Area Forecast Discussion
Issued by NWS Portland, OR

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388
FXUS66 KPQR 231122
AFDPQR

Area Forecast Discussion...
National Weather Service Portland OR
316 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...Rather chilly this morning, with
nearly all of the region, including the coast, in the lower to middle
20s. Temperatures in the Cascades are colder, mostly in the lower
teens to single digits. At 3 am, coldest was zero deg at McKenzie
snotel, just south of Santiam Pass.

Our next system is now approaching from the northwest. Mid and high
clouds will increase across the region this am. 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 middle to upper 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 help to boost temps 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 the interior north of a Forest Grove to Estacada line.
Overall 1000 to 700 mb thicknesses not all that impressive, and
suggest snow levels will be slow to rise today and tonight for areas
north and east of the north Willamette Valley. 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 from
Washington/Multnomah counties, and northward. But due to light nature
in precipitation, think only minor accumulations (less than an inch)
in spots down low, with main accumulations staying above 500 to 800
feet.

Have issued Winter Weather Advisories for 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 6 inches tonight through Sat.
Cascades will see heavier snowfall, with 4 to 8 inches tonight, and
again on Saturday.

Not much chance tonight 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.    Rockey.

.LONG TERM...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 thresholds between 03-09z Saturday. There
is a decent chance precipitation will begin as snow or a rain/snow
mix in the Willamette Valley, but no accumulations are anticipated
before it switches to rain.

PDX AND APPROACHES...A cold and dry airmass 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 thresholds towards 03-09z Saturday.
There is a decent chance precipitation will begin as snow or a
rain/snow mix, but no accumulations are anticipated before it
switches to rain later in the evening. /Neuman


&&

.MARINE...The first in a series of fronts dropping southeastward
across the northeast Pacific will arrive Friday afternoon and
result in Small Craft Advisory level wind gusts of 25 kt. Winds
may temporarily ease early Friday night before increasing again
late Friday night and Saturday morning. Expect widespread wind
gusts to 30 kt. There may be a few gusts to 35 kt, but they
should be rather sporadic and mainly tied to the strongest
showers early Saturday so have opted extend the Small Craft
Advisory through Saturday afternoon with this mentioned in it.
The next front will push through the waters Saturday night and
Sunday and result in a near repeat of gusty winds. This extended
period of gusty northwest winds will result in seas building
into the low teens by late tonight and into the mid to upper
teens for Saturday and Sunday. Another similar storm system will
likely arrive early next week before we start to enter a pattern
that stands a better chance of bringing widespread Gale Force
winds of 35 to 45 kt towards the middle to latter half of next
week. /Neuman


&&


.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory for...
        North Oregon Cascades... 6 pm today through 4 am Sunday.
        Lane County Cascades...  6 pm today through 4 am Sunday.
        North Oregon Cascade Foothills... 3 pm today to 4 am Sat.
        Lane County Cascades Foothills... 3 pm today to Noon Sat.
        Coast Range of Northwest Oregon... 3 pm today to Noon Sat.

     Winter Storm Watch for...
        North Oregon Cascades... 4 am Sun through Sun evening.
        Lane County Cascades...  4 am Sun through Sun evening.

WA...Winter Weather Advisory for...
        South Washington Cascades...  6 pm today through 4 am Sun.
        South Wash Cascades Foothills... 3 pm today to 4 pm Sat.
        Willapa Hills....        3 pm today to noon Sat.

     Winter Storm Watch for...
        South Washington Cascades...  4 am Sun through Sun evening.


PZ...Small Craft Advisory for winds from noon today to 3 PM PST
     Saturday for Coastal Waters from Cape Shoalwater WA 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
     Florence OR out 60 NM.

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

&&


$$

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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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