Sneakerwave in Oregon, January 5 to 6, 2024

Area Forecast Discussion
National Weather Service Medford OR
350 AM PST Fri Jan 5 2024

.SHORT TERM...A sharp front has moved through leaving mostly clear
mid and upper level skies behind it. This has allowed fog to form
in the valleys west of the Cascades in Southwest Oregon, and a
tattered stratus deck in NorCal west of the Cascades. Road cams
do not show the fog being overly dense and visibility restricting,
but we will keep an eye on things for if a dense fog advisory is
needed. A few showers in the mountains along the Cal/OR border and
Lake of the Woods area are still resulting in some light
accumulations from snow flurries. While the fog will last until
shortly after sunrise, the snow flurries are expected to stop in
the next 2-3 hours.

Today the region will experience a calm before the storm. For
areas where the fog breaks, temperatures are forecast to be normal
(to the degree for most sites!), which sit in the mid-to-upper 40s
for valleys west of the Cascades and upper-30s east of the
Cascades. Things will change once the sun goes down.

After sunset today the first in a series of storms will progress
through the region. The front will reach the coast between 10 pm
and midnight, and the Cascades by 3 to 5 AM Saturday morning. At
the coast, the leading edge of the front is expected to be
accompanied by gale force winds driving 0.3" to 0.7" of rain onto
the coast. During this time conditions will be at their worst,
but because it is coming in the middle of the night, impacts
should be very low. Rain will continue through Saturday night and
storm total accumulations are expected to reach between 1" to
1.5". Thunderstorms will also be possible all day on Saturday,
with chances highest along the coast (20-30%), but 10-15% chances
are forecast as far inland as Roseburg and Cave Junction.
Thunderstorms would add the risk of accumulating graupel,
lightning, and a very low chance for a water spout. I say
waterspout because the lapse rates are summer time strong, shear
has both a little speed and direction difference, and enough CAPE
to sustain updrafts for a prolonged period. So if you're along
the coast viewing the high surf -safely from a distance...check
out the beach hazard discussion below on this- you should also
keep an eye out for a waterspout.

There were some questions if the front would arrive sooner to the
Cascades, but 75% of model solutions support the arrival in the
3-5 AM window. This stability in the model output resulted in
very little change to the forecast and an increase in confidence
for its impacts. Impacts from the snow are expected to cause
disruptions to daily life because of difficult travel conditions.
This impact will be most prominent from 4 AM to 10 AM when snow
showers and winds are expected to peak and snowfall has a very
high chance for coming in at 1 inch per hour while being blown
around by winds gusting up to 40 mph and causing visibility to be
reduced even further. Overall though, this storm is fairly typical
for the Cascades. And let's not forget the snow year has started
off slow and this snow will be quite beneficial for water resource
concerns and snow parks alike.

There has also been some discussion in our office about whether
snow will reach the valley floors west of the Cascades. If this
is going to happen, Saturday night seems the most likely time for
it given snow levels will be at their lowest. However, the chance
for precipitation in places like Cave Junction, Grants Pass, and
Medford is between 30-50% and so even if snow levels are
supportive, it is still not a sure thing. That said, about 25% of
models show the snow levels low enough for this to occur and a few
outliers even suggest up to a half an inch of snow could
accumulate, but 0.1" would be more likely (on cool grassy
surfaces at least...for a little while). However, this is tough to
achieve and 25% is not that large, so while some flakes might be
seen, accumulation is not expected for most places below 2000-2500
feet.

Snow is also expected east of the Cascades. The mountains will
chew up a lot of the moisture and the shadow effect will leave
places east of the Cascades looking at 1 to 4 inches. Some of the
mountain passes may observe some minor impacts to travel when
snowfall peaks Saturday morning, but it's really only the Warner
Mountains in Modoc County that could see impacts rise to the
moderate level. Moderate impacts in the Warners would be most
likely between 7 AM and noon, when snowfall peaks.

Sunday, showers will be lingering in the high terrain, mostly in
the Oregon Cascades, but there are low chances in western Siskiyou
too. Winds will calm for all areas except the higher terrain
above 6500-7000 feet where the winds will remain gusty. Then by
Sunday night, another front is still expected to move through.
-Miles

.LONG TERM...Sunday night through Thursday, Jan 7-11, 2024...
Active weather will continue next week. The extended forecast begins
Sunday night with a deep upper trough digging into the Four Corners
region, with an upper ridge axis near or just off the West Coast.
Our area will remain in a mostly west-northwest flow aloft, but with
a warm front that will be poised to move onshore Monday. So, after a
brief break precip-wise Sunday night, we'll get right back into it
on Monday. Snow levels are initially low, starting around 2000 feet
late Sunday night, but they'll rise on Monday back to higher pass
levels by evening (Siskiyou Summit/Lake of the Woods). Snow amounts
on Monday/Monday night don't appear to be too impactful with 4-8
inches a good bet from Highway 140 northward in the Cascades. We are
expecting lesser amounts in the mountains south of there (1-3
inches) and also for areas over the east side north of Highway 140
and west of Highway 31 (including Highway 97 north of Klamath
Falls).

The pattern really begins to shift again on Tuesday with a decided
turn toward colder, wintry weather through at least Thursday.
Guidance is showing a strong cold front sweeping through the area on
Tuesday. WSW flow aloft (around 45-55 kt) will likely bring a period
of gusty winds and moderate precipitation (heavy in the mountains).
Snow levels remain high in the 4000-5000 foot range in advance of
the front into Tuesday afternoon, but will drop sharply to around
1500-2000 feet by Wednesday morning. Post-frontal precipitation will
continue in the colder air Tuesday night into Wednesday. With the
flow aloft shifting back to the NW, we expect snow to really pile up
in the mountains and snow could once again make a visit to the
valley floors. Mountain snow will likely be measured in feet (good
news given the slow start to the winter season in terms of
snowfall).

Beyond that, there is some uncertainty with the depth of the long
wave trough and how strong upper ridge amplification will be
upstream into Alaska. Strong ridges amplifying into Alaska often
lead to some of our coldest outbreaks here in the PacNW. And, in the
12z deterministic ECWMF, this isn't too hard to imagine as a
possibility. However, right now, that solution is in the lower 20%
of all solutions. Ensembles and clusters favor a more progressive
upper trough (moving into the nation's midsection with a less
amplified upstream upper ridge. This scenario would allow the air
mass to modify a bit toward the end of next week. Even so and either
way, we'll remain in a NW flow pattern, so snow potential remains
high in the mountains. -Spilde

&&

.AVIATION...05/12Z TAFs...West side valleys and the coast are
seeing MVFR/IFR and local LIFR conditions. Conditions will then
gradually improve to VFR late morning into early afternoon. MVFR
conditions move inland with the front late tonight, with periods of
heavy rain bringing possible IFR conditions moving in from the coast
Saturday morning. -Sven

&&

.MARINE...Updated 330 PM Thursday January 4, 2024...Steep seas in
all waters will increase further this morning as a heavy long-period
swell arrives in all waters.

A strong frontal system arrives tonight into Saturday morning and
will bring building seas as well as strong gusty south winds
followed by gusty northwest winds. While winds are not expected to
reach gale force, it would not be surprising for a few isolated gale
gusts. Meanwhile, very high and very steep, west-northwesterly seas
are expected from Cape Blanco northward tonight into Saturday. These
conditions are expected to result in dangerous bar crossings as well
as high surf along the coast. Seas will gradually lower late
Saturday into Sunday but will continue to be high and steep. High
and steep seas will persist Saturday night into Sunday. Also with
this system, there is a slight chance for thunderstorms over the
coastal waters on Saturday and again on Sunday.

Seas will lower Sunday night into Monday as west-northwesterly swell
diminishes. However another front on Monday may bring gusty south
winds and areas of steep seas. Long term guidance shows conditions
deteriorating rapidly Tuesday morning as an impactful front moves
over the area. This front will bring a potential for strong, gusty
winds followed by very high seas as large westerly swell builds into
the waters. -Sven

&&

.BEACH HAZARDS...Updated 200 PM Thursday, January 4 2024...A
long period swell will arrive Thursday night into Friday then build
further on Saturday. Current projections show peak seas of 13-17
feet at 19 seconds on Friday. On Saturday, an approaching front will
bring gusty northwest winds and building seas, reaching 15-19 feet
at 17 seconds Saturday morning and continuing into the evening.
These conditions will result in dangerous high surf along the
southern Oregon beaches with large breaking waves of 20 to 24 feet.
A High Surf Advisory remains in place Friday and Saturday (please
see the CFWMFR for details).

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Warning from 4 AM to 10 PM PST Saturday above 2500
     feet for ORZ024>026.

     Winter Weather Advisory from 4 AM to 10 PM PST Saturday for
     ORZ029>031.

     High Surf Advisory until 10 PM PST Saturday for ORZ021-022.

     Winter Storm Warning from 4 AM to 10 PM PST Saturday for ORZ027-
     028.

CA...Winter Storm Warning from 4 AM to 10 PM PST Saturday above 2500
     feet for CAZ080.

     Winter Weather Advisory from 4 AM to 10 PM PST Saturday for
     CAZ081>083.

     Winter Weather Advisory from 4 AM to 10 PM PST Saturday for
     CAZ085.

PACIFIC COASTAL WATERS...Small Craft Advisory until 4 AM PST Monday
     for PZZ356-376.

     Small Craft Advisory until 10 PM PST this evening for PZZ350-
     370.

     Small Craft Advisory from 10 AM Saturday to 4 AM PST Monday for
     PZZ350-370.

     Hazardous Seas Warning from 10 PM this evening to 10 AM PST
     Saturday for PZZ350-370.

&&