Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 161719
AFDPQR

Area Forecast Discussion
National Weather Service Portland Oregon
918 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.

&&

.SHORT TERM...Today through Wednesday...It appears temperatures will
remain below freezing across most of our inland valleys Salem
northward as rain spreads into the region Tuesday morning. Thus, it
appears significant icing will occur across a much wider audience
than was previously expected.

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.  The south Willamette valley zone may be of concern as well
for a brief period of freezing rain Tue morning.  Should the rain
arrive as the GFS suggest FZRA would be more likely.

High Wind Watches have been posted for the beaches and headlands, and
a Flood Watch remains in effect for much of the forecast area.
Conversion to warning or advisories will occur this afternoon so we
can work out the details as new model information becomes available.
Confidence is high for freezing rain duration/amounts in the
Gorge/Upper Hood River Valley and east Portland/Vancouver metro area;
moderate confidence for the sheltered valleys of the Coast Range and
Cascade foothills; and the lowest confidence is for the Central
Willamette valley zone.

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. Despite the -11.6 mb KTTD-KDLS gradient
winds at the west end of the gorge have eased similarly to Sun.
Stronger winds may return again tonight, so for now will let the wind
advisory expire at 10 am.

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.  So thats about it.  Weagle /26

.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...VFR across the forecast area through at least 12Z Tue.
The primary exception will be in the Columbia Gorge east of KCZK
where offshore low-level flow will maintain an MVFR stratus layer.
Increasing mid-level clouds tonight should limit fog or IFR
stratus development in the South Willamette Valley. Strong
offshore gradient will continue to produce 35-45 kt gusts at the
west end of the Columbia Gorge. Increasing LLWS threat near the
Gorge after 08Z Tue as S-SW wind at and above FL020 increases to
30 to 35 kt by 12Z Tue. Initial precipitation expected to reach
the coast around 12Z Tue, with MVFR conditions likely by 15Z.
Initial precipitation reaches the interior between 14Z and 17Z,
likely as -FZRA in the North and Central Willamette Valley and in
the Gorge. Other isolated -FZRA pockets are possibly in the the
North Oregon Coast Range lower valleys and SW Washington interior
lowlands and lower slopes of the Cascades.

KPDX AND APPROACHES...Low level offshore flow will produce VFR
conditions through 12z Tuesday. East wind gusts 35-45 kt will
continue near the west end of the Gorge, including KTTD.
Precipitation expected to begin around 15Z Tue, as -FZRA.
Increasing LLWS potential after 08Z Tue. Weishaar

&&

.MARINE...Little change to the current forecast. Latest guidance
shows boundary layer wind speeds increasing to 20 to 30 knots
between 03Z and 08Z Tue. Have opted to push back the onset of the
gale warning in the north waters to 03Z Tue. Solid gales continue
over all waters overnight through about midday Tue. 12Z NAM shows
35 to 45 kt boundary layer wind speeds between 18Z Tue and 00Z
Wed. The GFS is a bit stronger with solid 40-45 knots across all
waters with coastal jet enhancement over the inner waters. Will
maintain the current Storm Watch. Models suggest wind speeds
diminish Wed evening. However, another deep low pres area moves
through the NE Pac Fri for potential strong gales. The GFS is
weaker with this low than the ECMWF, but would still result in
gale force wind for the waters.

Seas remain below 10 ft today. By 12Z Wed combined seas to 25 feet
can be expected. Given these seas will be produced by wind moving
parallel to the coast, do not foresee any issues with high surf
criteria. However, high surf conditions may develop late in the
week as a potentially strong westerly wind fetch sets up over the
waters. It appears the main energy will be directed at the central
and south Oregon waters but it is still too early to pinpoint the
details. Weishaar

&&

.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 until 7 PM PST this evening 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 7 PM this evening 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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