Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 161417
AFDPQR

Area Forecast Discussion
National Weather Service Portland Oregon
617 AM PST Mon Jan 16 2017

.SYNOPSIS...Temperatures will remain well below normal today for much
of SW Washington and NW Oregon, as brisk east winds continue to
supply the Willamette Valley with very cold Columbia Basin air. Snow
cover remains several inches deep in the Portland metro area, adding
to the persistence of the cold air. The low-level cold air will come
into play as a very moist Pacific frontal system spreads rain across
the forecast area Tuesday and Wednesday. This will likely result in a
major ice storm in the west and central Columbia Gorge, with
significant icing also likely Tuesday for much of the Portland and
Vancouver metro areas. Significant icing is also likely in some Coast
Range and Cascade valleys, and may extend as far south as Salem. This
system is expected to bring very windy conditions to the coast and
higher terrain in the Coast Range. Occasionally heavy rain may also
cause some flooding, especially in areas impacted by last week`s
snowstorm. A return to colder weather is expected by the end of the
week, with additional systems bringing the threat of snow again at
least down into the hills.

&&


.UPDATE...Brief update since the main portion of this discussion will
be delayed. This is because we have undertaken a major shift in the
direction of the forecast, with Winter Storm Watches forthcoming for
the Portland/Vancouver Metro area, Salem, Columbia Gorge, and the
valleys of the S WA/N OR Cascades (including the Hood River Valley)
and N OR Coast Range. These watches are for the potential of a
significant ice event Tuesday morning...with the greatest threat
being closer to the Gorge.

Additionally, high wind watches have been posted for the coast
Tuesday afternoon through Wed morning. We also still appear on track
for at least *some* flooding later Tue into Wed, even though QPF has
decreased slightly over the past 24 hours.

Will explain in greater detail the reasoning behind the shift in the
direction of the forecast shortly, but for now we need to get the
watches out for the morning news cycle. Thanks for your patience.
Weagle

&&

.SHORT TERM...Today through Wednesday...Major changes to the
direction of the forecast with this morning`s update. It appears
temperatures will remain below freezing across most of our inland
valleys Salem northward as rain spreads into the region early
Tuesday. Thus, it appears significant icing will occur across a much
wider audience than was previously expected. Have issued numerous
watches on this shift, including a Winter Storm Watch for the
northern half of the Willamette Valley, including Salem and the
PDX/Vancouver metro area. The Winter Storm Watch also reaches into
the nearby valleys of the Coast Range and Cascades, where cold air
will likely remain trapped for a while. Of course, the Columbia Gorge
and Hood River/Wind River Valleys are included as well...as these
will be the last areas to scour out the low-level cold air that has
become so well established over the past few days. Additionally, High
Wind Watches have been posted for the beaches and headlands, and a
Flood Watch remains in effect for much of the forecast area.

The main impetus behind the change was increased confidence in the
slower timing of clouds and precipitation associated with an incoming
very moist Pacific frontal system, presently crossing 145W. This
system is being fed by a 200 kt jet stream which spans the western
and central Pacific. However, this jet is expected to develop another
wave of low pressure along the trailing portion of the front later
today, which will in turn slow down the progress of the incoming
front. The slower timing of the front will slow down the onset of
clouds and rain...allowing more time this evening for radiational
cooling.

Temperatures have struggled to get above freezing under our strong
but shallow valley inversions, and have failed to get above freezing
4 out of the last 5 days at PDX. The air mass is warming aloft; in
fact the 12z Salem sounding shows an 850 mb temperature of about +7
deg C this morning. While the warm air aloft is pushing temperatures
into the 40s and 50s in the higher terrain, it is having little to no
impact under the inversions in the inland valleys. With generally 3
to 6 inches of snow still on the ground across the Portland metro
area and acting as a refrigerant, it is unlikely valley temps today
will be much different than they have been the last couple days.
Adding to the cold air is the strong easterly flow through the
Columbia Gorge, which prompted a Wind Advisory when it gusted as high
as 55 mph last night at Troutdale. The combination of these factors
will likely keep temps near or below freezing across the PDX metro
area and many inland valleys through Tuesday morning, while rain
associated with the incoming frontal system approaches from the
Pacific.

As is often the case, the actual pressure gradient across the
Cascades is easing much slower than depicted in the last 2-3 days`
worth of computer models. As of 5 AM, the KTTD-KDLS gradient was near
-11 mb, which has powered gusts to 74 mph in Corbett and 83 mph at
Crown Point. Earlier guidance had suggested offshore gradients would
slacken enough to allow for faster moderation in temperatures west of
the Columbia Gorge. It now appears that will not be the case, as
gradients remain decidedly easterly through the Columbia Gorge until
the main cold front plows through the Pac NW late Wed afternoon or
Wed night.

Our prolonged, near-historic stretch of cold weather has primed roads
and sidewalks to be receptive to ice accumulation. It may take a
couple hours after temps climb above 32 deg F for icing problems to
stop. For inland areas, this should occur first in the central and
southern Willamette Valley midday Tuesday, with the cold air in most
Coast Range valleys likely to be scoured out by late Tuesday
afternoon. Outside of the Gorge, the Portland metro will probably be
last to warm above freezing Tuesday, with areas east of I-205 being
the most stubborn. With easterly pressure gradients continuing to
resupply the Gorge with cold air, see no reason temps will climb
above freezing in Hood River until later Wed afternoon, despite most
model soundings suggesting it occurs much sooner than that. That
means nearly this entire storm - which has enough rainfall to spark
flood concerns - will add about an inch of ice to the already
astounding 18-24 inches of snow and ice on the ground in Hood River.
This brings up concerns about weaker roofs caving in due to the
extreme buildup of snow and ice. The usual power outages and travel
concerns are expected with this event as well.

While the flood threat is still very real across SW WA/NW OR Tue-Wed,
QPF appears a little lighter than it did 24-48 hours ago. One reason
for this is that the vast majority of the forecast area will probably
miss out entirely on any rainfall from the first strong wave...which
is also the system slowing down the overall progress of the front.
This system is expected to intensify rapidly but curl north, dumping
the heaviest rain on Vancouver Island and the Olympic Peninsula. The
next two shortwaves still look like formidable rain-producers, as
this system taps into abundant subtropical moisture. Overall Tue-Wed
rain totals of 3-7 inches are still likely for the coast, Coast
Range, and S WA Cascades, with general 1.50 to 2.50 inch totals
elsewhere. With plenty of snow still on the ground across the inland
valleys north of Salem, this will likely still be enough to cause
urban and small stream flooding issues as heavy rain and melting snow
combine to overwhelm drainage systems. Icing from the beginning of
the event and a frozen ground will exacerbate the situation as
well...with much of the Portland and Vancouver metro area expected to
be a slushy, sloppy mess by the afternoon commute Tuesday. The last
shortwave and cold front will bring one last dose of heavy rain
Wednesday before precip tapers to showers behind the cold
front...which ironically will likely be the mechanism which finally
scours out all the low-level cold air across the region.

One last item of concern...wind along the coast and in the higher
terrain of the Coast Range. A High Wind Watch remains in effect
primarily for the beaches and headlands, for gusts up to 70 mph
Tuesday night and Wednesday. This looks to be in good shape, as
models have actually increased 850 mb winds a bit over the past few
runs. For example the 06z NAM shows 850 mb winds 70-80 kt along the
coast 12z Wed. Gradients are a little too easterly to bring the
strongest wind to the coastal communities. The cold air entrenched
east of the Coast Range will help keep this the case throughout the
event. However, the strong SW winds will probably surface in the
higher terrain in the Coast Range beginning as early as Tuesday.
Future shifts may want to consider adding the Coast Range to the
existing watch or future High Wind Warnings.

Eventually this strong wind aloft will chip away at the low-level
cold air despite easterly pressure gradients through the Columbia
Gorge. Strong westerly flow aloft will induce lee-side troughing over
the Columbia Basin; the resulting Chinook effect will put a big dent
in the cold air over the basin. Hood River will probably be one of
the last spots to lose the low-level cold air...sometime around late
Wed afternoon or evening and just in time for it to be replaced by
the next batch of cold air behind the front Wednesday night and
Thursday.  Weagle

.LONG TERM...Wednesday night through Sunday...The active weather
pattern continues during the extended forecast. Models continue to
show the jet shifting south of our forecast area by Wednesday night,
with the upper level trough shifting inland by early Thursday. The
jet shifting south should help keep the majority of the precip aimed
at southern Oregon and northern California. However, the upper level
trough will help to maintain cool and showery conditions over the
Pac NW on Thursday and Friday, with snow level lowering to around
2000 ft by Friday. A larger and colder upper level trough looks to
impact the region over the weekend, with an outside chance for low
elevation snow possible on Saturday and Sunday.

&&


.AVIATION...Continued dry northerly to easterly flow will produce
primarily VFR conditions at most taf sites. The main exceptions
to this will be at KHIO and KEUG where occasional mvfr and ifr
conditions may occur through ~18z Monday. Warm moist air overrunning
cold dry air flowing out of the Columbia River Gorge will likely
produce a period of freezing rain for at least the PDX metro taf
sites beginning near or shortly after 12z Tuesday. KPDX and
especially KTTD could see significant ice accumulations.

KPDX AND APPROACHES...Low level easterly flow will produce VFR
conditions through 12z Tuesday. However, warm moist air overrunning
cold dry air flowing out of the Columbia River Gorge will likely
produce an extended period of freezing rain beginning near or
shortly after 12z Tuesday. /Neuman

&&

.MARINE...The first half of today will be the last day of
relatively quiet marine weather as we will enter an extended
period of dangerously large seas with occasional bouts of strong
winds.

Easterly winds across the waters early this morning will
begin shifting to a more southerly direction and then increase
from north to south across the waters this afternoon with gusts of
20 to 30 kt developing. Winds will likely increase further into
low end gale force criteria of 35 to 40 kt across the northern
waters this evening or near midnight and then spread into the
central Oregon waters after midnight. Given model agreement
upgraded the Gale Watch to a Gale Warning. Pressure gradients will
strengthen further Tuesday, which will support even stronger winds
to develop across the waters. Widespread 40 to 50 kt wind gusts
will then develop with a coastal jet strengthening late Tuesday
afternoon and evening. This will likely produce an area of even
stronger winds within 20 nm of the coast with gusts of ~55 kt. As
a result, a Storm Watch was issued to highlight this threat. These
strong winds look to linger across the waters through most of
Tuesday night, which should result in seas climbing well above 20
ft and likely into the 25 to 30 ft range. Given these seas will be
produced by winds moving parallel to the coast, do not foresee any
issues with high surf criteria.

Pressure gradients relax Wednesday, which should result in winds
and seas subsiding during the day. Models have remained somewhat
consistent that weak high pressure may lead to quieter weather
Thursday before another large low pressure system brings a series
of strong fronts across the waters Friday and over the weekend.
Given the position of these low pressures, these will stand a
better chance of bringing a large westerly swell that could result
in high surf criteria being met. Either way, expect periods of
Gale to Storm Force winds over the waters over the next week,
which will produce dangerously large seas at times. /Neuman


&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Flood Watch from Tuesday morning through late Wednesday night
     for Central Columbia River Gorge-Coast Range of Northwest
     Oregon-Greater Portland Metro Area-Lower Columbia-North
     Oregon Coast-Upper Hood River Valley-Western Columbia
     River Gorge.

     Winter Storm Watch from Tuesday morning through Wednesday
     afternoon for Central Columbia River Gorge-Upper Hood
     River Valley-Western Columbia River Gorge.

     Flood Watch from Tuesday evening through late Wednesday night
     for Cascade Foothills in Lane County-Central Coast Range
     of Western Oregon-Central Oregon Coast-Central Willamette
     Valley-Northern Oregon Cascade Foothills-South Willamette
     Valley.

     Winter Storm Watch from Tuesday morning through Tuesday
     afternoon for Northern Oregon Cascade Foothills-Northern
     Oregon Cascades.

     Wind Advisory until 10 AM PST this morning for Greater Portland
     Metro Area.

     Winter Storm Watch from late tonight through Tuesday afternoon
     for Greater Portland Metro Area.

     High Wind Watch from Tuesday afternoon through Wednesday
     morning for Central Oregon Coast-North Oregon Coast.

     Winter Storm Watch from late tonight through Tuesday afternoon
     for Central Willamette Valley.

     Winter Storm Watch from late tonight through Tuesday afternoon
     for Coast Range of Northwest Oregon-Lower Columbia.

WA...Flood Watch from Tuesday morning through late Wednesday night
     for Central Columbia River Gorge-Greater Vancouver Area-I-
     5 Corridor in Cowlitz County-South Washington Cascade
     Foothills-South Washington Cascades-South Washington Coast-
     Western Columbia River Gorge-Willapa Hills.

     Winter Storm Watch from Tuesday morning through Wednesday
     afternoon for Central Columbia River Gorge-South
     Washington Cascades-Western Columbia River Gorge.

     Wind Advisory until 10 AM PST this morning for Greater
     Vancouver Area.

     Winter Storm Watch from late tonight through Tuesday afternoon
     for Greater Vancouver Area.

     High Wind Watch from Tuesday afternoon through Wednesday
     morning for South Washington Coast.

     Winter Storm Watch from late tonight through Tuesday afternoon
     for South Washington Cascade Foothills.

PZ...Small Craft Advisory for winds from 10 AM this morning to 4 PM
     PST this afternoon for Coastal Waters from Cape Shoalwater
     WA to Cascade Head OR out 60 nm.

     Storm Watch from Tuesday afternoon through Wednesday morning
     for Coastal Waters from Cape Shoalwater WA to Florence OR
     out 60 nm.

     Gale Warning from 4 PM this afternoon to 1 PM PST Tuesday for
     Coastal Waters from Cape Shoalwater WA to Cascade Head OR
     out 60 nm.

     Small Craft Advisory for winds from 1 PM this afternoon to 10
     PM PST this evening for Coastal Waters from Cascade Head
     OR to Florence OR out 60 nm.

     Gale Warning from 10 PM this evening to 1 PM PST Tuesday for
     Coastal Waters from Cascade Head OR to Florence OR out 60
     nm.

     Small Craft Advisory for Rough Columbia River Bar until 9 AM
     PST this morning.

     Small Craft Advisory for Rough Columbia River Bar from 5 PM
     this afternoon to 4 PM PST Tuesday.

&&

$$

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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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