Winter Weather Advisory 39, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
934 PM PST Sun Dec 26 2021

.UPDATE...
Updates were made to the forecast this evening to add a Freeze
Warning for portions of the coastal area that typically
experience impacts from sub-freezing temperatures in the winter. A
slight chance of thunderstorms was also added to the forecast for
this evening through 7 AM PST Monday morning for the coastal
waters areas and areas inland from the coast including and just
inland from the coastal mountains. Probabilities for measurable
precipitation for tonight through Monday were also refined.

&&

.DISCUSSION...
Cold low pressure in the upper levels to the tune of -30 to -35C
at 500mb continues to spin over the area this evening leading to
widespread snow showers for most locations at elevations at or
above 500 feet above sea level. However, a look at observations
and the air mass moving in from the northwest tonight indicates
that some locations below 500 feet MSL are likely already snowing
and we do expect snow to fall and accumulate tonight into Monday
morning all the way down to or very close to the coast. A Winter
Weather Advisory remains in effect for the coastal zones through
10 AM PST Monday morning. We've also added a Freeze Warning to
portions of the coastal zone that can experience some agricultural
impacts this time of the year from sub-freezing temperatures.

Elsewhere, snow has been very persistent this afternoon into this
evening and continues to come in showery waves at most locations.
While temperatures this evening are quite close to what they were
24 hours ago for most locations, we'll be experiencing
significant cold air advection across the forecast area over the
next 24 hours as the western portion of the upper level trough
that's been over the area swings through. In fact, 850mb/4850ft
MSL temperatures above Medford are expected to fall from -5C this
evening to -10C tomorrow evening. Thus, the lower level air mass
is expected to cool about 9F in the next 24 hours. What this means
is that areas that have been marginal for snow accumulation and
have seen mostly water on roadways are likely to see what moisture
is liquid freeze tonight through Monday evening.

Showers are expected to continue for most location overnight and
then will gradually taper off across the area Monday morning.
There will likely be some showers over WNW upslope favored
locations, primarily the Oregon Cascades and Warner Mountains
Monday afternoon and evening.

Forecast snowfall amounts in tonight through Monday's forecast
look on track with the usual caveats- For locations that are at
or below freezing now, amounts should be pretty accurate. The
challenge is for locations that are currently above freezing, as
accumulations will be primarily snowfall rate driven until
temperatures fall to below freezing. All in all, the guidance
indicates showers continuing, with a notable shortwave trough
pushing in on the front side of the incoming surge of colder low
level air late tonight into Monday morning for areas along and
west of the Cascades. Thus, expect an increasingly snowy
situation to continue into Monday morning for those locations
followed by colder temperatures than we've seen, thus far, during
this multi-day snowfall event.

A lighter round of snow is likely Monday night through Tuesday
night as a warm front pushes in from the coast. ~BTL

&&

.AVIATION...27/06Z TAFs...Showers, mainly snow except along the
coast and coastal waters where some rain or a wintry mix is
expected, will continue through tonight, with another burst of
activity expected between 06 UTC and roughly 12-15 UTC as an upper
level wave passes overhead. Expect MVFR/IFR conditions with terrain
obscurations, with local LIFR ceilings and visibilities within areas
of moderate to heavy snow. Showers will decrease in coverage and
intensity, and flight conditions should improve at least slightly
Monday morning behind the departing energy, but showers are expected
to continue throughout the TAF period with VFR/MVFR prevailing in
most areas by afternoon. Blowing snow may decrease visibility
further across the East Side and over higher terrain, where gusty
winds are expected to continue through the next several hours. -BPN

&&

.MARINE...Updated 900 PM PST Sunday, 26 December, 2021...
There is a slight chance of thunderstorms across the coastal
waters area tonight into early Monday morning as a very cold air
mass moves through. Gusty northwest winds will gradually diminish
through tonight, with stronger wind gusts expected within showers.
Very steep and hazardous seas are peaking this evening but will
persist across most all the waters through Monday morning, then
seas will gradually lower but remain steep into Monday evening.
Widespread showers will continue into Monday with periods of mixed
rain and snow showers possible tonight and Monday morning.
Additional fronts this week may bring periods of gusty winds and
steep to very steep seas. ~BTL/BPN

&&

.PREV DISCUSSION... /Issued 305 PM PST Sun Dec 26 2021/

DISCUSSION...Showers continue across the region, and with
extensive snow cover over almost all of the region except for very
close to the coast, and a cold air mass over the area,
temperatures and snow levels remain very low. This has kept almost
all of the showers inland with temperatures significantly lower
than MOS guidance has shown. Numerous accidents and power outages
are being reported across the region with the low elevation snow
causing hazardous driving conditions and the heavy wet snow
causing tree and tree limbs to topple, some onto power lines,
causing the outages.

Another strong short wave swings down in the upper trough over the
region tonight bringing another round of snow, briefly heavy at
times, to lower elevations tonight. With the continuing cold
temperatures with temperatures dropping below freezing at almost
everywhere inland from the immediate coast, the snow on untreated
road surfaces will bring increasing icy driving conditions into
Monday morning.

The upper trough then shifts east with a break in the action into
Monday evening before another front brings possible advisory
amounts to the west side, with low elevations once again Monday
night into Tuesday. With the current warnings over the region,
will wait until the current warnings are close to expiring before
issuing to reduce confusion with the already multiple warnings and
advisories in effect. -Sven

LONG TERM...Wednesday (Dec. 29) through Sunday (Jan. 2)...
The remainder of the week will be characterized by frigid
temperatures and nuisance fronts.

While areas west of the Cascades generally have a high (85+%) chance
of getting above freezing each afternoon, areas east of the Cascades
do not. At night, overnight lows will drop back below freezing for
all locations, including portions of the coastal lowlands. This
will make it possible for ice to form each morning resulting in
potential slipping hazards. A pronounced change in this cold
pattern does not look likely until the Monday of the following
week (Jan. 3) at the earliest.

Wednesday morning a front will be exiting the region and snow
showers will be on the decline. Snow levels Wednesday morning should
be around the west side valley floors (1000 feet). Then there will
be a quiet period until another front moves through around Thursday
evening (Dec. 30). With this front most likely moving through during
the overnight timeframe and arctic air trapped in the valleys, snow
down to the valley floors is possible (for 0.1" of snow or more the
probability is generally 70%, but for 1" or more of snow the
probability is generally 55%). This front won't bring heavy mountain
snow. 24 hour totals should be less than a foot. The main range that
is expected to observe the most snow is the Cascades from Lake of
the Woods north, with most other ranges expected to observe quite a
bit less snow (around 8 inches or less).

Another break is expected Friday before a second front should move
through on Saturday. The timing of this front still varies between
arriving in the morning or later in the afternoon. For now this also
looks like a fairly light front as far as amounts. Snow levels
should lingering near the valley floors to start, but with warmer
characteristics than any of the proceeding front this one is likely
to raise snow levels as it moves onshore. This could create it's own
complications because a near surface cold layer with warm air aloft
could make for icing issues along the snowline if the front moves
through later in the day. But being a week out more time is needed
to see how the atmosphere evolves.   -Miles

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 7 AM PST Monday for ORZ029>031.
     Winter Storm Warning until 7 AM PST Monday for ORZ029>031.
     High Surf Advisory until 10 PM PST this evening for ORZ021-022.
     Winter Storm Warning until 8 AM PST Monday for ORZ027-028.
     Winter Weather Advisory until 10 AM PST Monday for ORZ021-022.
     Freeze Warning from 1 AM to 10 AM PST Monday for ORZ021-022.
     Winter Storm Warning until 7 AM PST Monday for ORZ023>026.

CA...Winter Weather Advisory until 7 AM PST Monday for CAZ084-085.
     Winter Storm Warning until 7 AM PST Monday for CAZ085.
     Winter Storm Warning until 10 AM PST Monday for CAZ080-082-083.
     Winter Weather Advisory until 10 AM PST Monday for CAZ081.

Pacific Coastal Waters...
     Small Craft Advisory until 10 PM PST Monday for PZZ356.
     High Surf Advisory until 10 PM PST this evening for PZZ350-356.
     Hazardous Seas Warning until 1 PM PST Monday for
     PZZ350-356-370-376.
     Small Craft Advisory from 1 PM to 10 PM PST Monday for
     PZZ350-370-376.

BTL/BPN/CC

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE MEDFORD OR
1106 PM PST SUN DEC 26 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0800 PM     SNOW             1 SSE ASHLAND           42.18N 122.69W
12/26/2021  M7.5 INCH        JACKSON            OR   TRAINED SPOTTER

            24 HOUR TOTAL. STROM TOTAL NOW 11.5 INCHES.

&&

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE MEDFORD OR
1223 AM PST MON DEC 27 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0800 PM     SNOW             3 E MEDFORD             42.34N 122.80W
12/26/2021  M2.8 INCH        JACKSON            OR   TRAINED SPOTTER

            8AM TO 8PM. SNOW DEPTH 5.5.

&&

CZS

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE MEDFORD OR
1228 AM PST MON DEC 27 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1225 AM     SNOW             3 SW MEDFORD            42.31N 122.89W
12/27/2021  E3.0 INCH        JACKSON            OR   PUBLIC

            STORM TOTAL STARTING ON THE 24TH. MOST OF
            THE SNOW FELL ON THE EVENING OF THE 24TH.

&&

CZS