High Wind Watch 7, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
1155 AM PST Wed Dec 23 2020

Updated aviation discussion.

.DISCUSSION.../Issued 920 AM PST Wed Dec 23 2020/ There is some
localized dense fog at the Medford airport, but does not appear on
other road cameras. Therefore have withheld issuing a freezing fog
advisory this morning.

Pressure gradients from Medford to Redding continue to support
advisory speed winds in the Shasta valley Christmas day and is
consistent with the most recent hi-res model run output. The snow
amounts across Siskiyou county in the Winter Storm Watch are
between the 50th and 75th percentile of NBM amounts, and so
confidence is increasing that we are describing the most probable
scenario. With the forecast on track please see the previous
discussion below for expanded details. -Miles

&&

.AVIATION...For the 23/18Z TAFs...VFR conditions will persist
through the day with strong east winds aloft.  Satellite shows a lot
of dry air lingering around California and that should persist
through the TAF period.  However,  we'll see another round of LIFR
visibilities with freezing fog west of the Cascades Thursday
morning.  As for East of the Cascades, it's looking like fog is
unlikely Thursday morning.

-Smith

&&

.MARINE...Updated 920 AM PST Wednesday 23 Dec 2020... Relatively
calm, sub-advisory conditions will continue into Thursday morning.

A strong front will approach the region late Thursday afternoon.
Southerly winds will strengthen, possibly to gales, and wind driven
seas will become very steep and hazardous. A gale watch has been
issued for late Thursday afternoon into Christmas evening to cover
this. Wind driven seas from this front look to peak Friday afternoon
and evening, however, seas will remain very steep overnight into
Saturday. A very high and long period swell is possible Saturday
morning. Guidance indicates a decrease in a very high and very long
period swell, now showing of 16 to 18 ft at 14 seconds arriving
Saturday morning and peaking Saturday afternoon and evening.
However, this is a significant departure from previous runs and will
have to monitor further runs. Seas will gradually diminish Saturday
night into Sunday, but will remain high and steep through the end of
the weekend.

&&

.PREV DISCUSSION... /Issued 335 AM PST Wed Dec 23 2020/

DISCUSSION...Satellite imagery is showing some low clouds and fog
across portions of the Umpqua Valley, including at Roseburg. Visibilities
are at a couple of miles for now, and will become even more
limited during the early morning and sunrise. Will continue to
check for the need for a dense fog advisory, but none is needed at
the writing of this discussion. It also looks as if the fog and
low stratus has made it to Grants Pass, but elsewhere clear skies
and cold temperatures are occurring. Other inland valleys (the
Rogue) will see bouts of freezing fog later this morning while
other areas, particularly near the coast, will see heavy frost.

High pressure over the area will set up east winds and drying over
the ridges with less drying in the valleys due to the inversion.
Without clouds, expect the inversion to weaken during the
afternoon and some meager mixing to occur. Still the inversion
will not completely break, since the influence of high pressure is
so strong. This pattern will continue into Christmas Eve. If you
must travel, through Thursday evening is the perfect time to do
so. Please note, if you're traveling south on I-5 Thursday evening,
conditions may be breezy in the Shasta Valley ahead of the next
system coming in.

Models have remained steady with the incoming front, and Santa
should be able to clear the Pacific Northwest before precipitation
reaches the coast in the early morning. He will have to deal with
increased southerly winds ahead of the front, which will continue
to be breezy, and become strongest on Christmas day, particularly
for the Shasta Valley, Summer Lake, and our other traditionally
windy spots. Have not issued any products for it, but guidance is
suggesting that advisory level winds will be possible for these
areas. The southern end of the Rouge Valley may even see some
breezy winds as well. With the increase in winds overnight, do not
think fog and stratus will be a big problem except across the
Umpqua Basin. Still, Santa should make sure that Rudolph is rested
up so he can lead the way, just in case.

Then, on Christmas day, precipitation arrives ahead of a frontal
boundary and associated area of low pressure. This system looks
like a traditional storm with an influx Atmospheric River-like
moisture with southwest flow. This will bring at least moderate if
not spots of heavy rain for the coast and coast range near
Brookings. Rain rates appear to be below debris flow thresholds
for now, but will need to keep monitoring this--especially when
the high resolution data comes in.

Otherwise, the aforementioned southerly/southwesterly winds will
bring snow levels up to around 5000 to 6000 feet west of the
Cascades in Oregon and west of I-5 in California. For areas east,
snow levels will remain closer to 4000 feet. The front moves
through Friday afternoon and evening, bringing the snow levels
back down to around 4000 feet before further dropping them to near
3000 feet Saturday morning. Thus, some decent snowfall will be
possible in the Cascades and Siskiyous. A winter storm watch
remains in effect for portions of northern California above 4000
feet. Please see the NPWMFR for more details. Winter weather
advisories may be needed for the Cascades/Siskiyous, but am not
confident enough to make those decisions--especially since the
snow amounts will be dependent on timing of the front and location
of the moisture and low pressure system moving through.

Otherwise, showers will linger into Saturday as the low moves
across the Pacific Northwest. Preliminary amounts of snow for the
Friday System appear to be up to a foot of snow on the highest
Cascade Peaks with Highway 140 seeing 4 to 8 inches of snow over a
36 hour period. Mt. Shasta City could see 2 to 6 inches, and
Interstate 5 could see 1 to 4 inches of snow. Meanwhile, I-5 on
Siskiyou Summit looks to escape significant impacts with up to 3
inches of snow. Please note that these amounts are bound to change
as new information comes in.

A brief break is expected on Saturday night, and then a second
low will spin up and move through Central California bringing
additional rain and snow showers. Snow levels will be much lower
on this one since we are on the northern side of the system and
will have had a cold airmass already come through. The ECMWF is
more progressive with this system while the GFS holds onto showers
through Monday night. Have gone with the National Blend of Models
as a good way of blending the solutions with a calibrated
weighting system, which is showing a 10 percent chance of showers
into next week. -Schaaf

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...None.

CA...Winter Storm Watch from Friday morning through Saturday morning
     above 4000 feet in the for CAZ080-082-083.

Pacific Coastal Waters...Gale Watch from Thursday afternoon through Friday afternoon for
     PZZ350-356-370-376.