Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 231813
AFDPQR

Area Forecast Discussion...
National Weather Service Portland OR
1013 AM PST Fri Feb 23 2018

.SYNOPSIS...Chilly air mass will remain entrenched over the region
today into early next week. Another disturbance over the Gulf of
Alaska will spread precipitation across the region later today and
tonight, with snow levels still rather low. But, much stronger system
arrives on Sunday, with rain for the lowlands, and very heavy snow
and winds in the Cascades. This cool and active weather continues
next week.
&&

.SHORT TERM...Today through Sunday...Skies did finally clear out last
night, which combined with the very cold airmass in place, allowed
overnight temps to plummet to some of the coldest that we have seen
so far this winter. The interior lowlands were mainly in the low 20s,
and some outlying areas and the gorge and Upper Hood River Valley got
down into the teens. Temperatures in the Cascades were well down into
the single digits earlier this morning. The area saw several daily
low temperature records set or tied.

We will remain mainly dry through early this afternoon. However, high
and mid level clouds will be thickening up ahead of our next  system
approaching from the northwest. This, combined with cold start to the
morning, will tap down our high temperature potential. Suspect most
areas will see inland afternoon temperatures in the mid 30s, but
lower 40s along the coast. Models have indicated some minor boost of
southerly winds across the region in advance of incoming system. But,
not all that strong of a gradient, and seems may be too southeasterly
to get much warmth. So, would not expect much of a temperature boost
from the winds.

Now, precipitation will spread into region from the northwest this
afternoon. Should be cold enough to support precipitation falling as
snow over much of the interior lowlands to start the event. Overall
1000 to 700 mb thicknesses not all that impressive, and suggest snow
levels will be slow to rise today PDX/Vancouver metro area northward.
Areas to the south will get a minor boost in the freezing level, with
snow level rising to 1000  to 1500 feet. But, temperatures will be
key to whether or not snow accumulates. Current thought is that may
see minor accumulations on below 500 feet to valley floor, but best
chance of such accumulations will be farther north, such as the
Cowlitz River Valley, where temps will be cool due to clouds will
thicken earlier.

Bottom line: precipitation will begin as snow. But due to light
nature in precipitation, think only minor accumulations (a dusting up
to an inch for the lowest elevations), with the best chances over the
northern tier of the fcst area. Slightly higher amounts possible for
the lowlands areas above 500 ft. A Winter Weather Advisory was issued
at the end of the midnight shift for the interior lowlands from the
central Willamette valley northward for late this afternoon through
the first half of the overnight period.

Winter Weather Advisories are also in effect for the Willapa Hills
and North Coast Range above 700 feet, as well as the Cascades and
Cascade foothills, starting later this afternoon. Total snowfall in
the Coast Range and Willapa Hills of 2 to 5 inches tonight through
Sat, with the highest amounts above 1500 ft. Cascades will see
heavier snowfall, with 4 to 10 inches tonight, and again on Saturday.

Not much change overnight into Sat am, as showers will continue in
the northwesterly flow. Snow levels may dip down from time to time,
with spotty brief accumulations below the main snow level not out of
the question with any heavier shower. But overall, snow levels
gradually rise on Saturday, generally up to 1000 to 2000 feet.

Showers decreasing later Saturday afternoon and Sat evening. But next
system will be arriving later Sat night into early Sun, with rain, or
snow above 1500 feet, increasing early Sun am. This system will be
packing more of a punch, with modest shot of precipitation along with
rather gusty winds over higher terrain. Current models showing fair
degree of consistency, so confidence in solutions fairly good. Should
see rain for lower elevations, but snow above about 2000 feet. And a
lot of it. Strong potential of life threatening winter conditions in
the Cascades, with 1 to 2 feet, yes feet, of snow on Sun. With the
snow will come gusty westerly winds, with gusts 45 to 60 mph,
resulting in near blizzard conditions at times on Sun. Have posted
Winter Storm Watch for the Cascades for Sun and Sun evening.

If planning travel into or over the Cascades this weekend, continue
to monitor weather forecasts, and think about your safety. Before you
go, ask yourself if you are prepared for winter driving conditions,
or being stuck or stranded on roadside.    Pyle/Rockey

.LONG TERM...No changes. Previous discussion follows...
Sunday night through Thursday...Fairly active pattern
expected will continue through the long range. GFS ensembles for 12Z
Mon indicate cold upper level troughing to remain over Southwest
Washington and Northwest Oregon. Operational runs of the GFS and
ECMWF in good agreement showing 500 mb north flow over the area,
with the upper trough axis extending from NE Washington to extreme
NW California. The bulk of the dynamics will have shifted to
Southwest and South Central Oregon by that time. Model 1000-500 mb
thickness values Sun night forecast to be near 520 dm amd 850 mb
temps still -6 to -8C. This translates to continued low snow levels.
Going with near the surface in the Central Columbia Gorge and Upper
Hood River Valley to around 500 ft in the North Willamette Valley
and Clark County in SW Washington. Snow levels to gradually rise the
south of the aforementioned areas rise to around 1000 ft.
Substantial snowfall is likely for the Cascades and foothills and
higher elevations of the Oregon Coast Range and also SW Washington
Willapa Hills through Mon morning.

North flow aloft continues Mon and will push the upper trough axis
well to the south. A few of the GFS ensemble members want to close
off a 500 mb low near the Central California Coast Mon. Maintained
climo or slightly above climo POPS Mon due to the potential for weak
short-waves within the north flow aloft to migrate through the
forecast area. The ECMWF appears to be slightly faster with the next
short-wave Mon night and Tue. Over-running warm-frontal precip
expected to slide south into SW Washington and far NW Oregon
sometime Tue. GFS indicates slight moderation in 850 mb temps Tue,
but still around -5C. Snow levels Mon night and Tue hover in the 750
to 1500 ft range. Models then show another upper level trough
carving out over the NE Pac Wed through Thu. Details differ, but the
general gist is for continued below normal daytime temperatures,
relatively low snow levels and unsettled conditions. This pattern
will bring additional snow to the Cascades, which will continue
the trend of building snowpack. Weishaar
&&

.AVIATION...A cold and dry airmass currently in place will give
way to an incoming front and attendant band of precipitation late
this afternoon and evening. This will result in VFR conditions
deteriorating into MVFR tonight and Saturday, possibly even
locally IFR. Precipitation inland will likely start as flurries
this afternoon, with snow increasing this evening, turning to
rain by Saturday morning. Coastal locations may see a rain/snow
mix this afternoon and evening before changing to rain. Best
chance of snow accumulations tonight will be KSLE northward.

PDX AND APPROACHES...VFR conditions today, though clouds will be
increasing with VFR cigs this afternoon and possibly some
flurries. The snow will increase this evening with 1-2 inches
possible before changing to rain by sunrise Saturday. Conditions
will lower to MVFR and possibly IFR tonight. pt
&&

.MARINE...The next system will be dropping southward from the
north across the waters during the day today. The main low
pressure area stays north of the waters near Vancouver Island
today through Saturday with Small Craft Advisory threshold winds
over our waters, except gales in the northern waters late tonight
and Saturday morning, especially over the outer waters. There is
a bit of a lull in the winds later Saturday and Saturday evening
before another system drops south and brings more Small Craft
Advisory threshold winds Sunday except gales are again pretty
likely over the northern waters later Saturday night and Sunday
morning.

This series of systems from today through the weekend produces a
nice northwesterly fetch off the B.C. coast that will likely lead
to seas in our coastal waters being well up in the teens through
the weekend. pt
&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 4 PM this afternoon to 8 AM PST
     Saturday for Central Columbia River Gorge-Western Columbia
     River Gorge.

     Winter Weather Advisory from 3 PM this afternoon to 4 PM PST
     Saturday for Northern Oregon Cascade Foothills.

     Winter Weather Advisory from 6 PM this evening to 4 AM PST
     Sunday for Cascades in Lane County-Northern Oregon
     Cascades.

     Winter Storm Watch from late Saturday night through Sunday
     evening for Cascades in Lane County-Northern Oregon
     Cascades.

     Winter Weather Advisory from 6 PM this evening to 4 PM PST
     Saturday for Cascade Foothills in Lane County.

     Winter Weather Advisory from 4 PM this afternoon to 4 AM PST
     Saturday for Greater Portland Metro Area.

     Winter Weather Advisory from 2 PM this afternoon to 4 AM PST
     Saturday for Lower Columbia.

     Winter Weather Advisory from 3 PM this afternoon to noon PST
     Saturday for Coast Range of Northwest Oregon.

     Winter Weather Advisory from 6 PM this evening to 4 AM PST
     Saturday for Central Willamette Valley.

WA...Winter Weather Advisory from 4 PM this afternoon to 8 AM PST
     Saturday for Central Columbia River Gorge-Western Columbia
     River Gorge.

     Winter Weather Advisory from 3 PM this afternoon to 4 PM PST
     Saturday for South Washington Cascade Foothills.

     Winter Weather Advisory from 6 PM this evening to 4 AM PST
     Sunday for South Washington Cascades.

     Winter Storm Watch from late Saturday night through Sunday
     evening for South Washington Cascades.

     Winter Weather Advisory from 4 PM this afternoon to 4 AM PST
     Saturday for Greater Vancouver Area.

     Winter Weather Advisory from 2 PM this afternoon to 4 AM PST
     Saturday for I-5 Corridor in Cowlitz County.

     Winter Weather Advisory from 3 PM this afternoon to noon PST
     Saturday for Willapa Hills.

PZ...Small Craft Advisory for winds from noon today to 1 AM PST
     Saturday for Waters from Cape Shoalwater WA to Cascade
     Head OR from 10 to 60 NM.

     Gale Warning from 1 AM to 10 AM PST Saturday for Waters from
     Cape Shoalwater WA to Cascade Head OR from 10 to 60 NM.

     Small Craft Advisory for winds from 2 PM this afternoon to 3 PM
     PST Saturday for Coastal Waters from Cascade Head OR to
     Florence OR out 60 NM.

     Small Craft Advisory for hazardous seas from 1 AM to 3 PM PST
     Saturday for Coastal waters from Cape Shoalwater WA to
     Cascade Head OR out 10 NM-Coastal Waters from Cascade Head
     OR to Florence OR out 60 NM.

     Small Craft Advisory for Rough Columbia River Bar until 1 PM
     PST this afternoon.

     Small Craft Advisory for Rough Columbia River Bar from 9 PM
     this evening to 3 PM PST Saturday.

     Small Craft Advisory for winds from noon today to 3 PM PST
     Saturday for Coastal waters from Cape Shoalwater WA to
     Cascade Head OR out 10 NM.

&&

$$

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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.



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