Winter Weather Advisory 25, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
807 AM PDT Wed Mar 17 2021

.DISCUSSION...No updates are needed this morning. Another chilly
morning to start the day, although temperatures are about 5
degrees warmer this morning compared to this time yesterday
morning. The exception though is Alturas, CA were the 24 hour
difference is 20 degrees warmer thanks to a stratus deck covering
portions of Modoc, Lake and Klamath counties. There's also some
high level cirrus streaming over the area this morning, and this
is in advance of the next system expected late tonight into
Thursday. Dry conditions are expected today, along with seasonable
afternoon temperatures, and southerly winds will begin increasing
this afternoon and evening. For more details on the active weather
tonight into Friday, see the previous discussion below. /BR-y

&&

.AVIATION...For the 17/12Z TAF Cycle...VFR will prevail through
this evening. The exception will be localized areas of IFR/LIFR low
clouds and fog in some West Side valleys, especially in the Umpqua
Basin, early this morning. Ahead of the next front, breezy to gusty
winds are expected to develop late Wednesday afternoon and evening
along the coast, over the mountains, in the Shasta Valley, in the
southern Rogue Valley and east of the Cascades, with some light
showers just starting to move into the coast around the end of the
TAF period. -CC/BPN

&&

.MARINE...Updated 800 AM PST Wednesday 17 Mar 2021...NDBC buoy data
continues to trickle back in to NWS systems. Buoy data is being read
over NOAA weather radio and is updated every hour, but the NDBC
website remains down. Information from the Umpqua Offshore Buoy 229
is available at cdip.ucsd.edu.

South winds will increase today as another strong front approaches
the area. Strong gales and very steep wind-driven seas are expected
beginning this evening and continuing into Thursday until the front
moves onshore. Then, the fresh swell from the gales will mix with a
very high and very steep long period west swell Thursday night into
Friday. Large breaking waves around 22 to 26 feet are possible, with
area bar conditions becoming hazardous.

Additional fronts keep winds moderately strong through Friday into
Saturday morning. Then, high pressure will bring a break in the
frontal activity Saturday into Sunday, then another front will move
onshore Sunday night into Monday. -BPN/Spilde

&&

.PREV DISCUSSION... /Issued 504 AM PDT Wed Mar 17 2021/

DISCUSSION...Today starts with the upper level ridge over the
region shifting east into the great basin ahead of a system that
brings precipitation to the coast late tonight. Southerly winds
increase today, feeding moisture into the region ahead of the
approaching front. Southerly winds ahead of the front increase at
the coast this evening, but with an offshore component to the BL
winds, winds parallel to the coast up to 700 MB, and a weak south
to north gradient a coastal jet is not expected to form and
Warning strength winds are not expected on the coast. Inland the
Redding to Medford gradient tightens up by sunrise and gusty
south winds spread from the Shasta Valley into the Rogue Valley in
Jackson county. The gradient is indicating high end advisory
gusts, and the MOS guidance for Weed is indicating this also, and
have issued a wind advisory for both areas, East of the Cascades a
broad area of southwest 700 MB winds in the 50 to 60 KT range is
also indicating advisory strength winds and RAWS MOS guidance is
in line with this also. Have issued a wind advisory in this area
as well.

As this system moves in it will also draw in a weak to moderate
atmospheric river at mid levels to augment the low level moisture
feed from the Sacramento River valley. The strong southerly flow
will bring very strong orographic lift to southern Siskiyou
county, especially in the Mt. Shasta area, squeezing out the bulk
of the moisture in that area through the day. With a southwesterly
component to the steering winds the coastal ranges and western
Siskiyou county will also see one to two inches of precipitation
with this system and a following shortwave on Friday. Snow levels
will generally be around feet in the Mt. Shasta are, but may drop
briefly as low as 3000 feet Thursday morning. Wet snow will
combine with strong winds, and may cause broken branches and even
possible power outages.

A cold air mass moves in at 500 MB, with temperatures dropping
below -30C in the coastal waters, and near the coast by Thursday
evening. This cold air mass aloft remains over this area into
Friday, bringing a slight chance of thunderstorms to the coast
waters and near the coast and inland a bit with steering winds
from the southwest at around 30 KT moving any that develop inland
until they weaken. An active pattern remains into the weekend.
Sven

Coastal Hazards...Updated 200 AM PST Wednesday 17 Mar 2021...
Following the mid-week storm system, long period westerly swell is
expected to enter the southern Oregon coastal waters late
Thursday through Friday. Latest wave guidance has this swell
peaking at around 19 feet at 15 seconds Friday morning, which
would produce breaking seas of around 23 feet along area beaches
and shorelines, thus prompting the need for a high surf advisory.
Area rocks and beaches will become hazardous, and erosion could be
a concern. -BPN

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory from 8 AM to 11 PM PDT Thursday for ORZ030-031.
     Frost Advisory until 9 AM PDT this morning for ORZ021-022.
     High Surf Advisory from 5 PM Thursday to 5 PM PDT Friday for
     ORZ021-022.
     Wind Advisory from 5 AM to 8 PM PDT Thursday for ORZ026.

CA...Wind Advisory from 8 AM to 11 PM PDT Thursday for CAZ085.
     Winter Storm Watch from late tonight through Thursday evening
     above 4000 feet in the for CAZ082-083.
     Wind Advisory from 5 AM to 8 PM PDT Thursday for CAZ081.

Pacific Coastal Waters...Gale Warning from 8 PM this evening to 11 PM PDT Thursday for
     PZZ350-356-370-376.
     Hazardous Seas Warning from 11 PM Thursday to 11 AM PDT Friday
     for PZZ350-356-370-376.

MNF/SBN/BPN