Area Forecast Discussion
Issued by NWS Portland, OR
FXUS66 KPQR 260433
Area Forecast Discussion
National Weather Service Portland OR
830 PM PST Sat Feb 25 2017
.SYNOPSIS...A northerly flow aloft into the Pacific NW through the
early part of the coming week will bring a couple of shortwaves
through for a continuation of cool weather with low snow levels and
chances for showers. By midweek weak high pres ridging will push
most precipitation chances north into WA, with temperatures
moderating some. A cold front late in the week is expected to bring
another round of rain and mountain snow back to the region.
.SHORT TERM...Tonight through Tuesday...No changes, as all still
looks on track for cool rainy conditions later tonight into Sunday,
with snow levels between 800 and 1000 feet. A northerly flow aloft
over the Pacific NW is expected to continue into Tue, with a couple
more shortwaves moving down the coast. This will keep the region in
a cooler than normal pattern, with snow levels generally down into
the Coast Range and lower Foothills. The first of the shortwaves
coming down late tonight and Sunday moves through by Sunday evening.
Good dynamic lift expected with this system, and is accompanied by
fairly deep moisture, so pops will be categorical. With dynamics,
moisture and favorable wind direction for orographic lift, expect the
Coast Range, Cascades and Foothills to see solid snow advisory
amounts despite the quick moving nature of the shortwave. At lower
elevations, models suggesting the snow level sets up somewhere
around 1000 feet, lowest in the north. Given the mild temperatures
today and appearance that clouds may flatten out and spread this
evening, there is some question as to whether snow levels will get
quite as low as promised, although dynamics may tend to help lower
snow levels. For now, will put a mention for some light
accumulations in the lower northern zones above 1000 feet late
tonight and Sunday, but hold off on any headlines due to the small
areal coverage of terrain above 1000 feet.
The showers will drop off with the passage of the shortwave in the
latter half of Sunday, but as the low level flow settles in out of
the nw Sun night and low levels remain moist, it will be difficult
to shut off showers completely, especially for the orographically
favored OR Cascades and Coast Range. The second shortwave come
through late Mon. This system will be similarly cold, but likely not
as strong and not with the same deep moisture, so pops will not be
quite as high. This could also result in some snow flakes being seen
at lower elevations, but snow accumulations will again be limited
more to the mountains, and should be lighter than the Sunday system.
A weak onshore flow continues behind the second trough Mon night
into Tue, but with air mass likely beginning to modify slightly as
upper heights begin to come up. Will show a diminishing trend in
pops in this time period, but still keep chance pops going into Tue.
Snow levels likely to rise slowly, more into the range of 1500 feet.
.LONG TERM...Tuesday night through Saturday...Models develop a
rather flat upper level ridge over the region midweek, tending to
push the best chances for precipitation up into WA. With the surface
ridge setting up across southern OR though, will need to keep some
low pops down into nw OR Wed and Thu due to weak but moist low level
onshore flow and isentropic lifting. Fri GFS GEM and EC all drive a
cold front through with a trough originating in the Gulf of Alaska.
With good agreement in models as well as strong dynamics and good
moisture, will go with likely pops as the front moves through. Post
frontal showers continue Sat as a broad upper trough over the ne
Pacific spreads inland.
.AVIATION...Widespread VFR conditions continue this evening with
weak high pres over the region. A surface just offshore Vancouver
Island will drop into the area early Sunday morning. The
associated front is currently off the Washington coast and is
forecast to reach the Oregon coastal areas between 12Z and 15Z
Sun and to inland TAF locations by 18Z Sun. MVFR conditions will
develop from north to south starting around KAST by 06Z,
reaching KONP and KEUG between 15Z-18Z. Cannot rule out some
snowflakes mixing in with the rain to start the day, but think
that surface temps will be a bit too warm. Rain tapers to showers
from north to south during the late morning and early afternoon
hrs, with conditions generally improving to VFR. Occasional MVFR
still possible through the afternoon in heavier showers.
KPDX AND APPROACHES...VFR conditions continue through most of
tonight. Rain will increase after 12Z, with MVFR cigs likely
developing by around 15Z. Rain tapers to showers by early
afternoon, with conditions returning to mainly VFR with
occasional MVFR in heavier showers through the evening. Pyle /mh
.MARINE...A surface low pres system near Vancouver Island will
dive south into the area and move along the Washington and north
Oregon coast Sunday and Sunday night. Southwesterly winds will
pick later this evening and overnight, becoming 15 to 20 kt with
gusts 25 to 30 kt. Forecast appears to be on track as winds over
the Washington coastal waters are gusting to 20 to 25 kt.
Winds will turn to the northwest Sunday morning, and will
continue to gust to 25 kt at times through Sunday afternoon.
Northwest winds will continue Mon and may remain near low-end
small craft advisory thresholds through much of the day. Weak
high pres will build over the NE Pac for a good portion of next
week, keeping conditions rather quiet from Tue through Thu. The
next front is currently modeled to arrive late Thu or Fri. It
looks fairly strong at this point, bringing a good chance for
Seas are running in the 5 to 7 ft range and will be increasing
overnight as a long period westerly swell arrives. There will
also be some wind waves and fresh swell generated by the incoming
low pressure system. Expect combined seas to build into the low
teens late tonight and remain at or above 10 ft through at least
Sunday night. Seas will gradually subside early next week, then
remain 10 ft through most of the week. Pyle
OR...Winter Weather Advisory for late tonight through Sunday
afternoon or early evening for...
North Oregon Cascades and Foothills
Lane County Cascades and Foothills
Coast Range of Northwest Oregon
Coast Range of West Central Oregon.
WA...Winter Weather Advisory for late tonight through Sunday
afternoon or early evening for...
South Washington Cascades and Foothills.
PZ...Small Craft Advisory for winds until 4 PM PST Sunday for
Coastal Waters from Cape Shoalwater WA to Cascade Head OR
out 60 nm.
Small Craft Advisory for hazardous seas from 4 AM Sunday to 4
AM PST Monday for Coastal Waters from Cape Shoalwater WA
to Florence OR out 60 nm.
Small Craft Advisory for winds from 1 AM to 4 PM PST Sunday for
Coastal Waters from Cascade Head OR to Florence OR out 60
Small Craft Advisory for Rough Columbia River Bar from 2 AM to
7 AM PST Sunday.
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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. The area is
commonly referred to as the forecast area.