High Wind in Oregon, December 18, 2023

Area Forecast Discussion
National Weather Service Medford OR
1003 PM PST Sun Dec 17 2023

.DISCUSSION...

Radar is showing some showers across southern Oregon
and northern California this evening. The rainfall is generally
really light, and snow levels are above pass levels. The main
question of this evening update is whether to expire the air
stagnation advisory due to the rain. Mixing heights at the time of
the sounding were around 300 feet or so, and the rain is
particularly light. However, winds are relatively strong; so
expect mixing to improve over the course of the night. Have
cancelled the air stagnation advisory, but will leave the wind
advisory in place. Will need to continue monitoring the wind
forecast to continue checking trends. -Schaaf

&&

.AVIATION...18/06Z TAFs...As warm air surges into the area,
ceilings appear to be for most of tonight with mainly VFR ceilings.
The valleys could fog in, although that seems fairly unlikely
tonight given the ample ceilings and unsettled boundary layer.
There could be periods of MVFR visibilities and ceilings under some
of the heavier rain.  If rain is a little heavier than anticipated,
than fog seems more likely in the valleys.

Otherwise, mainly VFR ceilings and and visibilities are anticipated
on Monday. The only exception appears to be east of the Cascades
where models are expecting IFR to LIFR ceilings early in the morning
for most of the Klamath Basin.  We don't have that in the KLMT TAF,
although it seems reasonable based on the situation.

-Smith

&&

.MARINE...Updated 800 PM Sunday December 17, 2023...
Southerly swell will continue to build into the region overnight,
increasing a few more feet, as south winds and wind seas also build.
Conditions will be at their worst late Monday afternoon through
Monday night.

There is high confidence in gales south of Cape Blanco from shore
out to 60 nm, and the gale watch has been accordingly upgraded to a
warning. Due to the low pressure system weakening as it moves north,
areas northward of Cape Blanco will be less windy, and gale-force
winds don't seem likely with 30% chance or less to see maximum gusts
reach 40 kts in this area. However, a combination of steep to very
steep south swell and wind waves will create hazardous conditions
through the entire marine region.

Seas will remain high and steep Tuesday as the fresh southerly swell
moves through, but winds will begin to ease with the low pressure
moving onshore. Then, a west swell is expected to build into the
waters Wednesday morning at around 10 to 12 feet at 13 to 14
seconds, which should maintain at least advisory level seas across
most of the area. Improving conditions are expected Thursday, but
the pattern is likely to become active again late in the week into
next week. A strong storm is possible shortly after Christmas.
/BR-y/CSP

&&

.PREV DISCUSSION.../Issued 236 PM PST Sun Dec 17 2023/

SHORT TERM...Tonight through Wednesday Night...The ridge that has
been in place over our area for several days is finally
retreating to the east, allowing a closed low to approach us from
the southwest. The increased mid to high level clouds today have
kept temperatures much cooler in the valleys today, while over the
ridges and across the East Side, where winds have mixed the
atmosphere more, temperatures have lived up to expectations. These
valley inversions are likely to hold on through tonight for most
areas, so the air stagnation advisory remains in effect until
tomorrow morning. However, valleys that see increasing winds
tonight like those at the coast, the Shasta Valley, and the
portion of the Rogue Valley from Ashland to south Medford may mix
out a bit sooner.

A large closed low will approach the region from the eastern
Pacific tonight into Monday morning. Precipitation, which is
already visible just to our east and southeast, will spread west
to east tonight, and winds will begin to increase across the area
as the low nudges ever so much closer to the coast. By Tuesday
night, the low has exited to the north, but a second closed low
is expected to dig in to take its place just offshore by Tuesday
morning. This low will then meander slowly south, arriving
offshore of central California by Wednesday night.

Winds will be the primary concern with this pattern, peaking
Monday afternoon and evening as the low passes just offshore.
Model 700mb winds range from 50 to 70 kt during this time, almost
directly and consistently from the south, although at times with
an eastward component. This will lead to gusty winds in the Shasta
and Bear Creek valleys (particularly at Weed and Ashland) and
across the East Side. Wind Advisories have been issued for these
areas to account for this, and details can be found at PDXNPWMFR.

Snow is of almost no concern at all throughout the first part of
the week, as snow levels will remain well above pass level, with
the possible exception of the Mount Shasta and Trinity Alps area,
where strong upslope precipitation rates could drag snow levels
down to around 5000 feet or a bit lower, although still above the
major thoroughfares.

Rain will be of some concern, and there will be a bit of a trick
to it. Low level flows will remain out of the south to southeast
as the first low tracks through Monday, then remains mostly south
to southwest as the second low arrives and digs to the south.
This scenario is likely to result in significant downsloping for
much of the area, especially during the periods of south to east
winds. The NBM, whose solutions over the past several days has
appeared to resemble a west to east frontal passage and not the
pattern we are actually expecting, is finally starting to trend
towards a solution more consistent with a southerly wind event.
As a result, precipitation chances and amounts have trended down
for much of the area, mainly the valleys and lee-side slopes that
will see a lot of downsloping winds with this flow direction. This
includes Medford, which may only end up with a few hundredths of
an inch through Wedensday, should downsloping continue unabated.

That being said, that same south flow is known to produce strong
upslope precipitation, namely along the Curry County coast, the
Klamath Mountains, and the Mount Shasta City/Dunsmuir/McCloud
area. Precipitation amounts and chances remain higher in these
areas, along with a few smaller areas here and there across the
region. We do not expect an overabundance of rain or any
hydrological concerns through Wedensday, although full ditches and
rapid rises on small streams are possible. Precipitation will
slack off through Tuesday night and Wednesday as our new offshore
low drifts farther south and its influence on our area wains. BY
Wednesday night, only a few linger showers may remain in our far
southern areas. -BPN

LONG TERM...Thursday morning through Christmas Eve...The extended
period will be relatively quieter than the short term. Low
pressure off the northern California coast will track
south/southeastward along the coast before eventually moving
inland into southern California on Friday. As this pattern
transitions, dry conditions are expected to continue Thursday and
into at least Friday afternoon. In the meantime, our region will
be between the cutoff low to the south and zonal flow to our north
and we'll likely see the return of valley fog across the West
Side Valleys both Thursday and Friday mornings.

By Friday afternoon, the next trough approaches the region from the
northwest and this is when model solutions begin to diverge on the
details. Though they diverge in details, each model and respective
ensembles remain consistent in their trends and both have trended
farther south than previous runs. The EC continues to be the
farthest south solution while the GFS is now slightly faster, but
farther north. Either solution does result in a chance of
precipitation in the area Friday afternoon into Saturday, but
details vary regarding precipitation amounts and the inland extent
of measurable precipitation. The EC is by far the wetter solution as
the next system moves right overhead and this results in widespread
precipitation potential. The GFS depiction maintains just the tail
end of the front passing through and favors precipitation chances
across the northwestern portion of the forecast area. Given the
northwest to southeast trajectory, this system will be colder than
the southerly flow system in the short term, and this will bring
lower snow levels that could impact some of the higher passes.
Current forecast has snow levels down to 4000-5000 ft, and guidance
suggests they could lower down to 3000 ft towards the tail end of
the precipitation. In addition to the lower snow levels, there could
be some gusty winds that accompany this trough, and we could be
looking at another round of wind headlines. This is something we'll
have to monitor over the coming days to really nail down the details
and potential impacts.

Conditions should trend drier on Saturday as shortwave ridging
passes through the region and quiet conditions continue through the
weekend, including Christmas Eve. Aside from some night and morning
fog on Christmas Eve (looks like Santa could use Rudolph's help) and
morning fog on Christmas Day, the weather leading into Christmas
should be relatively benign. The week following Christmas, however,
the weather looks to be much more active (beginning as early as
Christmas Day). The Climate Prediction Center (CPC) maintains
increased risks for heavy rain, strong winds and heavy mountain snow
in our region. Stay tuned as this time drawers nearer and details
become more clear. /BR-y

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory from 10 AM Monday to 1 AM PST Tuesday for ORZ026.

     Wind Advisory from 4 PM Monday to 7 AM PST Tuesday for ORZ030-
     031.

CA...Wind Advisory from 10 AM Monday to 4 AM PST Tuesday for CAZ081.

PACIFIC COASTAL WATERS...Small Craft Advisory until 4 PM PST Monday
     for PZZ350-356-370-376.

     Gale Warning from 4 PM Monday to 7 AM PST Tuesday for PZZ356-
     376.

     Hazardous Seas Warning from 4 PM Monday to 7 AM PST Tuesday for
     PZZ350-370.

&&