Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 230437
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
829 PM PST Mon Jan 22 2018


.SYNOPSIS...A series of storm systems will bring periods of mountain
snow and valley rain to northwest Oregon and southwest Washington
through the weekend.


&&


.SHORT TERM...Tonight through Thursday...No Changes...Previous
discussion follows...Showers decreasing as transient ridging advances
from the west under zonal flow. A broad upper low centered in the
Gulf of Alaska will deepen while a short wave currently along the
southern boundary get pushed eastward. Models show the short wave
attempting to close off into a low circulation positioned of the BC
coast by tomorrow morning. The broad baroclinic leaf between 140 and
130W implies it will be a distinct possibility, but none of the
current models actually complete the circulation.

Nonetheless, there is a sizable moisture tap on the warm side of the
upper jet pointed toward the region. This jet alignment makes snow
levels a bit tricky given the upper pattern`s zonal flow and
trajectory of the shortwave/potential upper low. The surface
component of the feature in the NAM tries to spin up a surface low,
however, and that looks to bring an easterly component to the low
level winds. This easterly component could bring a modest amount of
snow to the area above 1000 feet between the upper Hood River Valley
and Mt Adams early tomorrow. Precip cloud be enough to bring 1 to 4
inches of snow before noon and just shy of an advisory level average
of 3 inches. Back aloft, there will be warmer air aloft pushed
northward as the upper feature drifts north off the NoAM coast and
that will serve to lift snow levels for a significant period
Wednesday. Still, it appears there will be enough accumulation
across the higher Cascades to warrant and advisory for the North
Oregon and Lane County Cascades above 5500 feet, and then a Warning
for the South Washington Cascades above 5000 feet.

Recent model runs (both larger scale and finer scale hi-res) hold
off on forming the aforementioned closed surface circulation.
Regardless, looks like a somewhat weak coastal jet will have time to
develop Tuesday night, but offshore pressure gradients should limit
winds to the headlands and pushing on 55-60 mph at best. That`s far
too marginal to justify issuing a warning since they will be
southerly winds and not likely to generate much impact, if at all.

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. /JBonk

.LONG TERM...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...Showers have become pretty sparse with conditions
mainly VFR though isolated MVFR conditions are possible. 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.

PDX AND APPROACHES...Mostly VFR conditions expected through the
period. A stray shower possible this evening. Steady rain and
gusty southerly winds will arrive Tuesday morning and continue
through the day. Expect deteriorating conditions through the day
with a start at VFR, eventually becoming MVFR by the late
morning to early afternoon. pt


&&


.MARINE...An active pattern coming up with a series of rather
strong systems to affect the waters much of the time through the
forecast period. First up is a strong low that is supposed to
deepen to around 985 mb and move up off the B.C. coast later
tonight and Tuesday. The trailing cold front will spread into
the waters later tonight and Tuesday, then stall over the waters
and not move onshore until early Wednesday. Look for high end
gales with gusts to 45 kt developing later by Tuesday continuing
through much of Tuesday evening. Our wave models are forecasting
that seas could get close to 20 ft on Tuesday 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. pt

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 10 AM Tuesday to 8 PM PST
     Wednesday for Cascades in Lane County.

WA...None.
PZ...Gale Warning from 1 AM Tuesday to midnight PST Tuesday night
     for Waters from Cape Shoalwater WA to Florence OR from 10
     to 60 NM.

     Small Craft Advisory for hazardous seas until 1 AM PST Tuesday
     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 Tuesday.

     Gale Warning from 6 AM Tuesday 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 Tuesday
     for Coastal Waters from Cape Shoalwater WA to Florence OR
     out 10 NM.

&&


&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 10 AM Tuesday to 8 PM PST
     Wednesday for Cascades in Lane County-Northern Oregon
     Cascades.

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

PZ...Gale Warning from 1 AM to 10 PM PST Tuesday for Waters from
     Cape Shoalwater WA to Florence OR from 10 to 60 NM.

     Small Craft Advisory for hazardous seas until 1 AM PST Tuesday
     for Waters from Cape Shoalwater WA to Florence OR from 10
     to 60 NM.

     Small Craft Advisory for Rough Columbia River Bar until 6 AM
     PST Tuesday.

     Gale Warning from 7 AM to 10 PM PST Tuesday for Coastal Waters
     from Cape Shoalwater WA to Florence OR out 10 NM.

     Small Craft Advisory for hazardous seas until 7 AM PST Tuesday
     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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