Area Forecast Discussion
Issued by NWS Portland, OR

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000
FXUS66 KPQR 180445
AFDPQR

Area Forecast Discussion
National Weather Service Portland Oregon
836 PM PST Tue Jan 17 2017

.SYNOPSIS...A moist front will bring widespread moderate to heavy
rain with freezing rain to the Columbia River Gorge and the Upper
Hood River Valley.  High winds will affect the coast and coast
range tonight and Wednesday. The rain will turn to showers
Wednesday night and Thursday as snow levels lower down to the Cascade
passes. An other moist front is expected Thursday night and Friday
with a series of lows maintaining showers through next weekend.

&&

.SHORT TERM...Tonight through Friday....Satellite shows a 972 mb low
off the north Vancouver Island coast heading to the northeast
with an occluded front spreading moderate to heavy rain across
the region. Precipitable water amount per satellite observation puts
between 1" to 2.2" just off the Oregon/Washington coasts. Strong
southwesterly 850 mb winds is pumping this moisture inland to support
rain rates of around 1 inch per 3 hours for the coastal mountains
and south Washington Cascades. Rain rates should pickup a little more
as the front moves through.  Current QPF forecast is a bit
overdone for the 6 hours ending at 10 pm for the coastal mountain
locations. Coastal rivers have just begun to respond and are close to
stage forecasts. Coastal rivers are of concern tonight and Wed.
Johnson Creek in the Portland are is one to watch also.  Current
forecast keeps it below flood stage though.

Now to winter weather.Freezing rain is all but gone in the
Willamette Valley and southwest Washington lowlands. Only the
eastern Portland/Vancouver metro area (around Troutdale & Camas)
has temperature to support freezing rain. Temperature
will take longer to moderate as the icy cold wind continues to spill
out of the Gorge.  The TTD-DLS pressure gradient remains around -10
mb and has a slight weakening trend.Ice accumulation rate at
Troutdale airport automated instrument showed 0.12" in the past hour
and and estimated 0.40" in the past 3 hours.    The Gorge continues
to get freezing rain or sleet in the deeper colder air near Hood
River.  A spotter report near Beacon Rock WA at 600 ft had 0.75" ice
in the last 6 hours.  For this update will continue the Ice storm
warning in the Portland/Vancouver metro are but confine it to near
the Gorge overnight.  No changes to the Gorge & Upper Hood River
Valley warning for now.

Previous discussion follows...  When the colder air will scour out of
these areas and freezing rain turns to rain depends on when the east
surface pressure gradient through the Columbia River Gorge weakens.
The gradient has weakened a couple of mb since this morning, but will
remain above 6 mb into Wednesday morning. This will likely keep the
cold air (and freezing rain) for the lower areas of the Portland
metro area influenced by the Columbia Gorge going through late
tonight and possibly into Wednesday morning. The Gorge and Upper Hood
River Valley areas will continue freezing rain through early
Wednesday evening. Have increased qpf and ice accumulation a bit for
tonight and Wednesday to better represent the high moisture observed
offshore this afternoon.

What is causing this rain? An occluded front which is forming nicely
off of the Washington and Oregon coast this afternoon. This front has
quite a bit of moisture with it offshore with satellite derived
precipitable water shows a swath of precipitable water values
between 1.25 and 2.2 inches. The air mass will lose some of this
moisture as the front begins to move inland this evening, but the
precipitable water will be much higher than the climatological daily
normal. For perspective, the seasonal daily normal precipitable water
for Salem today is 0.58 inch. A low moving along the front overnight
tonight will slow the inland progress of the front, and maintain rain
through late Wednesday evening.

Due to the high moisture observed offshore and the prolonged period
of rain, have increased qpf for tonight and Wednesday morning. From
tonight through Wednesday evening, expect 3 to 7 inches of rain for
the coast range, 2 to 4 inches for the Cascades and the Cascade
foothills, 2 to 4 inches for the coast, and 1 to 2 inches for the
Willamette Valley. This rain will likely result in flooding of some
of the coastal rivers, small streams, and possibly urban flooding in
low-lying areas or areas with clogged drains.

South winds are increasing along the coast ahead of the front with
Cape Disappointment reporting gusts around 60 mph, Abernathy
reporting gusts around 75 mph, and Clatsop Spit around 45 mph. These
winds will continue to increase this evening and remain strong into
Wednesday morning, and am not going to make any changes to the High
Wind Warning. Valley winds will increase this evening too with south
gusts up to 35 mph possible for the South and Central Willamette
Valley. The east winds through the Gorge will delay the south winds
for the North Willamette Valley, and do not expect as noticeably
gusty south winds for the Portland/Vancouver area.  However, the east
winds will be quite gusty for East Portland, Fairview, and Troutdale.

Rain will turn to showers behind the front Wednesday night with an
upper level trough continuing showers through Thursday. Snow levels
which will be high (above 8000 feet) tonight will slowly lower
Wednesday and be down to 4000 feet by Thursday morning for some light
snow accumulations for the Cascade passes. Another potentially moist
front arrives early Friday morning. The airmass will be colder with
this front and snow levels will be between 2500 and 3000 feet. ~TJ

.LONG TERM...No Changes. Previous discussion follows...Friday night
through Monday...Models in good general agreement on keeping a closed
low off the WA and southern BC coast from Fri night through Sun
night, rotating a series of shortwaves through the region. The
shortwaves however show a tendency to split...with some energy
lifting ne through the Pacific NW and some energy headed down into
northern CA. identity of the waves quickly becomes difficult to
correlate between the models, so will generally have to limit pops to
high chance to low likely range at any given time. Models, EC in
particular, showing a tendency to hang onto an offshore flow through
the gorge, and it is not clear that the cold air ever truly gets
scoured out of the basin east of the Cascades. As such, will keep a
chance of freezing in for the gorge and hood river valley. Mon and
Tue models trend towards ridging aloft approaching the coast,
resulting in decreasing pops. EC is drier of the models, and as a
result tends to colder temps, and as such will still need to hang
onto some chance for freezing rain in the Gorge and Hood River Valley
all the way into Tue.

&&

.AVIATION...Steady precipitation continuing across the region,
now RA for most of area, but -FZRA continues from KPDX/KVUO east
through Columbia Gorge. Strong south winds continue along the
coastal strip with gusts to 50 kt tonight. Mix of flight
conditions will persist with MVFR or IFR vsbys continuing. Cigs
remain primarily MVFR at coast but more variable inland, but
expect MVFR cigs to eventually work into interior overnight. KPDX
will be the first to warm up and switch over to RA, likely by 12z.
KTTD may keep freezing rain well into Wed morning. Also, with the
gusty easterly winds at the surface and strong southerly winds
aloft, low-level wind shear will be a concern through tonight from
KPDX east through KTTD and the gorge.

KPDX AND APPROACHES...Expect freezing rain to continue through
12Z, but temperature will slowly warm with switch to RA after. Expect
MVFR vsbys to continue through tonight, becoming IFR at times.
Also expect cigs to lower to MVFR later this evening or overnight.
Finally, with the gusty east winds at the surface and strong
southerly winds aloft, low level wind shear will be a concern
through tonight. Cullen

&&

.MARINE...High end gale to low end storm force gusts continue to
occur over the waters this evening. Expect wind gusts to persist
around 40 kt this evening until a second round of storm force
gusts over the waters, particularly within 20 nm of the coastline
as a coastal jet develops. The cold front will push onshore Wed
afternoon, allowing winds to subside from north to south during
the day. Expect winds to generally be 25 kt or less by late Wed
afternoon. Winds then remain much lighter Wed night and Thu,
although a surface trough moving through Thu may require a small
craft advisory for winds. Another system on Fri may bring the
potential for additional gales. The active weather pattern
continues through the weekend and into early next week, although
details remain uncertain at this point.

Seas are steadily increasing, with observed seas in the 21 to 23
ft range this evening. Expect seas to peak between 23 and 26 ft
overnight. After a prolonged period of 20 ft, seas will gradually
subside along with the decreasing winds during the day Wed. Expect
that they will be in the low teens by Wed night or early Thu. Seas
will increase to near 20 ft again with the Fri system. The model
guidance then suggest that a large long- period westerly swell
arrives over the weekend. If things continue to track as expected,
high surf conditions will likely meet warning criteria over the
weekend. Cullen

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Ice Storm Warning until 6 PM PST Wednesday for Central Columbia
     River Gorge-Upper Hood River Valley-Western Columbia River
     Gorge.

     Flood Watch through late Wednesday night for Cascade Foothills
     in Lane County-Central Coast Range of Western Oregon-
     Central Oregon Coast-Central Willamette Valley-Coast Range
     of Northwest Oregon-Greater Portland Metro Area-Lower
     Columbia-North Oregon Coast-Northern Oregon Cascade
     Foothills-South Willamette Valley.

     Ice Storm Warning until 10 PM PST this evening for Greater
     Portland Metro Area.

     High Wind Warning until 4 PM PST Wednesday for Central Coast
     Range of Western Oregon-Coast Range of Northwest Oregon.

     High Wind Warning until 10 AM PST Wednesday for Central Oregon
     Coast-North Oregon Coast.

WA...Ice Storm Warning until 6 PM PST Wednesday for Central Columbia
     River Gorge-South Washington Cascades-Western Columbia
     River Gorge.

     Flood Watch through late Wednesday night for Greater Vancouver
     Area-I-5 Corridor in Cowlitz County-South Washington
     Cascade Foothills-South Washington Coast-Willapa Hills.

     Ice Storm Warning until 10 PM PST this evening for Greater
     Vancouver Area.

     High Wind Warning until 10 AM PST Wednesday for South
     Washington Coast-Willapa Hills.

PZ...Storm Warning until 10 AM PST Wednesday for Coastal Waters from
     Cape Shoalwater WA to Florence OR out 60 nm.

     Small Craft Advisory for Rough Columbia River Bar until 4 PM
     PST Wednesday.

&&

$$

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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.


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