Heavy Rain in Washington, March 15, 2017

Area Forecast Discussion
National Weather Service Portland OR
919 PM PDT Tue Mar 14 2017

.SYNOPSIS...A front draped across northwest Oregon and southwest
Washington will continue to bring periods of rain to the area through
Wednesday. Precipitation will turn showery Wednesday night with snow
levels temporarily lowering to near the Cascade passes. Drier weather
looks on tap for Thursday and early Friday before more wet weather
arrives late Friday and Saturday. Drier weather may once again return
for a day or two late in the weekend and early next week.

&&

.SHORT TERM...Tonight through Friday...Satellite imagery continues to
reveal an atmospheric river spreading moisture northeastward into the
Pacific Northwest. Closer to the surface, doppler radar and surface
observations continue to indicate light rain falling across most of
the region. The main exception to this so far today has been across
the far southern Willamette Valley and eastern Lane County where it
has remained mainly dry so far.

Infrared satellite imagery earlier this evening did depict a modest
baroclinic leaf develop near 130W and push northeastward towards
western Washington. This gives credence to the idea a weak surface
low pressure developed and is moving northeastward towards the
Washington coast. Models suggest this may shift the steadiest rain
northward late this evening and overnight and radar and surface
observations seem to be trending towards this idea. The inherited
forecast handled this well so not a whole lot of change, but did
reduce PoPs and QPF towards Lane County given trends so far.

A shortwave trough currently wrapping around the south end of the
large low pressure system in the Gulf of Alaska will shift eastward
over the next 24 hours and provide a mechanism to finally push this
band of rain plaguing the region to our south and east. Before this
occurs there may be a burst of heavier rain Wednesday midday or
afternoon across the region, but precipitation should turn showery
thereafter. QPF amounts still look to remain rather modest across the
region so most rivers will be handle the rain over the next 24 hours
with few problems. Our slower responding rivers will continue to
nudge upwards and will maintain the Flood Watch. It should also be
noted the Columbia River at Vancouver and Longview may reach flood
stage later this week, but impacts at this point, are expected to be
relatively minor. Nonetheless, the forecast has trended higher over
the past 24 hours as dam releases and water flows in rivers upstream
across eastern WA, eastern OR and Idaho become more certain so it
bears watching. Near the bottom of each river forecast webpage
located on our Advanced Hydrologic Prediction Services webpage, you
will see a Flood Impacts & Photos section that describes the known
impacts at different river heights.

Snow levels will eventually lower below the Cascade passes Wednesday
night. Models are in good agreement that shortwave ridging will begin
to push into the region quickly enough that Winter Wx Advisory snow
accumulations in the Cascades may be difficult to achieve. Perhaps
the more important news is that the shortwave ridging will be strong
enough that we should temporarily dry out for the second half of
Thursday and the first half of Friday.

The next storm system is currently well south of the Aleutians, but
will stream east-northeastward over the next couple of days and bring
the next round of rain...initially quite light...to the area
beginning Friday afternoon or evening. /Neuman

.LONG TERM...Friday night through Tuesday...Previous discussion from
Tuesday afternoon follows...The weather will quickly again turn
active by the weekend with an upper level trough solidly over the
Gulf of Alaska and a rather energetic impulse surging across the
Pacific. Models are in reasonably good agreement that a developing
surface low will move through the coastal waters before pushing
onshore in the northern Washington coast. As a result, another round
of fairly heavy rainfall will push through late Friday night through
Saturday, with some gusty winds possible along the coast and in the
Coast Range. However, the system is backed by a fairly strong jet, so
the system should be rather fast moving. Transient upper ridging in
between systems may bring a period of drier weather Sunday, but
models do differ in the evolution of the pattern and some guidance
suggests at least some showers across the northern reaches of the
forecast area are possible. Regardless, the next system will approach
the forecast area early next week for another round of rain.
Cullen

&&

.AVIATION...A stalled frontal boundary continues to bring steady
rain to SW Washington and far NW Oregon, and it will move very
little through Wed morning. Expect the coastal TAF sites to
remain IFR or low MVFR during this time. The northern interior
TAF sites will also likely remain MVFR with occasional IFR. The
central and southern Willamette Valley will remain south of the
heavier rain and look like a good bet to remain VFR overnight.
The front will slowly move further inland Wed afternoon, bringing
the deteriorating conditions through the rest of the interior
lowlands. The front will push into the Cascades Wed evening.
Expect conditions to gradually improve behind the front, with
rain transitioning to scattered showers.

KPDX AND APPROACHES...Predominantly MVFR conditions with steady
rain through Wed afternoon. There may be brief breaks of VFR, but
they should be short-lived. A cold front will move through during
the late afternoon, with rain tapering to showers behind the
front. Expect conditions to improve to VFR behind the front. Pyle

&&

.MARINE...A stalled frontal boundary will remain over the coastal
waters through Wed afternoon. Conditions will be fairly benign
overnight with southerly winds 10 to 15 kt. A low forming along
the front will move toward the central OR coast Wed morning. This
will bring an uptick in the winds over PZZ255/275, and decided to
issue a small craft for winds for these zones through much of the
day Wed. Expect winds generally 15 to 20 kt with gusts to 25 kt,
with the strongest winds south of Newport. Winds will be much
lighter over the northern waters. The front will finally move
onshore late Wed afternoon. Winds will turn westerly Wed evening,
and will likely gust 20 to 25 kt. A small craft advisory may end
up being needed for a few hrs Wed night. Then winds will then
subside later Wed night through early Fri as weak high pres
builds over the waters. Another stronger system will approach
from the SW late Fri into early Sat. The fcst models have been
pretty consistent with this system over the past several days,
bringing a surface low onshore over the WA coast. It appears
likely that there will be a period of gale force winds over the
WA and OR coastal waters with this system.

Seas will remain below 10 ft through the rest of the week.
However, the strong system late Fri into Sat should push seas
back into at least the mid teens. Pyle

&&

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

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

PZ...Small Craft Advisory for winds from 6 AM to 5 PM PDT Wednesday
     for Coastal Waters from Cascade Head OR to Florence OR out
     60 nm.

     Small Craft Advisory for Rough Columbia River Bar until 9 PM
     PDT this evening.

&&

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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.