Heavy Snow in California, February 28, 2018

Area Forecast Discussion
National Weather Service Medford OR
907 AM PST Wed Feb 28 2018

.UPDATE...Light snow continues this morning as a warm front pushes
in from the west. Precipitation will remain light through the
morning and into the afternoon, with levels rising to about 3000
feet as the warm front fully arrives. The big event begins
tonight, with the cold front arriving and pushing snow levels back
down. Numerous winter weather headlines remain in effect, and
details on timing and snowfall amounts can be found at WSWMFR.

Once the cold front passes, the upper level trough will dig south
and set up over our forecast area. Although no winter weather
advisories or warnings are currently in effect for Thursday night
into Friday, this will be the period where low elevation snow is
expected. The focus of today's forecast efforts will be to
maintain the current headlines, then to expand headlines as
necessary for the Thursday night to Friday period. Expect an
update by this afternoon with this new information.

For more information on this and the rest of the forecast, see the
previous discussion below. -BPN

&&

.AVIATION...28/12Z TAF CYCLE...Current VFR conditions will gradually
lower to MVFR as the morning progresses as a significant winter
storm approaches. A warm front will move through this morning and
light precipitation is possible. Given the cold temperatures, it's
possible to see some snowfall KMFR, but accumulations should be very
light, if at all. Freezing levels will rise later this morning and
precipitation will transition to rain west of the Cascades. East of
the Cascades, freezing levels will remain low and any precipitation
will fall as snow.

Areas of MVFR and local IFR cigs/vsbys in low clouds and rain/snow
will spread inland to the Umpqua Basin this morning and to the
Cascades later this morning. Expect terrain to become obscured.
Southerly winds will increase later this morning into the afternoon
with strongest winds at the coast, east of the Cascades, in the
Shasta Valley, and over the higher terrain. Strong winds aloft will
lead to LLWS concerns for valley locations west of the Cascades
starting late this afternoon and going into the overnight hours as a
strong cold front moves inland. BR-y/FB

&&

.MARINE...Updated 830 AM PST Wednesday 28 Feb 2018...A cold front
will bring heavy rain and increasing south winds late this morning.
Hazardous seas are expected north of Bandon and beyond 5 nm from
shore later this morning with small craft conditions expected
elsewhere. Hazardous seas will expand in coverage late this
afternoon to affect all of our marine zones, with south gales within
30 NM of the coast. Initially gales will affect areas down to
Bandon, but will expand to cover much of our marine zones.

Winds will diminish early Thursday morning after the front moves
onshore, but a steep and high northwest swell will follow Thursday
into Friday evening. Swell dominated seas of 15 to 18 feet at 13 to
15 seconds are expected during this interval. In addition, a low
will move into the waters Friday, which will bring in another round
of gusty south winds. Gales may occur with this low.  Conditions
will improve Friday night into Saturday, but it will remain cold and
unsettled through the weekend. -JRS

&&

.PREV DISCUSSION... /Issued 422 AM PST Wed Feb 28 2018/

DISCUSSION...Models continue on track for March to enter like a
lion, a big snowy lion.

When I came in to work at 11 PM it was looking like a bust with
the low temperatures with just a thin cirrus layer and a
temperature of 34 degrees. However, the clouds became thicker and
the low dropped to 32 degrees around 1:30 before rising back to 34
degrees as the clouds became overcast and are now continuing to
lower as the virga descending into the dry layer beneath forces
its way slowly to earth. The warm front that is bringing this
precip sheild to the region will move north today ahead of the
warm front that will approach the coast tonight. A moderate, brief
atmospheric river event continues to be indicated with this
system, but with a cold air mass in place over the region and with
the 700 mb flow just a bit south of westerly snow levels will
remain lower than is usual with an atmospheric river event. A
current temperature of 24 F is a good indicator of what can be
expected in Northern California with this system.

Models are showing precipitation moving into the Cascades by 18z,
but at lower levels there will probably be a lag until the low
level dry layer is finally saturated enough for precip to make it
to the ground. Precipitation rates will increase through the day
as an 85 kt jet moves over the region and southwesterly 700 mb
winds increase to 50 kt over portions of the forecast area. With
increasing southerly BL winds funneling moist air from the Shasta
valley strong orographic lift will increase precip rates in
Northern California overnight into Thursday morning, and the
increased precip rates will drop snow levels below 2500 feet at
times in the Mt Shasta area. As we head into Thursday morning the
left exit region of a very strong +130 kt jet bumps up the dynamic
ante as the jet moves inland. With a much stronger jet on the
back side of the closed upper low I think there is a good chance
that this system might stall longer than models are indicating
into Thursday evening. The latest run is now indicating the upper
low lingering off the coast of the Pacific Northwest into the
weekend.

Sven

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 1 PM this afternoon to 1 PM PST
     Thursday for ORZ029>031.
     Gale Warning from 7 PM this evening to 1 AM PST Thursday for
     ORZ021.
     Winter Weather Advisory from 7 PM this evening to 1 PM PST
     Thursday above 2000 feet in the for ORZ021>026.
     Winter Storm Warning from 1 PM this afternoon to 1 PM PST
     Thursday for ORZ027.
     Winter Weather Advisory from 1 PM this afternoon to 1 PM PST
     Thursday for ORZ027-028.

CA...Winter Weather Advisory from 1 PM this afternoon to 1 PM PST
     Thursday for CAZ084-085.
     Winter Storm Warning from 1 PM this afternoon to 1 PM PST
     Thursday for CAZ080.
     Winter Storm Warning from 1 PM this afternoon to 1 PM PST
     Thursday for CAZ082-083.
     Winter Weather Advisory from 1 PM this afternoon to 1 PM PST
     Thursday for CAZ080-081-083.

Pacific Coastal Waters...Small Craft Advisory until 4 PM PST this afternoon for
     PZZ350-356-370-376.
     Small Craft Advisory for hazardous seas from 1 AM Thursday to 10
     AM PST Saturday for PZZ350-356-370-376.
     Gale Warning from 7 PM this evening to 1 AM PST Thursday for
     PZZ350-356-370-376.
     Hazardous Seas Warning from 4 PM this afternoon to 1 AM PST
     Thursday for PZZ350-356-370-376.

BPN/JRS/MNF/SBN