Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 231305
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
505 AM PST Tue Jan 23 2018

.SYNOPSIS...Low pressure moving into British Columbia will push a
cold front slowly across western Washington and Oregon today through
Wednesday. Strong south winds will develop along the beaches and
headlands this afternoon as the front inches toward shore. This
system will also spread plenty of rain across the forecast area, with
snow in the Cascades. Rain will slowly taper to showers and isolated
thunderstorms Wednesday, which will linger through Thursday. Snow
levels will lower to the foothills as a cold upper trough moves in
Thursday and Friday. Another strong system may bring more rain, wind,
and Cascades snow this weekend.

&&


.SHORT TERM...Today through Thursday...Apologies for how late this
discussion is getting out; we`ve had our hands full with a Tsunami
Watch that was issued for a significant earthquake off the coast of
Alaska. This watch has since been cancelled for the Oregon and
Washington coast.

Low pressure is heading into the British Columbia this morning, with
an attendant frontal system slowly approaching the Pac NW coast. Rain
is just getting started along the coast as isentropic lift develops
ahead of the occluding frontal system. Models show good agreement
that a fairly steady rain will spread across the forecast area
throughout the early morning hours, then linger across the majority
of the forecast area through Wed morning.

Perhaps the trickiest part of this forecast is how long cool air
banked up against the Hood River Valley and much of Skamania County
will last before being overcome by warm advection aloft. Higher
resolution models such as the UW WRF-GFS and 4 km nested NAM suggest
the cold air will be more stubborn than some of the relatively lower
resolution models such as the GFS, NAM and GEM suggest. With pressure
gradients expected to remain easterly through the Columbia Gorge
through late this afternoon or even early evening, it appears snow
levels will be slower to rise there versus the rest of our forecast
area. However, eventually strong and increasingly southwesterly flow
aloft will chip away at the eastside cold air, changing the snow to
rain below 3500-4500 feet this afternoon and evening. By then, it
appears 2-4 inches of snow and perhaps locally more will fall in the
Upper Hood River Valley. Therefore, we issued a Winter Weather
Advisory for the Upper Hood River Valley this morning until 1 PM this
afternoon.

The warmest part of this system for the Cascades will likely be this
evening as the occluding front inches across the forecast area. At
this point, snow levels look to be around 4000 feet for the S WA
Cascades, 4500 to 5500 feet for the North Oregon Cascades (lowest
north), and above 6000 feet for the Lane County Cascades. Areas above
these snow levels stand to get a substantial amount of snow, likely
measured in feet rather than inches due to the atmospheric river
involved with this system. The passes will escape the most
significant impacts, with rain mixing with and/or changing to snow
tonight before snow levels come back down later Wed/Wed night. With
the higher ski resort elevations of Mount Hood likely to remain
mostly snow, we decided to upgrade the Winter Weather Advisory to a
Warning for that zone. With most precip expected to fall as rain
below 6000 feet for the Lane County Cascades, we cancelled the
advisory for that zone.

As mentioned above, this system has abundant moisture associated with
it, with 2 to 4 inches of rain possible for the Coast Range and
Cascades and around 1 inch for the inland valleys. Latest river
forecasts suggest this will be enough to produce some decent rises on
area rivers, but not enough to cause flooding.

Winds will be strong along the beaches and headlands this afternoon
and evening as the front moves onshore. Once again, gradients appear
too easterly for the coastal communities to take the brunt of the
wind; this will likely be another marginal beaches and headlands
event with a few spots gusting to around 60 mph.

The associated cold front from the parent upper low will somewhat
stall out early Wednesday morning and then finally slide south
across the CWA Wednesday afternoon. This brings snow levels back
down to around 3000 feet for the evening. A secondary cold front
will then arrive Wednesday night to lower accumulating snow levels
to between 1500 and 2000 feet by Thursday morning. By then, the
heavier precip rates will have tapered off into a more showery
nature. However, the overall colder air mass will also bring
moderate instability and the possibility of small hail and
thunderstorms to the Willamette Valley and points westward including
the waters.  Weagle/JBonk

.LONG TERM...No Changes. Previous discussion follows...Thursday night
through Monday...Active weather pattern to continue through Friday as
a slow-moving trough keeps swinging moisture into the Pacific
Northwest, which looks to be followed by a very weak trough that will
pushes through during the weekend.

Snow levels hold to around 1500 ft Thursday through Friday morning.
850-mb temps lowering to around -5C on both the GFS and ECMWF.
Looking at the 1000-500-mb thicknesses, under onshore flow, we
typically need thicknesses below 522 dam for snow levels below 1000
ft (local study). Both the GFS and ECMWF have thickness around 525
Thursday morning, but increasing to around 530 dam through Friday.
The GFS is the lower of the two, with lowest thickness around 525
dam Thursday evening. Currently, snow levels are above 1000ft, this
could change as the week progresses.

Snow levels start to come back up Friday evening, and continue to
rise through the weekend. A milder system starts to move in with
another surge of moisture. Saturday morning will be pretty wet, but
ridging starts to build Saturday afternoon, pushing the moisture
plume north by late Saturday night or early Sunday. The Euro has the
ridge pushing through the area and potentially drying us out by early

Monday. The GFS has the ridge pushing through, but maintaining a
moisture plume focused on our area through Monday. With this
uncertainty, have leaned toward wetter PoPs. -42

&&

.AVIATION...VFR prevailing with rain spreading towards the coast.
Steady rain will move in Tuesday morning with increasing
southerly winds with an incoming front. MVFR ceilings and
visibilities are expected to spread in through the day Tuesday
ahead of this front. The expectation is for conditions to trend
to VFR after 00Z inland with a mix of MVFR and VFR at the coast.

PDX AND APPROACHES...VFR prevailing with a good chance for MVFR
after 16Z through about 00Z. Then should see conditions improving
to mostly VFR after 00Z.  /mh

&&

.MARINE...An active pattern continues this week. First up is a
strong low that is supposed to deepen to around 985 mb and move
up off the B.C. coast today with the trailing cold front to
spread into the waters this morning and move ashore tonight. Look
for high end gales with gusts to 45 kt developing through much
of Tuesday evening. Our wave models are forecasting that seas
could get close to 20 ft today as well, not falling much to
around midnight.

The low center off B.C. will then dumbbell around until it is
centered off Vancouver Island Thursday, with at least high end
Small Craft Advisory winds that could get close to gale force.
Seas Thu will initially be in the 10 to 15 ft range but an
upstream fetch arrives at the coast later Thursday and Thursday
night that could push seas back up into the 15 to nearly 20 foot
range Thursday night.

Winds and seas will ease a bit Fri, but another potentially
strong system could bring another bout of strong winds and high
seas by Saturday, though model details still vary quite a bit at
this point. /mh pt

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning from 1 PM this afternoon to 7 PM PST this
     evening for North Oregon Coast.

     Winter Weather Advisory from 6 AM this morning to 1 PM PST this
     afternoon for Upper Hood River Valley.

     High Wind Warning from 1 PM this afternoon to 10 PM PST this
     evening for Central Oregon Coast.

     Winter Storm Warning from 7 AM this morning to 8 PM PST
     Wednesday for Northern Oregon Cascades.

WA...High Wind Warning from 1 PM this afternoon to 7 PM PST this
     evening for South Washington Coast.

     Winter Storm Warning from 7 AM this morning to 8 PM PST
     Wednesday for South Washington Cascades.

PZ...Gale Warning until midnight PST tonight for Waters from Cape
     Shoalwater WA to Florence OR from 10 to 60 NM.

     Small Craft Advisory for Rough Columbia River Bar until 6 PM
     PST this evening.

     Gale Warning from 6 AM this morning to 1 AM PST Wednesday for
     Coastal Waters from Cape Shoalwater WA to Florence OR out
     10 NM.

     Small Craft Advisory for hazardous seas until 6 AM PST early
     this morning for Coastal Waters from Cape Shoalwater WA to
     Florence OR out 10 NM.

&&

$$

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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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