Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 162252 AAA
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
237 PM PST Tue Jan 16 2018

.SYNOPSIS...Showers will decrease today as low pressure moves north
into the Gulf of Alaska. Another low will approach the coast tonight
and Wednesday then stall offshore the British Columbia Coast Thursday
and Friday. This low will bring a dry warm front to the area
Wednesday followed by a moist cold front Wednesday night and
Thursday. Steady showers are expected Thursday and Friday with snow
levels below the Cascade passes. Showers continue this weekend as the
low weakens and moves inland, with a stronger front providing for
heavier mountain snow on Saturday and Sunday.

&&

.SHORT TERM...Tonight through Friday. The next low pressure system
and associated front are visible well offshore on latest satellite
imagery. This system will track across the northeastern Pacific and
bring a more active period of weather to the forecast area. Some
light rain is possible overnight as the leading warm front crosses
the region, but expect rather minimal rainfall mostly along the coast
and in portions of southwestern Washington; elsewhere, expect the
Willamette Valley and Oregon Cascades south of Mount Hood to remain
mostly dry overnight. Snow levels will rise above the Cascade passes
as warm air moves into the region Wednesday behind this front, and
afternoon high temperatures may remain above seasonal normals across
most of the region.

The cold front associated with this disturbance will push onshore
late Wednesday afternoon and push inland to the Cascades by Wednesday
night. This will bring a round of heavy rain near the front as it
moves across the region. Expect around one inch of rain at the Coast
and in the Willamette Valley, with up to 1.5 inches in the Cascades.
A burst of gusty winds can be expected along the coast as the front
pushes onshore - primarily along the beaches and headlands - with
gusts to 55 mph possible. However, the alignment of the pressure
pattern remains a bit unfavorable for high winds to mix down and
develop across a widespread area of the coastal towns, so generally
think speeds will remain below warning criteria. At the same time, a
large swell in the coastal waters could bring high surf to the
beaches late Wednesday and Thursday. Those who will head to the
coastline should exercise caution and remain a safe distance away
from the beaches and jetties as potentially dangerous large waves can
move onshore with little notice.

Behind the front, a colder air mass aloft will filter in to the
region and bring snow levels back down to near the Cascade passes on
Thursday. Snow levels will continue to lower further, down to around
2,500 feet by later Thursday through Friday. The parent low pressure
will stall off the British Columbia coast, which will maintain a
decent flow of moisture across the region into Friday. While no
single time period Thursday or Friday looks to receive an excessive
amount of snow, expect rather steady rounds of light snow with
anywhere from 1 to 4 inches per 6 hour period. At this point, expect
that accumulations will remain below advisory or warning criteria
through Friday, though localized heavier amounts are probable at the
higher elevations (above 6,000 feet). At the lower elevations, fairly
widespread showers will continue Thursday and Friday with another 1
to 1.5 inches of rain likely to accumulate. Afternoon high
temperatures will trend much closer to seasonal normals on Friday and
remain in the mid 40s. Cullen

.LONG TERM...Friday night through Tuesday...Active weather pattern
continues through the weekend. Wave after wave of energy will push
into the Washington and Oregon coast from a broad trough of low
pressure over the Gulf of Alaska. With this moisture, snow levels
will be down 2000 to 3000 ft, so this will be a great pattern to
build the snowpack in the Cascades.  This will also, however, mean
those planning travel through the Cascades this weekend will need to
be prepared for snow, which could be heavy at times. Rain on the
Coast and in the Willamette Valley will mean rises on rivers across
the area as well, though we are starting out with fairly low river
levels for this time of year.

There`s some uncertainty in the pattern early next week, but general
consensus is that we may get a break in the precip late Monday into
Tuesday, with either broad or amplified ridging over the West Coast.
Left a slight chance for PoPs because with a more broad ridge, can`t
rule out precip completely. -McCoy

&&

.AVIATION...Generally VFR across the area at 21Z, except for
areas of low-end MVFR stratus in the Central Columbia River
Gorge. Expect VFR to prevail across the forecast area through at
least 12Z Wed. Warm-frontal precipitation develops along the
coast during the evening and then spreads inland overnight.
Likely to have MVFR stratus in the Central Columbia Gorge this
evening through tonight as offshore low-level flow intensifies.
May see areas of MVFR along the coast after 17Z Wed.

PDX AND APPROACHES...VFR to be the dominant flight category at
the terminal through 18Z Wed. Brief cigs AOB 040 through 01Z Wed.
East wind at the west end of the Gorge will intensify after 03Z
Wed. Weishaar

&&

.MARINE...Wind will begin backing to more southeast early this
evening. A strong surface low center is forecast to deepen to
960 to 965 mb and be near 45N 140W by 12Z Wed. The 12Z NAM has
come more in line with the GFS and ECMWF regarding the central
pressure. The low moves northeast Wed and will send a warm front
across the waters late tonight with a strong trailing cold front
late Wednesday afternoon. Expect solid gales to develop over the
far outer waters around 06Z Wed and then spread east overnight
and into Wed morning. The 12Z guidance shows the potential for a
relatively short period of 50-55 kt gusts late Wed afternoon into
Wed evening, primarily over the outer waters. The NAM suggests
the possibility of coastal jet enhancement Wed afternoon, with up
to 75 kt wind speeds at 925 mb. The forecast surface isobar
orientation 00Z Thu shows some southeast component, which is not
favorable for enhanced mixing. Therefore, not confident a coastal
jet will develop, but feel it is prudent to mention the
possibility.

Increasing confidence that storm force gusts will occur and have
upgraded the gale warning to a storm warning valid 18Z Wed
through 03Z Thu. Strongest wind likely to occur over the outer
waters between 20Z and 00Z Thu and an hour or two later for the
inner waters. Post frontal gales to then continue for the outer
waters later Wed evening into Thu. 12Z GFS and 12Z ECMWF have
trended weaker with the late Sat/Sun system. Will go with
marginal gale force wind Sat evening.

Seas will continue in the lower teens today then build to the
upper teens by mid-day Wednesday. The highest seas are expected
to move inside 60 nm by daybreak Thursday. The 12Z spectral
guidance and ENP graphical output have come more in line with the
earlier ECMWF runs indicating peak wave heights near 30 ft late
Wed night/Thu morning. The ECMWF remains about 2-4 ft lower.

There is growing confidence that areas of the shoreline will be
highly impacted beginning late Wed night by the expected large
waves. The local Coastal Flood Index tool gives an output value
of 23. Coastal Flood Warning criteria is 22. The forecast value
assumes a slightly higher tidal anomaly. In any event, 30-33 ft
west swell with 15-18 second periods will contain an enormous
amount of energy. One thing to keep in mind is there will likely
be isolated higher waves. Seas diminish Fri, but will remain
above 20 ft through Fri night, then ease to the upper teens Sat.
Weishaar

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Coastal Flood Warning from midnight Wednesday night to midnight
     PST Thursday night for Central Oregon Coast-North Oregon
     Coast.

WA...Coastal Flood Warning from midnight Wednesday night to midnight
     PST Thursday night for South Washington Coast.

PZ...Small Craft Advisory until 2 AM PST Wednesday for Waters from
     Cape Shoalwater WA to Florence OR from 10 to 60 NM.

     Storm Warning from 10 AM to 7 PM PST Wednesday for Coastal
     Waters from Cape Shoalwater WA to Florence OR out 60 NM.

     Gale Warning from 2 AM to 10 AM PST Wednesday for Waters from
     Cape Shoalwater WA to Florence OR from 10 to 60 NM.

     Small Craft Advisory for Rough Columbia River Bar until 4 AM
     PST Thursday.

     Small Craft Advisory until 10 AM PST Wednesday 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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