Flooding in Washington, March 15 to 31, 2017

Area Forecast Discussion
National Weather Service Portland OR
247 PM PDT Wed Mar 15 2017

.SYNOPSIS...An atmospheric river continues to feed rain into
southwest Washington and northwest Oregon along a cold front
which is slowly moving through the local area this afternoon. Behind
the front, precipitation will turn to showers and snow levels will
temporarily lower to below the Cascade passes. Drier weather on tap
for Thursday and early Friday before more wet weather arrives later
Friday and Saturday. Drier weather may once again return for a day or
two late in the weekend and early next week.

&&

.SHORT TERM...This afternoon through Saturday...Radar shows the most
intense rain generally over the Cascades and foothills this
afternoon, just ahead of a cold front which is slowly moving east
across the area. Expect the front to finally push through the area
later this afternoon. Showers behind the front will be decent for a
while tonight as snow levels lower to near or below the Cascade
passes. We could see a few inches of snow as a result, but it looks
like the moisture will dry up fairly quickly, so don't expect enough
snow to have a lot of impact.

As far as potential river flooding goes, will continue to keep a
close eye on the Columbia River in particular, but mitigation efforts
are underway to keep it from reaching minor flood stage, so will just
keep the Flood Watch for now. Haven't heard much about areal flooding
and it doesn't seem like rain has been intense enough to cause fast
rises on small streams or significant additional lowlands flooding, so
will also just continue to keep an eye on trends for that.

Thursday is looking a little bit drier in the latest model runs, so
decreased POPs some, especially from about Salem and Newport south.
Showers taper off for the entire local forecast area by Thursday
evening, so expect everyone to see at least some sun Thursday before
it sets. Much like the last couple of sun breaks we've seen recently,
this one will be fairly brief.

After some possible morning fog early Friday, moisture from the next
system should start to spread in from south to north starting late
Friday morning or Friday afternoon as a weak surface low moves north
up the Oregon coast. The main slug of moisture will be with a cold
front trailing the low on Saturday. Although the entire area will see
some rain with this system, right now it looks like the focus for
rain intensity and amounts will be near the coast and in the Coast
Range. Snow levels rise back up to 7,000 to 8,000 ft as the low
passes Friday, so once again the Cascades will see mostly rain with
this system. Bowen

.LONG TERM...Saturday night through Wednesday...Rain tapers to
showers Saturday night and then the forecast becomes temporarily dry
again as transient upper ridging develops Sunday. Models are in good
agreement that Sunday will be dry, so removed POPs for the entire
area. Dry conditions look like they'll continue into Monday for most
areas, but rain will return again from south to north starting as
early as Monday morning. The associated surface low will move north
off the coast through Tuesday for continued rain. However, models
diverge with this system, with some bringing the low inland and some
showing it passing by off the coast, so confidence in timing and
rainfall amounts is low. There is continuing and increasing
disagreement among the models after Tuesday, although it looks like
we will remain in a progressive and likely wet pattern. Bowen

&&

.AVIATION...Areas west of I-5 how now settled in to MVFR ceilings
with IFR ceilings along the coast while east of I-5, VFR
conditions are dominating. In addition, south winds have been
gusty ahead of the front. Expect these gusty winds to continue
for a few more hours and weaken once the cold front moves through
in the next few hours.

KPDX AND APPROACHES... Mainly VFR conditions through the period
with gusty south winds weakening this evening. Scattered showers
will come to an end by mid day tomorrow with skies becoming
mostly sunny by the evening. /Bentley

&&

.MARINE...Cold front has almost finished moving through the
marine waters and is expected to move through in the next hour or
two. Therefore, have kept the 4PM expiration for the SCA for
winds for the inner waters. Behind the front, west winds will
gust close to 25 knots through the evening hours and into the
overnight hours. These stronger winds will be confined to the
northern outer waters by early Thursday morning and will remain
through Thursday evening. The next system of interest will move
into the area on Friday afternoon and bring an area of gale force
winds for most of the waters. We will need to monitor this system
as some forecast models (GFS) show a compact area of stronger
storm force winds.

Seas will remain mostly benign this week until the stronger low
arrives at the end of the week. Regardless of the strength of
this low, seas are likely to reach above 10 feet with this
system, but if some of the stronger solutions verify, seas may
approach the upper teens. /Bentley

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Flood Watch from 11 PM PDT this evening through Saturday
     afternoon for Central Willamette Valley-Greater Portland
     Metro Area-Lower Columbia.

WA...Flood Watch from 11 PM PDT this evening through Saturday
     afternoon for Greater Vancouver Area-I-5 Corridor in
     Cowlitz County.

PZ...Small Craft Advisory for winds until 2 AM PDT Friday for Waters
     from Cape Shoalwater WA to Cascade Head OR from 10 to 60
     nm.

     Small Craft Advisory for winds until 11 PM PDT this evening for
     Waters from Cascade Head to Florence OR from 10 to 60 nm.

     Small Craft Advisory for winds until 4 PM PDT this afternoon
     for Coastal waters from Cascade Head 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. The area is
commonly referred to as the forecast area.