Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 161058

Area Forecast Discussion
National Weather Service Portland OR
258 AM PST Sat Dec 16 2017

.SYNOPSIS...Cool cloudy weather will continue this weekend, along
with areas of rain, though rain threat mainly north of a Tillamook to
Mt Hood Line. Not much change in the pattern until Tue, then will see
much stronger front arrive, with rain and mountain snow increasing.
Then will turn colder but dry later next week.


.SHORT TERM...Today through Monday...Main weather challenge this am
is the areas of fog and low clouds along with patchy freezing fog. As
temperatures dropped below freezing Fri evening, the moisture on the
roads froze, resulting in slick roads due to what is common called
`black ice.` Now, with fog in place, likely will continue to see
slick spots into this morning, primarily in the rural areas from the
coast mountains across the interior lowlands to the Cascades

Otherwise, not much change to the current weather pattern. Moist
westerly flow maintain steady stream of moisture aloft in the Pac NW,
but most of the precipitation will be over British Columbia and
western Washington, and just brushing northwest Oregon at times this
weekend. Seems highest threat of precipitation will be late tonight
into Sun am. Appears areas south of a Newport-Albany line will
generally be dry, but expect decent amount of valley fog in the
overnight and early morning hours.

Snow levels will hover between 2500 and 3500 feet today and tonight,
then slowly rise back to 4000 to 5000 feet later Sun. But since not a
lot of QPF expected, suspect will generally only get a few inches of
new snow for the Cascades, with 2 to 5 inches over south Washington
Cascades. Not enough for advisories, but since been a while since had
snow, motorists should be prepared for snowy and slick conditions at
times over the Cascades, from Santiam Pass northward.     Rockey.

.LONG TERM...Monday night through Friday...We start next week under
relatively flat, zonal flow with a broad ridge off the coast of
California and an approaching trough coming out of the Gulf of
Alaska. We will see our first widespread rain in a while with this
system starting Tuesday morning and continuing through Wednesday
morning, with rainfall amounts around an inch for most of the area.
Snow levels will be falling through this event, starting around 5000-
5500 ft Tuesday morning, and falling to around 1500 ft by Wednesday
morning. Showers will be tapering off midday Wednesday. 1000-500-mb
thicknesses drop down to 523 dam, and 850 temps only lower down to
-4C while precip is falling, indicating no chance for snow down to
the valley floor with this system, though locations in the Cascade
Foothills and Coast Range could see a dusting of snow down to 1500
ft on Wednesday as precip is coming to an end. Good news for skiers
is that with ~1 inch of liquid equivalent, and snow levels starting
around 5500 ft, the resorts could see a foot of snow with this

Beyond Wednesday, ridging builds back up over the eastern
Pacific and along the West Coast. Ensemble members generally agree
on some sort of a ridge over the eastern Pacific, though the exact
positioning is in flux. Most models suggest another dry spell
continuing into the weekend, with the return of east winds.  Only
difference from this last dry spell may be that model temperatures
in the Columbia Basin are looking a little colder, suggesting colder
air filtering into the Willamette Valley as we go into next weekend.


.AVIATION...Mostly VFR conditions across the area with a mix of
IFR to LIFR in fog for the interior. Also sub-freezing
temperatures for some locations inland is likely leading to some
freezing fog south of KUAO and near KHIO. Fog development is
complicated by approaching warm front sending mid level clouds
into the area so expect variable conditions through this
morning. An increase in wind and thickening clouds may help
dissipate the fog, but weak warm air advection will bring low VFR
or MVFR cigs and rain chances later today and probably tonight
as well.

PDX AND APPROACHES...Moderate chances for fog to bring vsby
restrictions near or even at the terminal this morning. Otherwise
with advancing warm front, MVFR conditions becoming likely
later this morning and probably continuing through this evening.


.MARINE...High pressure offshore bringing NW winds across the
waters this morning. Winds will back later today as a warm front
approaches. South to southwest winds increase tonight with gusts
to 25 kt expected west in zone PZZ270. May see wind gusts to 25
kt over PZZ250 Sun. A stronger front is modeled to arrive on
Tue, and looks like it has the potential to bring gale force
winds. Another front may arrive Wed. Then weak offshore flow is
expected to develop in the late week time period.

Seas are sitting just 8 to 9 ft and are slowly subsiding. However
with periods around 8 to 10 seconds, choppy steep seas are
occurring. Seas will continue to subside through the morning.
Seas remain in the 5-7 ft range later today through Mon. The
stronger Tue front will likely push seas into the low to mid
teens before they drop off again during the second half of the
week. /mh




PZ...Small Craft Advisory for winds from midnight tonight to
  7 am Sunday on coastal waters from Cape Shoalwater
  to Florence from 10 to 60 nm offshore.
     Small Craft Advisory for hazardous seas for all coastal
  waters until 9 am today.
     Small Craft Advisory for Rough Columbia River Bar from 2 PM
     this afternoon to 6 PM PST this evening.



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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. This area is
commonly referred to as the CWA, or forecast area. is the U.S. government's official web portal to all federal, state and local government web resources and services.