High Wind in Oregon, November 21, 2018

Area Forecast Discussion
National Weather Service Medford OR
916 AM PST Wed Nov 21 2018

.DISCUSSION...KMAX-88D radar reveals rain progressing well
through the west side and latest observations reveal a quarter to
a half inch of rain has fallen across the coast region through 9
am. Very light and spotty amounts inland of I-5 so far with just a
dusting of snow above about 6000 feet at Crater Lake and Mt.
Ashland. The cold front will push across the Rogue Valley and
eastward between about 2 and 6 pm with an associated increase in
rain expected and lowering snow levels. The forecast has things
pretty well in hand and no major shifts in the forecast are
planned early on today. Thanksgiving wind event looks on track
with vigorous short wave moving into the Oregon coast and
associated surface low to track across Newport by noon or so.
Initial thoughts are that high wind watches will be upgraded to
warnings with some additional advisories possible. Stavish

&&

.AVIATION...For the 21/12Z TAFs...VFR conditions will persist
through early this morning for most areas ahead of an incoming
front. Cigs will gradually lower to MVFR from west to east as the
front makes it's way inland later this morning. IFR conditions are
expected during periods of heavy rain, especially along the coast
and in the Umpqua Basin. Gusty south to southeast winds ahead of
the front will continue across the higher terrain, east of the
Cascades, the Shasta Valley and the southern end of the Rogue
Valley. These gusty winds will downslope off the Siskiyous,
maintaining VFR conditions in the Rogue Valley, including KMFR,
and likely holding off any precipitation until late this
afternoon. Winds will ease tonight, but another stronger front is
expected by early Thursday morning. /BR-y

&&

.MARINE...Updated 200 AM PST Wednesday 21 November 2018...Active
weather is expected through Saturday, with periods of heavy rain,
strong southerly winds and choppy wind driven seas combined with
moderate swell. The first in a series of fronts is moving through
the waters this morning, and southerly winds have increased in
response to this front. Seas will peak later this morning with
very steep hazardous seas expected mainly beyond 5 nm from shore,
and continued steep small craft advisory level conditions
elsewhere.

Another, stronger front is expected Thursday, and models continue to
show sustained gales developing over much of the waters. While we're
pretty confident in widespread gales with isolated areas of storm
force gusts, the latest model runs are showing a brief period of
possible sustained storm force winds developing in the coastal jet.
This is the first run to show this, so have decided to maintain the
watch for now and let the next shift see how the 12z model runs
handle this before upgrading. Regardless, conditions will be quite
hazardous Thursday morning and afternoon with very steep wind driven
seas of 14 to 16 feet peaking Thursday afternoon and evening.

Winds will diminish briefly Thursday night before another quick
moving system moves through the waters early Friday morning,
bringing another round of high-end small craft winds. Winds will
diminish thereafter through the weekend, but seas will remain
elevated through Saturday due to a mix of southwest and west-
northwest swell combined with wind chop. A break in the weather is
expected Saturday into Sunday, with gradually improving conditions
expected. /BR-y

&&

.PREV DISCUSSION... /Issued 350 AM PST Wed Nov 21 2018/
DISCUSSION...21/00Z NAM/GFS/EC in. A long wave trough axis will
move to 140W later today and southwest flow aloft is increasing
along the west coast. Short waves diving down the backside of the
trough will slow its progress. Those short waves will eject from
the trough and move onshore today into Friday. This will make for
a wet and occasionally windy pattern during this interval.

First up, a front that will move onshore today. Precipitation is
now starting along the coast, and will spread inland through
today. Most of the CWA will see significant, but not excessive,
precipitation from this system Wednesday into Wednesday night.
Given the low antecedent streamflows, flooding is not expected.

Winds have increased along the coast, in the Shasta Valley, and
over the west side ridges. They will increase soon over the east
side and the Rogue Valley as well. A Wind Advisory is in effect
for portions of these areas.

As far as snow goes, snow levels will be generally near or above
pass level, with some snow accumulating in the mountains above
about 5500 feet. Snow levels will be a bit lower near Mount
Shasta, but still mostly above 4500 feet.

Today is going to be a big travel day and the winds and
precipitation will impact travelers. Slick roads will be an issue
as this is the first precipitation in quite a while and there
will be oil buildups on the roads.

Second up, the wave on Thursday will be the strongest of the
string, and it will be quickly followed by another Thursday night
into Friday. The associated front will move onshore late Thursday
morning, and Thursday will be a wet and windy day across the
area. The winds will be strongest at the coast and along Highway
101 as well as on Interstate 5 in the Shasta Valley late Wednesday
night through Thursday. The strong winds will spread to the east
side Thursday morning lasting through Thursday night, impacting
most of the roadways there. High Wind Watches are in effect for
the coast, the Shasta Valley, and the east side. Wind advisories
will likely be necessary over some of the remaining portions of
the CWA.

As far as precipitation goes, this will be the first big rainfall
event this fall for southern Oregon and northern California.
Rain amounts of 3 to 6 inches will be common along the coast with
local amounts higher than that in areas that typically have
orographic enhancement, like the Coast Range. The Mt. Shasta area
could see some moderate to heavy rainfall as well. The heaviest
rainfall rates will occur during the day Thursday. There is
concern about precipitation on recent burn scars. A Flash Flood
Watch is in effect for the Klondike, Chetco Bar, Eclipse, and
Natchez burn scars for Thursday and Friday. Outside of those
areas, no flooding is expected other than minor high small stream
flows.

Several inches of snow will accumulate with the first storm
Thanksgiving Day, which could impact passes near Lake of the
Woods, but the heaviest snow will fall above 5000 feet. There will
be a surge in snow level up above 6000 feet late Thursday night
into Friday as the next storm moves onshore, but heavy snow could
continue up near Crater Lake. Snow amounts of 1 to 2 feet are
possible above 5500 feet through Friday night.

Long term discussion from the Tuesday afternoon AFD...Friday
Night through Tuesday...The Friday system will pass to the east
Friday night, and as a result, winds will taper off and
precipitation will diminish from west to east. By daybreak, some
showers may linger in the area, especially over the Cascades where
some light snow accumulations are possible. For the most part,
the region will be dry as upper level ridging builds in overhead.

An upper level impulse is expected to slide by to our north on
Sunday, but the bulk of the energy and moisture associated with the
system will remain well to our north. There is a possibility for some
rain and high mountain snow, particularly along the coast and into
the Umpqua Basin and Cascades, but amounts should remain light as
the front just grazes by the forecast area.

Until this point, the operational models and their respective
ensembles have been in fairly good agreement, but heading into early
next week, differences in timing, track, and strength of individual
systems make for a rather low confidence forecast. Guidance does
resolve another system arriving in the area Monday night or Tuesday,
and this could be another significant rain and wind producer. Have
kept precipitation chances high through this period as rain seems a
sure thing, but have kept winds modest in response to the wide
spread in guidance. Whatever the timing, the wet and active pattern
is expected to persist through next week. -BPN

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Watch from Thursday morning through late Thursday
     night for ORZ029>031.
     Wind Advisory until 10 PM PST this evening for ORZ031.
     Flash Flood Watch from Thursday morning through Friday afternoon
     for ORZ022-024.
     High Wind Watch from late tonight through Thursday afternoon for
     ORZ021-022.
     Wind Advisory until 4 PM PST this afternoon for ORZ026.

CA...High Wind Watch from Thursday morning through late Thursday
     night for CAZ084-085.
     Flash Flood Watch from Thursday morning through Friday afternoon
     for CAZ080.
     Wind Advisory until 4 PM PST this afternoon for CAZ081.
     High Wind Watch from Thursday morning through Thursday evening
     for CAZ081.

Pacific Coastal Waters...Gale Watch from late tonight through Thursday
     afternoon for PZZ350-356-370-376.
     Small Craft Advisory until 4 AM PST Thursday for PZZ350-356-376.
     Hazardous Seas Warning until 4 PM PST this afternoon for
     PZZ350-356-370-376.
     Small Craft Advisory from 4 PM this afternoon to 4 AM PST
     Thursday for PZZ370.