavy precipitation will be in the form of snow. Moisture
transport values are in the top 10 percent of what one might
expect for this time of year, and this is especially notable given
the cold air mass. Usually the highest moisture systems have
higher snow levels.

All of this adds up to heavy snow later today into Wednesday, up to
2 inches per hour this evening through tonight, impacting portions
of Interstate 5 from south of Dunsmuir north through Weed to around
Grenada. Strong winds will accompany the snow at times, especially
in the vicinity of Weed. Most of Siskiyou County is in a Winter
Storm Warning, and we have Warnings out for the higher terrain of
Joesphine County as well. Heavy precipitation rates and the
aforementioned cold air mass could deliver significant snowfall to
valley floors in the Illinois Valley, portions of Douglas County
above 1500 ft, the Klamath River Valley (including Seiad Valley,
Happy Camp), and elevations above 1500 ft in Jackson County. We
have Winter Weather Advisories out for these areas. There's no
doubt the south winds will dry out portions of Jackson County as
is typical, so some areas will see little to no snowfall. Still,
it only takes a few hours of snow to cause impacts, and some areas
won't see as much drying with the south wind such as the higher hills
east of Talent and Ashland, and we'll leave the Advisory out for
those reasons. East of the Cascades can be even trickier in these
situations, and most areas will see snowfall into Wednesday. We've
highlighted the most significant snowfall areas with a Winter
Weather Advisory including portions of Highways 140, 97, and 139.
The Cascades will see significant snow as well, particularly near
Crater Lake.

Winds, as mentioned, will be hefty and will compound any impacts
that snowfall delivers, especially in the Weed area along Interstate
5...and in areas east of the Cascades. A High Wind Warning has been
issued for portions of the coast--specifically for areas south of
Cape Blanco--with the latest data pinging Gold Beach and Port Orford
with gusts to 60+ mph. Additionally the high resolution HRRR model
is showing very gusty conditions in the Rogue Valley today,
especially in the area foothills. These impacts have been rolled
into the Winter Weather Advisory for that area.

The winter weather impacts continue Wednesday into Thursday night as
low pressure remains just west of the CA/OR coastline and spreads
deep southerly flow and snowfall into many of the same areas that
see heavy snowfall later today into tonight. Snowfall rates, and
thus impacts, won't be as significant as today/tonight, but
continued heavy, wet snow in areas will add weight to
trees/powerlines, etc. Snow levels will also increase some during
this time as the air mass warms some, but not by much. Continued
gusty south winds are expected as well. A shortlived reprieve from
precipitation can be expected Friday. The longer-term discussion
follows. Keene

Long-term discussion from the previous AFD for Friday through
Sunday...The next system will be arriving from the west by Friday
evening and Friday night.

Like the previous storm systems, this one will likely be
arriving from the north with a cooler airmass. However, ensembles
suggest this weekend storm system isn't as anomalously cold as the
storm on the 24th and the upcoming one on the 26th.  Snow levels
will likely be around 3500 feet.  We see little change in the snow
levels based on the distribution of the ensemble members this
weekend.

The other important feature of this storm is that it has some
important components of an atmospheric river.  About 66% percent of
the global ensemble forecast system(GEFS) members incorporate a
weak to moderate atmospheric river this weekend.  None of the GEFS
members show exceptionally high water vapor transport for this
weekend. Overall, impacts are not expected to be as high because the
potential for low level snow and flooding is low with the storm
system this weekend.

AVIATION...For the 26/12Z TAFs...VFR conditions should prevail for
most areas, with the exception of several West Side valleys, mainly
the Umpqua, Coquille, Rogue, and Klamath River valleys, where
periods of LIFR ceilings are expected through this morning.

A strong low will reach the coast Tuesday afternoon then swing
through the region Tuesday evening with rain and snow, mainly MVFR
conditions, and increasing gusty low level southerly winds. This
includes a potential for low level wind shear over the Coastal Range
during late afternoon and early evening. The freezing level will
fall from around 2500 feet to 1800 feet MSL. By tonight, IFR/LIFR
conditions will likely prevail for most areas, with widespread low
ceilings, and snow and blowing snow above 1500-2000 feet MSL.
-DW/BPN

MARINE...Updated 200 AM PST Tuesday, 26 Jan 2021...Steep, swell
dominated seas this morning will quickly give way to worsening
conditions later this morning and this afternoon as a very strong
cold front approaches the coast. Expect rapidly increasing southerly
winds to reach storm force by this afternoon, with wind-driven seas
becoming very steep and hazardous. winds will weaken with the
passage of the front this evening, then very steep fresh swell will
follow Tuesday night into early Wednesday morning.

South winds increase again Wednesday with the next front. Winds will
be much weaker than the Tuesday system, and are only expected to
reach advisory levels at 20 to 30 kt. But, very steep seas build
again Wednesday into Wednesday evening, peaking around 14 to 18 ft.
Conditions then improve Thursday into Friday. But, the pattern will
remain active with worsening conditions expected late Friday into
the weekend. -DW/BPN

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from noon today to 4 AM PST Wednesday
     for ORZ030-031.
     Wind Advisory from 1 PM this afternoon to 7 AM PST Wednesday for
     ORZ030-031.
     High Wind Warning from noon today to 6 PM PST this evening for
     ORZ021-022.
     Winter Weather Advisory until 10 AM PST Wednesday below 1500
     feet in the for ORZ024.
     Winter Storm Warning until 10 AM PST Wednesday above 1500 feet
     in the for ORZ024.
     Winter Weather Advisory until 4 AM PST Wednesday above 1500 feet
     in the for ORZ023.
     Winter Weather Advisory from noon today to 4 AM PST Wednesday
     above 1500 feet in the for ORZ026.
     Winter Weather Advisory from noon today to 4 AM PST Wednesday
     for ORZ027-028.

CA...Winter Weather Advisory from noon today to 4 AM PST Wednesday
     for CAZ083-085.
     Winter Storm Warning from noon today to 4 PM PST Wednesday for
     CAZ081>083.
     Winter Weather Advisory until 10 AM PST Wednesday below 1500
     feet in the for CAZ080.
     Winter Storm Warning until 10 AM PST Wednesday above 1500 feet
     in the for CAZ080.
     Winter Weather Advisory from noon today to 4 AM PST Wednesday
     for CAZ081.

Pacific Coastal Waters...Storm Warning until 5 PM PST this afternoon for
     PZZ350-356-370-376.
     Hazardous Seas Warning from 5 PM this afternoon to 2 AM PST
     Thursday for PZZ350-356-370-376.

MAS/MAS/MAS