Winter Storm Warning 12, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
1004 AM PST Wed Mar 8 2023

.UPDATE...The AVIATION discussion has been updated. -Spilde

&&

.AVIATION...08/18Z TAFs...IFR conditions lasted until an hour or so
ago at Roseburg and are still impacting the terminal at Medford, but
expect these to lift to MVFR or better in the next hour or so as the
atmosphere destabilizes again. This will bring mostly a mix of
MVFR/VFR to the area this afternoon with areas of higher terrain
obscured and showers, though showers will be less numerous than
recent days. Snow levels are expected to rise to around 2000 feet
this afternoon, but snow pellets and/or graupel could be mixed in
with any of the showers down to around 500 feet. A break is expected
later tonight with mainly VFR, but patchy valley IFR possible. Upper
level winds will increase along the coast and eventually just inland
this evening and overnight. With mostly light surface winds, there
is the potential for low-level wind shear at North Bend and
Roseburg into Thursday morning. -Spilde

&&

.MARINE...Updated 800 AM Wednesday, March 8, 2023...Low pressure
will bring showers and brief gusty winds at times today, but winds
generally remain sub-advisory with a brief period of calmer seas
this morning. Conditions deteriorate rapidly early this evening as a
strong low brings south gales and very steep and high seas tonight
through Thursday night. Seas will likely remain high and very steep
behind the front Friday, while the next front approaches. Winds on
Friday are forecast at advisory strength, but a minority of models
do indicate gales. Briefly improving conditions are likely Saturday
ahead of the next front on Saturday night or Sunday. -DW/CSP

&&

.PREV DISCUSSION.../Issued 847 AM PST Wed Mar 8 2023/

UPDATE...Made a small update to the forecast adding some areas
fog through 10 AM that are reducing visibilities to between a
half and quarter mile in the Phoenix/Medford and Grants Pass
areas. A stratus deck around 300 feet has also formed in the
Umpqua basin that is causing some foggy conditions along roads
leading away from the basin floor. So far, no fog in the Illinois
or Coquille Valley.
-Miles

DISCUSSION...The heaviest snow showers are over the Cascades and
spreading across the east side. On the road cameras this is
showing roads becoming snow covered or limited visibility due to
heavy snowfall rates.

The first of the winter weather headlines to come down is the
advisory in the Shasta Valley. This is likely to be followed by
the rest of the headlines at 10AM. This won't be the end of the
snow in the region. Shower activity will persist this afternoon as
the current wave departs and another area of low pressure drops
out of the Arctic and parks itself offshore of the PNW.

Temperatures along the S. Oregon coastline where we had some
concern about frost/freeze so far tonight have remained above
freezing in our observations. Temperatures currently range between
34 and 37 degrees. This would imply some frost is possible for
locations well sheltered but, winds aren't totally calm and shower
activity remains ongoing. This would suggest there is enough
turbulence to stall frost formation for all but the most well
sheltered areas. Some of these showers may become intense enough
along the coast to produce a few lightning strikes too once the
sun warms things up this afternoon.

Despite my choice of words in 'warm' -everything is relative to
the cold of the night and to the East Side of the Cascades- we are
forecasting a few cold temperature related records on Wednesday
to be challenged. The low-maximum record temperatures at Alturas
(30 degrees in 2002), Mt. Shasta City (33 degrees in 1999), and
Medford (43 degrees in 2006) are forecast to tie or be within a
degree of the records.

Temperatures are expected to remain cool enough through the
afternoon that as snow showers increase in SW upslope flow at Mt.
Shasta this evening, there could be impactful accumulations from
Mt. Shasta City south on the roads. Amounts from 10PM onward range
from 2.5" to 5", which would meet advisory criteria. Given the
current headline, the break this afternoon, and a more potent
system to follow Thursday, we will not be issuing a headline on
this shift for this period of snow. Be on the lookout for a short
notice advisory in the afternoon update if models continue to show
the shower activity tonight.

Thursday morning that area of low pressure compresses and focuses
the energy within it along a corridor of moisture originating
from the tropics. This atmospheric river (AR) will have
significant impacts to our south, as we remain on the northern
edge. Being on the north side will yield a mix of hazards and
reduce the damage to the snowpack. The hazards include snow,
winds, and potential flooding.

The afternoon shift put out high wind watches yesterday and those
still remain. Tonight, winter storm watches have been put out for
the initial AR. More detail on the snow concern can be found in
the watch text product (WSWMFR). Then that leaves the flooding
potential. Both mainstem, small stream, and areal flooding will be
possible. The two mainstem rivers that are forecast to reach
action stage by the NW River Forecast Center, are the S. Fork of
the Coquille at Powers and Myrtle Point, then downstream for the
Coquille at Coquille, and Deer Creek at Roseburg. These rivers are
forecast to reach action stage at this weekend.

Some added detail on what is driving the flooding potential is
below-
The snowpack is well above normal, ranging from an inch or
two only a few hundred feet above the valley floors up to above
120 inches in the mountains. At Crater Lake, the depth is 23
inches above the average (as of March 7th). The snow water
equivalent is a little closer to normal at 100 to 110 percent
because the snow has been drier than we are used to observing.
This is important because the incoming rain on snow is of concern
for one, degrading our snowpack, and two, causing some flooding
concerns as it melts. The dry snow is also cold though and as
discussed before, we remain cold right up to this AR's arrival.
This cold snowpack will also help prevent a rapid and widespread
melt-off.

When snow levels lift to 4000 to 5000 feet Thursday night, the
highest snow levels are forecast over the coast range and Siskiyou
County. This means that there will be rain falling on the low to
mid-elevation snow. This could lead to rapid snow melt causing
small stream flooding and standing water. This is especially true
for the second AR sweeping through on Sunday because as described
above our current snowpack will offer some initial resistance.

The Sunday AR still varies in timing but, it is well agreed upon
to occur. The highest snow levels are forecast to be 5000 to 6000
feet. And this will double the rain total in the coast range at 5
to 7 inches (liquid) from Sunday into Tuesday. While the threat
of small stream flooding may be present as both systems pass
through the first system has a lower chance as it may serve more
to warm up the snowpack. Whereas the second system brings heavier
rainfall, will be a bit warmer, and could tap into the inch or two
of latent water existing in the snow pack. This combination is
more likely to cause more widespread impacts from standing water
and small stream flooding.
-Miles

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Watch from Thursday afternoon through Friday
     afternoon for ORZ029>031.

     High Wind Watch from Thursday evening through Friday evening for
     ORZ030-031.

     Winter Storm Warning until 10 AM PST this morning above 1500
     feet for ORZ022.

     Winter Weather Advisory until 10 AM PST this morning above 500
     feet for ORZ022.

     Winter Weather Advisory until 10 AM PST this morning above 1500
     feet for ORZ023-026.

     Winter Storm Warning until 10 AM PST this morning for ORZ024.

     Winter Weather Advisory until 10 AM PST this morning for ORZ024.

     Winter Storm Watch from Thursday afternoon through Friday
     afternoon for ORZ027-028.

CA...Winter Storm Warning until 10 AM PST this morning for CAZ080.

     Winter Weather Advisory until 10 AM PST this morning for CAZ080.

     Winter Storm Watch from Thursday morning through Friday
     afternoon above 4000 feet for CAZ080.

     High Wind Watch from Thursday afternoon through Friday afternoon
     for CAZ081.

     Winter Storm Warning until 10 AM PST this morning for CAZ082-
     083.

     Winter Storm Watch from Thursday morning through late Thursday
     night for CAZ082-083.

     Winter Weather Advisory until 10 AM PST this morning for CAZ085.

     Winter Storm Watch from Thursday afternoon through Friday
     afternoon for CAZ085.

     High Wind Watch from Thursday evening through Friday evening for
     CAZ085.

PACIFIC COASTAL WATERS...Small Craft Advisory from 4 PM this
     afternoon to 1 AM PST Thursday for PZZ350-356-370-376.

     Gale Warning from 1 AM Thursday to 4 AM PST Friday for PZZ350-
     356-370-376.

&&

MAS/MAS/MAS