Heavy Snow in Oregon, February 2 to 3, 2021

Area Forecast Discussion
National Weather Service Medford OR
244 PM PST Tue Feb 2 2021

.SHORT TERM DISCUSSION...Through Thursday morning...Low pressure
near Vancouver Island will move south, making its way inland
around Portland, then move into southern Idaho. The low will bring
a front out of the due north with fairly strong mid-level cold
air advection and an average PW for a storm of this trajectory
and time of year. Models have been incrementally compressing the
timeframe for when the heaviest precipitation will move through,
which is roughly between 1 AM and 7 AM PST. With the strong cold
air advection and heaviest precipitation coming at night, low
elevation snow cannot be ruled out. Have dropped snow levels with
the Winter Weather Advisory to 1500 feet. This has brought
slightly increased snow amounts to areas already included in the
advisory and has caused Hayes Hill and Jacksonville Hill to now be
included.

Also added California zone 82 to the Winter Weather Advisory. This
is because 3-5 inches of snow is forecast to fall along route 89,
which has already received a large amount of snow. Confidence is
not high that this much snow will fall, but the existing snow and
eastward trend of where the most moisture is has prompted its
inclusion. Also added was the Shasta valley from Gazelle north
along I-5. There is moderate to high confidence that the west side
of the valley will receive advisory level snow, the eastern half
may be slightly less likely.

The compression of when the heaviest precip moves through also
means that 1"/hr precipitation rates will be possible. At Siskiyou
Summit the NBM suggests the probability of these heavier rates
are around 80%. These heavy precip rates are most likely between
Siskiyou Summit and the eastern edge of Curry County. This puts
the Illinois Valley and route 199 under the bullseye for heavy
rates in the most recent three HREF runs. Here, localized wet
snow may mix down to valley floor Wednesday morning, but we are
not expecting it to accumulate. This also can't be totally ruled
out for the Rogue valley, but the probability of it happening is
low-to-very low.

Winds with this front have steadily come down with time. That
said, gusty winds will still occur along the coastal headlands
and higher terrain in the region.

Rain and snow showers will taper off fairly quickly Wednesday
afternoon, but we suspect the area will retain a fair amounts of
cloud cover. How much this cloud cover effects fog formation
Wednesday night and Thursday morning is probably minimal because
of the amount of moisture in the area and cold air that can settle
forming an inversion.

.LONG TERM...Thursday February 4th through Tuesday February
9th...Compared to recent weeks, the extended forecast will feature
much quieter conditions. Upper level ridging builds over the
eastern Pacific Ocean Thursday and persists through at least the
weekend. This will shift the storm track well to the north, with
the Pacific Northwest under northwest flow on the back side (east
side) of the ridge. This pattern will lead to stable conditions
with mostly dry weather persisting through at least the start of
the weekend. Valley fog and low clouds will be common at night and
during the morning hours, especially west of the Cascades but
freezing fog will be possible east of the Cascades as well.

With the area under northwest flow aloft, shortwaves riding over the
ridge will pass through the area on Friday and possibly again on
Sunday. Most of the area will remain dry, although a few hundreths
might get squeezed out across areas north and west of the Umpqua
Divide. For most of the area, these shortwave passages will be
marked by increased mid and upper level cloud cover and an increase
in afternoon breezes.

Upstream of the ridge, in the central Pacific and over Siberia, the
pattern remains active. Lows will continue to batter the western
portion of the ridge, narrowing and weakening it. This will
eventually lead to the breakdown of the ridge with the storm track
returning to the area early to mid week next week. There are still
significant timing differences in the GEFS and EC ensembles, but the
impact is expected to relatively low. The ensemble means are in
fairly good agreement that a shortwave will eventually bring a front
through our area Tuesday. The impacts for now can be summarized as
light rain along the coast and high terrain, and even lighter
amounts of snow with snow levels in the 3500-4000 ft range. /BR-y
&&

.AVIATION...For the 02/18Z TAFs...Mainly VFR conditions will prevail
today, although expect terrain obscurations to continue. Ongoing
showers will gradually taper off later this morning, then increase
along the coast late this afternoon and evening. Overall VFR
conditions will be the dominate condition, but MVFR conditions are
possible in any heavier showers.

Another front will move inland this evening and overnight, bringing
gusty winds and lowering conditions. There's a chance for low level
wind shear at Roseburg early Wednesday morning, but confidence was
not high enough to include it in the forecast at this time. /BR-y

&&

.MARINE...Updated 200 PM PST Tuesday 2 Feb 2021...A cold front
will move onshore tonight, bringing moderate southwest winds and
steep to very steep seas to the area. Winds will diminish after the
front moves inland, but steep to very steep seas will persist into
Wednesday night. Offshore high pressure will build Wednesday along
with a trough near the coast, and this will produce gusty north
winds and steep seas across the area, especially south of Cape
Blanco. These conditions will persist through the weekend.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 10 PM this evening to 4 PM PST
     Wednesday above 1500 feet in the for ORZ023>030.
     Hazardous Seas Warning from 4 AM to 4 PM PST Wednesday for
     ORZ021.
     Small Craft Advisory until 4 AM PST Wednesday for ORZ021.
     Small Craft Advisory from 4 PM Wednesday to 4 AM PST Thursday
     for ORZ021.
     Small Craft Advisory from 7 PM this evening to 4 AM PST Thursday
     for ORZ022.

CA...Winter Weather Advisory from 10 PM this evening to 4 PM PST
     Wednesday above 1500 feet in the for CAZ080>082.

Pacific Coastal Waters...Small Craft Advisory until 4 AM PST Thursday for PZZ356-376.
     Hazardous Seas Warning from 4 AM to 4 PM PST Wednesday for
     PZZ356-376.
     Small Craft Advisory until 4 AM PST Wednesday for PZZ350-370.
     Small Craft Advisory from 4 PM Wednesday to 4 AM PST Thursday
     for PZZ350-370.
     Hazardous Seas Warning until 4 PM PST Wednesday for PZZ350-370.