Small Craft Advisory 28, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
917 AM PDT Fri Mar 13 2020

...Updated Marine Discussion...

.DISCUSSION...No major updates needed this morning. Aside from
some scattered high level clouds and patchy areas of fog in
portions of the Coquille and Umpqua Basins, clear skies prevail
across the forecast area. Afternoon temperatures will be around 5
to 10 degrees cooler than previous days due to increasing cloud
cover moving into the area this afternoon. This will be the last
day of sunny skies with relatively warmer temperatures as today
will be a transition day to a much cooler and active weather
pattern that will be with us through at least mid-week next week.
An upper level low will dig offshore today, sending a cold front
through the area this afternoon and overnight. Snow levels will
sharply lower with this front, bringing the return of snow to
lower elevations. The focus for this afternoon's package will be
honing in on the details of the low snow levels and adjusting the
necessary winter weather headlines accordingly. Details on the
current forecast and expected weather can be found in the previous
discussion below. /BR-y

&&

.MARINE...Updated 915 AM PDT Friday 12 March 2020...Have updated the
marine weather warning to get rid of the mentions of gale
conditions, since they have subsided. The current forecast is rather
complex with gusty north winds and steep to very steep wind driven
seas continuing across the waters through this morning, with the
strongest winds occurring south of Gold Beach. The trough
responsible for these conditions will weaken late this morning and
seas will gradually subside, but only briefly. A front will move
across the waters later today. Steep to very steep swell dominated
seas will build into the area tonight and Saturday combing with
strong westerly winds that could see some isolated gale gusts north
of Cape Blanco beyond 10 NM. These seas will subside Saturday night
into Sunday, but should remain at small craft advisory levels.
Unsettled conditions will persist into next week, with north to
northeast winds Sunday night through Monday night and steep seas
developing Monday night. -Schaaf

&&

PREV DISCUSSION... /Issued 553 AM PDT Fri Mar 13 2020/

DISCUSSION...A frost advisory remains in effect for a portion of
the coast north of Port Orford early this morning. But, this is
the last vestige of our current, very soon to change weather
pattern of clear, cold nights and very warm, sunny days.

A cold front moving southeast out of the Gulf of Alaska will
bring a substantial, noteworthy shift in our weather this evening.
The conditions during the past few days would be typical for late
April while our wintry weekend weather will be similar to what
would usually be associated with mid-February. The associated
upper level low is moving relatively quickly down the Pacific
Northwest coast, but will slow down and persist over and then near
our region this weekend into early next week.

As stated, the cold front will track across the area tonight, but
a majority of the impact will be due to periods of showers and
snow showers in the long-lingering very cold post-frontal air
mass. The inherent hit or miss nature of showers and the effect
of small changes in the track/positioning of the closed upper
level low upon the snow level makes this a challenging forecast
of snow levels and precipitation/snow amounts. Thus, there is the
most confidence in the big picture of periods of higher elevation
snow and low elevation rain this afternoon through Sunday night.
Rain is likely to develop over the coastal waters, the coast from
Cape Blanco northward, and northern Douglas County northward
before the end of the afternoon. The front will sweep across the
remainder of our area tonight with the first in a series of post-
frontal disturbances already reaching the coast late this evening.

The main changes to the forecast were to expand in area and in
time the coverage of winter weather headlines (Please see the
PDXWSWMFR winter weather message for specific details). Also, the
addition of a slight chance of thunderstorms over a portion of
the southern coastal waters late Saturday night, then shifting
inland on Sunday. This is due to the upper low likely to be
centered offshore of Cape Mendocino, often a favorable location to
create convection, and good model agreement on a strong shortwave
rotating inland during peak heating Sunday afternoon.

Snow associated with the frontal passage will very likely be
initially focused on the higher peaks (above 4500 feet) of the
Cascades and Siskiyou Mountains from western Oregon into far
western Siskiyou County. But, the highest amounts of snow in our
area are very likely to be focused on the Kalmiopsis Wilderness in
far southwest Oregon as well as in western and southern Siskiyou
County with 7 to 15 inches forecast (with heavier amounts still to
the south, over the Sierra Nevada).

Snow levels are forecast to drop tonight to around 2000 feet by
early Saturday morning. But, warm soil/ground temperatures will
likely prevent any accumulation below around 2500 feet. The first
batch of post-frontal showers late tonight into Saturday morning
will also be focused roughly west of Interstate 5, except also
from northern Jackson County northward to include eastern Douglas
County. Meantime, at least a light accumulation of rain and snow
is likely across the area by the end of Saturday morning.

Snow levels will likely rise slightly during the day Saturday then
reach their lowest levels early Sunday morning at 1500 to 2000
feet, locally slightly lower over the coastal range of mountains.
Day-time heating should bring another modest increase of snow
levels during Sunday to around 2500 to 3000 feet. The main
episodes of showers are expected through Sunday evening.

Model agreement diminishes beyond Sunday evening with the low
expected to continue southward along the California coast. But,
uncertainty in the track/timing of shortwaves rotating around the
northern fringe of the low leads to uncertainty in just how far
north that new batches of showers/snow showers will occur during
Monday into Wednesday. The departure of the core of the upper low
will bring a modest warming trend during the early portion of the
work week, back up to around normal values.

The upper level pattern is likely to become more complex at mid-
week,continuing into the next weekend. The upper low is likely to
swing eastward into the Great Basin of Nevada while another broad, cold
upper trough moves south out of the Gulf of Alaska...likely
remaining farther offshore than this weekend's weather producer.
This will result in a frequent though mostly relatively low
probability of showers, with a currently low confidence in the
daily details.

AVIATION...For the 13/12Z TAF Cycle...VFR conditions will prevail
initially, but an incoming front will bring areas of MVFR cigs/vsbys
in low clouds and rain to the area. The lower conditions will
develop over the coastal waters soon, then move onshore late this
morning with higher terrain becoming obscured. The lower conditions
will spread inland to the Cascades and Siskiyous Friday afternoon
and evening, then prevail through Friday night. Gusty west to
southwest winds will develop ahead of the front, especially at the
coast, over ridgelines, and across the East Side, and mountain
turbulence is likely downwind of higher terrain. Freezing levels
will drop to around 3000 feet as the front passes onshore, dropping
further Friday night.

MARINE...Updated 230 AM PDT Friday 12 March 2020...Strong north
winds and steep to very steep wind driven seas will continue across
the waters through tonight, with Gales south of Gold Beach. The
trough will weaken Friday morning and seas will gradually subside,
but only briefly, since a front will move across the waters later
Friday. Steep to very steep swell dominated seas will build into the
area Friday night and Saturday, subsiding Saturday night into
Sunday. Unsettled conditions will persist into next week, with north
to northeast winds Sunday night through Monday night and steep seas
developing Monday night.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 8 PM this evening to 5 AM PDT
     Sunday for ORZ027-028.
     Winter Storm Watch from this evening through Sunday evening
     above 2500 feet in the for ORZ024.

CA...Winter Storm Watch from late tonight through Sunday evening for
     CAZ082-083.
     Winter Storm Watch from this evening through Sunday evening
     above 2500 feet in the for CAZ080.

Pacific Coastal Waters...Small Craft Advisory until 11 AM PDT Saturday for PZZ356.
     Hazardous Seas Warning from 5 PM this afternoon to 11 AM PDT
     Saturday for PZZ350-356-370-376.
     Small Craft Advisory until 11 AM PDT this morning for PZZ350.
     Small Craft Advisory until 5 PM PDT this afternoon for PZZ376.
     Hazardous Seas Warning until 11 AM PDT this morning for PZZ376.
     Small Craft Advisory until 5 PM PDT this afternoon for PZZ370.