Frost and Freeze in Oregon, December 30, 2021

Area Forecast Discussion
National Weather Service Medford OR
832 PM PST Wed Dec 29 2021

.DISCUSSION...Upper ridging is allowing the air mass to dry out
aloft, but plenty of low-level cloud cover remains this evening
in the valleys west of the Cascades. Clearing has occurred in
most of Klamath and northern Lake counties and, with the fresh
snow pack, temperatures are plummeting. Some readings are already
in the single digits. Model guidance just doesn't capture this
very well, so while most areas will have lows in the single digits
and teens over there, it won't be surprising to see some readings
near or below zero, if skies remain mostly clear overnight.
Klamath Falls is already 10F as of 8 pm with areas of freezing
fog. Cloud-cover also lingers over southern Lake and Modoc
counties, where temperatures probably won't get so blasted cold.

Meanwhile, cloud-cover is eroding over the Cascades, but we've
got a layer of low stratus here in the Rogue Valley and most
other inland valleys of the west side. These clouds are good
insulators, so many areas won't get quite as cold as would
normally be expected under clear skies. Those that do clear will
have lows well down in the 20s, but it'll be in the upper 20s to
around freezing where clouds remain. We do have a freeze warning
up at the coast. Current temperatures there are running from
32-36F, so this appears to be on track. We are expecting
increasing cloud cover, especially in northern areas overnight, so
temperatures may rise some overnight there.

In fact, the next disturbance isn't waiting around long. It'll be
here before we know it Thursday morning. Some flurries are even
possible across northern Klamath and Lake Counties late tonight.
Model guidance is breaking out precipitation near or just after
sunrise Thursday morning along the coast north of Cape Blanco
(temperatures will have risen above freezing by then) and also
northern Douglas County. So, expect some snow to develop near or
north of Roseburg Thursday morning, shifting east to the Cascades
by late morning and over to the east side, mainly from Highway 140
northward in the afternoon. We have issued a winter weather
advisory for zones 23 and 25 (above 2000 feet) and 26 (above 2500
feet) for 2 to 6 inches of snow beginning late Thursday morning
and going through Friday morning. Main impact areas will be the
Cascade foothills, but also portions of central Douglas County and
northern and eastern Jackson County. Please see WSWMFR for the
specifics and impact information. Some light snow is also possible
for the Rogue and Illinois valleys, but temperatures should rise
above freezing and roadways are most likely to remain wet through
late afternoon/early evening. Snow levels rise to 1500-2500 feet
depending on location. Rain/snow showers could linger in those
areas Thursday night into Friday morning with snow levels lowering
back to around 1000-1500 feet. -Spilde

&&

.AVIATION...30/00Z TAFs...Low ceilings have built into some of the
valleys this evening and we're forecasting IFR to LIFR ceilings to
persist overnight in some of the valleys.  in addition, pockets of
freezing fog are also possible in a few of the valleys, even east of
the Cascades.  By Thursday morning,  there will be some showers
moving through the forecast area.  Look for a mix of rain showers
west of the Cascades at snow showers east of the Cascades, leading
to a mix of MVFR to LIFR conditions as the showers move through.
Data suggests that the fog will build back in again Thursday night.
-Smith

&&

.MARINE...Updated 200 PM PST Wednesday 29 December 2021...Northerly
winds will remain at advisory strength through this evening with
steep mixed seas. A weak front will move onshore Thursday with
another episode of steep seas expected Thursday evening into Friday.
A much stronger front will move through Sunday night into Monday
with gale to storm force winds possible. The active weather pattern
will continue through the coming week. -Smith

&&

.PREV DISCUSSION... /Issued 249 PM PST Wed Dec 29 2021/

SHORT TERM...(12/29/21 Wednesday Night through Saturday night
01/01/2022) A few showers are lingering across portions of Lake
and Klamath County this afternoon, but they should taper off over
the next few hours. Meanwhile, there are some low stratus in some
of the West Side Valleys with a few cumulus clouds in Coos County
and offshore. Overall, a brief break in the active weather is
expected tonight with some cold temperatures expected everywhere,
including at the coast. Although some clouds could come back in
and moderate temperatures later Thursday morning, could easily see
the coast reaching temperatures at or just below freezing. As a
result, a freeze warning has been issued for the coast. Please see
the NPWMFR for details. Otherwise, the big issue overnight will
be the possibility for freezing fog in the West Side Valleys.

Otherwise, the next front comes in on Thursday with some light
precipitation. Snow levels will once again fall back down to
around 1500 feet with some cold air remaining at the valley floors
(especially in places that had freezing fog develop and stayed
for a while). Although snow amounts will generally be light, will
want to continue to monitor the situation should any advisories be
needed for travel purposes on Friday morning. Temperatures, with
clouds, will be somewhat warmer; however.

After the showers taper off on Friday, a shortwave ridge builds
into the area giving us another break in the active weather.
Expect this weekend to have cold low temperatures, especially east
of the Cascades, and for the West Side Valleys with NBM ensembles
showing ranges in the teens to upper 20s in Medford with single
digits to teens possible east of the Cascades. Additionally, some
freezing fog will be possible in the west side Valleys on Saturday
morning and likely on Sunday morning. The main message here is
that holiday travel should occur on Saturday if at all possible
since the long term portion of the forecast (Sunday) begins with
our next major active system that could produce additional periods
of heavy rain and snow, and strong winds. More information
available in the long term discussion below. -Schaaf

LONG TERM...Sat night 01/02 Jan through Wed night 05/06
Jan 2022.

At the beginning of the extended period, a long wave ridge near the
coast will be in the process of breaking down, and what's left of it
will break to the east of the forecast area Saturday night.
Southwest flow will develop over the area and the upstream trough
will approach the coast. This will open up the storm door again and
the area will be vulnerable to short waves ejected from the trough
and moving onshore.

The first of these waves will move onshore Monday, and the
associated front will move onshore late Sunday night into Monday.
Precipitation ahead of the front will begin to overspread the area
during the day Sunday. This looks to be a fairly strong front in
terms of wind, with gale force winds over the coastal waters and
strong winds over the ridges and east side Sunday night into Monday.
Snow levels will initially be very low but will rise up to the 2000
to 3500 foot range by the time significant precipitation begins. If
this forecast holds, there will be another heavy snow event in the
Mount Shasta area Monday into Monday night with significant amounts
over the Cascades and Siskiyous as well as the higher elevations of
the Kalmiopsis.

More short waves will follow. Tuesday will also be wet and windy.
Snow levels will be between 2500 and 4500 feet for the most part
with the lower snow levels to the north and the higher values to the
south.

Rainfall amounts Sunday night through Tuesday will also be
significant. While no widespread flooding is expected at this time,
urban areas and small streams may see some minor flooding. As is
typical in an event with strong southwest flow aloft, the heaviest
precipitation will occur from the Kalmiopsis south through western
Siskiyou county, then east into the Mount Shasta area.

The trough will dig offshore and will be slow to progress east, but
it will finally move onshore Tuesday. While there will be weak
ridging in the wake of the trough, short waves embedded in the flow
will continue to move onshore through Wednesday night. So showery
conditions will persist Tuesday night through Wednesday night, but
the heavier precipitation will be over by Tuesday night. Winds will
diminish Tuesday night. Snow levels will remain in the 2500 to 4500
foot range with the lower values to the north and the higher values
to the south.

AVIATION...29/18Z TAFs...Shower will gradually diminish through
this evening. All precipitation from the Cascades east will fall as
snow, but west side precipitation will run the gamut from rain to
mixed rain/snow to snow with a freezing level between 2000 and 3000
feet. Conditions will be highly variable across the area, ranging
from IFR to VFR. In general, lower conditions will accompany showers
and the higher terrain will mostly remain obscured. Some improvement
is expected Wednesday afternoon and evening, but lower conditions
will return tonight with areas of LIFR cigs/vsbys in fog and
freezing fog developing in the west side valleys. An incoming front
will bring precipitation to the coast by early Thursday morning,
spreading inland to the Cascades and Siskiyous during the day.
Overall, conditions with the frontal rains will be areas of MVFR
cigs with higher terrain obscured.

MARINE...Updated 200 PM PST Wednesday 29 December 2021...Northerly
winds will remain at advisory strength through this evening with
steep mixed seas. A weak front will move onshore Thursday with
another episode of steep seas expected Thursday evening into Friday.
A much stronger front will move through Sunday night into Monday
with gale to storm force winds possible. The active weather pattern
will continue through the coming week.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Freeze Warning from midnight tonight to 9 AM PST Thursday for
     ORZ021-022.
     Winter Weather Advisory from 10 AM Thursday to 10 AM PST Friday
     above 2000 feet in the for ORZ023.
     Winter Weather Advisory from 10 AM Thursday to 10 AM PST Friday
     above 2000 feet in the for ORZ025.
     Winter Weather Advisory from 10 AM Thursday to 10 AM PST Friday
     above 2500 feet in the for ORZ026.

CA...None.

Pacific Coastal Waters...
     Small Craft Advisory until 10 PM PST this evening for
     PZZ350-356-370-376.
     Small Craft Advisory from 4 PM Thursday to 4 PM PST Friday for
     PZZ356-376.
     Small Craft Advisory from 10 AM Thursday to 4 PM PST Friday for
     PZZ350-370.
     Hazardous Seas Warning from 4 PM to 10 PM PST Thursday for
     PZZ370.

MAS/CZS/BMS/JRS