Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 191643
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
928 AM PDT Thu Oct 19 2017


.SYNOPSIS...A series of storm systems will bring periods of valley
rain that may be heavy at times, mountain snow and coastal winds
through the weekend.


&&


.SHORT TERM...Today through Sunday...A modest atmospheric river
remains pointed at northwest Oregon and southwest Washington this
morning. The front that is pushing this slug of moisture southward
has been slow to move. As a result, 2 to 4" of rain fell across much
of the north Oregon and south Washington coasts and higher hills
while areas farther south towards Eugene are just seeing their first
drops of rain. Fortunately, the front is beginning to move resulting
in rain letting up across our northwest zones and rain is filling in
across Lane and Linn Counties. Overall, expect a wet day across much
of the Willamette Valley with totals likely coming out between 0.50-
1.00" of rain.

Precipitation should turn showery with some breaks in the clouds
later this afternoon and evening as a colder and more unstable
airmass spreads into the region. The best chance for thunderstorms
will be over the waters later tonight and early Friday morning, but
cannot rule out a clap or two of thunder Friday afternoon, mainly
east of Interstate 5 in the Willamette Valley.

Snow levels will likely lower to between 4-5kft late tonight and
Friday as the cool and more unstable airmass spreads across the
region. There should be some decent snow accumulations above 5000
ft, but temperatures appear marginal enough below that elevation
that impacts and criteria probably won`t warrant a Winter Wx
Advisory. Still would not be surprised to see between 4-8" of snow
around 6000 ft and above by the end of Friday. Expect showers to
decrease Friday evening with a tendency for some brief clearing.
Model soundings indicate this may result in snow levels temporarily
lowering closer to 2500-3000 ft across the Cascades, particularly
near the east side of Mt Hood and the lower terrain surrounding Mt
Adams. This could matter for a few hours towards daybreak on Friday
as the next round of widespread precipitation arrives. Any snow
accumulations at these lower elevations should be brief as a warm
front results in snow levels rising to at least 7000-8000 ft by late
Saturday.

The strong westerly flow at 850mb that will punch across the region
behind the warm front will likely enhance orographic precipitation
across the coast range and Cascades late Saturday. A distinct rain
shadow should then develop and result in rainfall rates dropping off
dramatically from the morning hours in the Willamette Valley. A
decently strong south to north pressure gradient will also set up
across the area, and likely produce at least breezy winds along the
coast and even in the Willamette Valley. Whether or not, it will be
strong enough to produce 58 mph gusts along the coast (warning
criteria) and 45 mph gusts in the Willamette Valley (advisory
criteria) remains unclear, but will certainly look at this in more
detail with the afternoon forecast package.

Models have and continue to suggest an atmospheric river will then
take aim at the Pacific Northwest. There is high confidence that a
period of more intense rainfall will drop southward across the area
Sunday. However, it remains unclear if it will sit over northwest
Oregon or western Washington in the 12-18+ hours preceding late
Sunday. This has big implications for our river forecasts given it
has the potential to produce 1" of rain in the valleys and 2-3" of
rain across the higher terrain in a 6 hour period if it remains
stationary. So the potential of 12-18 hours of that type of rain
would undoubtedly produce hydrologic issues even with it still being
early in the wet season, particularly across our burn scars and
smaller streams. Given model solutions over the past 48 hours for
the weekend period, suspect extreme northwest Oregon and southwest
Washington stand the best chance for having the atmospheric river
sit over it for an extended period of time, but it`s far from
guaranteed. Think the Hydrologic Outlook issued by the mid shift
looks good and will plan on carrying it forward. /Neuman



.LONG TERM...No Changes. Previous discussion follows...
Saturday night through Wednesday....Strong moist zonal flow will set
up over the region Saturday night through Sunday. Snow levels will
rise above 8,000 feet. Still some uncertainty as to whether or not
the heaviest rain will set up over western Oregon or western
Washington. Have kept PoPs high, but forecast details such as amount,
intensity, and duration are unclear.

Expect to see some rises on area rivers, especially in the coastal
basins, and if the heavy rain sets up over urban areas, some
localized urban flooding issues could develop. Also, locations with
significant burn scars should be aware of possible flash flooding and
debris flows.

Models are suggesting, short wave ridging will develop over the
Pacific NW early next week, which should bring at least a day or two
of dry weather. The Euro model suggests a weak front could push into
the region during the middle of next week while the GFS remains dry
and mild. Given the uncertainty, have trended PoPs towards
climatology. Bishop


&&


.AVIATION...A band of rain is expected to linger over the
region today before shifting off to the east tonight. Expect MVFR
conditions to be the dominant condition through around 03z this
evening, with a few areas of IFR conditions. As the main band of
rain shifts southeast tonight, some showers will linger. Flight
conditions are likely to improve to VFR esp over the northern TAF
sites. Areas of MVFR conditions will persist though after 03Z, most
likely across the southern part of the forecast area.

KPDX AND APPROACHES...MVFR conditions likely to prevail through 03z
this evening as an area of rain lingers over the region. The rain is
expected to ease up tonight, allowing VFR conditions to return after
03z, although there is still about a 30 percent chance of seeing
MVFR cigs remaining past 03z.


&&


.MARINE...Winds likely to keep gusting into small craft advisory
range, around 25 kt, today through Fri as a trough of low pres over
the waters lingers before giving way to a brief ridge of high pres
Fri afternoon. The next frontal system moves in Fri night and Sat,
with gales likely from late Fri night through Sat night.

Seas will build quickly around midday today as a large westerly
swell arrives. Manual decay of the swell from buoy 46005 matches
well with model expectations, which would bring seas a little over
20 feet in by early afternoon. The seas are likely to remain at or
a little above 20 ft through tonight, before slowly subsiding Fri,
and then dropping off a few more feet Fri night. This will bring
high surf to the coast as well this afternoon through early Fri.


&&


.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Surf Advisory from 1 PM this afternoon to 10 AM PDT Friday
     for Central Oregon Coast-North Oregon Coast.

WA...High Surf Advisory from 1 PM this afternoon to 10 AM PDT Friday
     for South Washington Coast.

PZ...Small Craft Advisory for winds until 6 PM PDT this evening for
     Coastal waters from Cape Shoalwater WA to Cascade Head OR
     out 10 NM-Waters from Cape Shoalwater WA to Cascade Head
     OR from 10 to 60 NM.

     Small Craft Advisory for hazardous seas until 11 PM PDT Friday
     for Coastal waters from Cape Shoalwater WA to Cascade Head
     OR out 10 NM-Coastal waters from Cascade Head to Florence
     OR out 10 NM-Waters from Cape Shoalwater WA to Cascade
     Head OR from 10 to 60 NM-Waters from Cascade Head to
     Florence OR from 10 to 60 NM.

     Small Craft Advisory for winds until 6 AM PDT Friday for
     Coastal waters from Cascade Head to Florence OR out 10 NM-
     Waters from Cascade Head to Florence OR from 10 to 60 NM.

     Small Craft Advisory for Rough Columbia River Bar until 10 AM
     PDT Friday.

&&


$$

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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 forecast area.


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