Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 260414
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
913 PM PDT Tue Apr 25 2017

.SYNOPSIS...Rain has generally tapered off a bit across NW Oregon and
SW Washington this evening, but any break will be brief for much of
the district. The system which brought a rainy afternoon today has
since moved across the Cascades, but a moist frontal system lingers
offshore in the NE Pacific. This system will bring a decent amount of
rain and high elevation snow later tonight into Wednesday, especially
to western Oregon. Snow levels are expected to lower back down to the
Cascade passes late tonight and Wednesday morning, where a few inches
of new snow are expected. Showers are expected to decrease later Wed
into Thu, with higher pressure leading to drier weather Fri and Sat.
The next weak frontal system is expected Sat night or Sunday,
bringing a chance for rain.

&&

.SHORT TERM...Tonight through Friday...The warm front which brought a
rainy afternoon for most of the forecast area has since moved into
eastern WA/OR this evening, leaving behind a fairly soupy and moist
air mass behind it across SW WA/NW OR. Meanwhile, water vapor
satellite imagery shows a nearly stationary baroclinic zone
stretching from the Oregon and Northern California coast west-
southwestward all the way to West Pacific Tropical Depression Three
east of the Philippines.

This tap to deeper tropical moisture is showing up in the latest
derived blended TPW imagery, which indicates a swath of 1.25-1.75
inch total precipitable water along the front just off the OR/CA
coast. Meanwhile, a decent upper level shortwave can be seen just
north of the frontal zone, crossing 140W and moving fast courtesy of
a 125 kt+ jet stream which is aimed toward the central and southern
Oregon coast. The jet and the shortwave appear to be organizing a
weak wave of low pressure along the front, with some signs of a
baroclinic leaf and cooling cloud tops inside 135W.

The result of all this will be an increase in rain spreading onshore
into western Oregon later tonight, while an increasingly strong jet
stream supports heavier precip along the Cascades. This, combined
with gradually cooling 850 mb temps, will likely cause the snow level
to slowly fall to Willamette and Santiam Pass level late tonight and
early Wednesday morning, while precipitation is still rather heavy.
Decided to issue a Winter Weather Advisory for our Oregon Cascades
zones above 4500 feet midnight-noon Wed. While the passes may come up
just short of the 6-inch advisory criteria, ski resort elevations
above 5000-5500` elevation could see as much as a foot of new snow
due to favorable orographics, moisture, and good jet dynamics
associated with this system. Lower elevations should get a good
soaking from this system later tonight into early Wed, especially
south of Salem.

As cold air continues to move into the region behind the front on
Thursday, snow levels may fall a bit further, but should remain above
3,000 feet. Little change in the pattern on Thursday, with ongoing
showers in the Cascades with orographic enhancement due to the
westerly flow aloft. However, a passing shortwave impulse may also
enhance shower coverage into the interior.

Finally, after another very wet week, models suggest a bit of hope on
the horizon of the short term. Heights begin to rise and flow aloft
turns a bit more northerly, leading to a drying trend. While showers
will still be around - especially in the Cascades, but also elsewhere
- there will likely be some breaks in the clouds at times. Best
chance at dry and clearing will be across the southern and western
portions of the forecast area.Weagle/Cullen

.LONG TERM...Friday night through Tuesday...Showers continue to
further diminish later Friday through Saturday as an upper level
ridge over the eastern Pacific will further amplify in between low
pressure systems both upstream and downstream. Not quite confident
enough yet to bring sky cover to mostly sunny wording, but did trend
things that direction. Meanwhile, temperatures will also respond
accordingly with high temperatures in the afternoon warming into the
mid 60s - all the way to around seasonal normals, at least for one
day! However, this will be yet another only short-lived intermission
in the season of steady rain. A weakening frontal system will bring
increasing round of clouds, another chance of rain, and slightly
cooler temperatures. Forecast models do diverge a bit the first half
of next week, but as we transition to May there is some potential
for a bit of shift in the pattern. At this point, trended the
forecast dry for Monday and Tuesday, but did so somewhat cautiously
for two reasons - first, ensemble members continue to suggest a
somewhat large spread in possible solutions; second, we`ve seen this
sort of pattern in the models several times in recent weeks, where a
seeming trend toward dry conditions and a building ridge fails to
appear. That said, this would be consistent with the latest CPC 6-10
outlook of warmer and drier than average May 1-5 period. An abundance
of words to say the forecast was trended this direction but with
lower than typical confidence. Cullen

&&

.HYDROLOGY...The lower Columbia River will be running above bankfull
for the next several days. Primary areas of concern are the Columbia
near Vancouver and downstream past the Columbia near Longview to the
mouth. Contributions to the high flow are coming from high
astronomical tides in addition to increased runoff from the Snake
River and Willamette River basins. Lower reaches of tributaries
flowing into the Columbia River may also experience periods of
elevated water during the high tide cycles. Please continue to
monitor the forecasts for the next several days. /JBonk

&&

.AVIATION...Widespread IFR to low MVFR conditions this evening
as a frontal system slowly moves across the area. Expect ceilings
to remain low overnight in the IFR to MVFR range. Conditions
improve during the day Wednesday with cigs lifting up to VFR with
showers around the area.

KPDX AND APPROACHES...IFR ceilings this evening lifting up to
MVFR later tonight. Conditions improve to VFR during the day on
Wednesday as the front moves east of the area with higher cigs and
showers in the area.

&&

.MARINE...Solid small craft winds early this evening beginning to
decrease. Expect winds to be 20 kt or less after 06z and remain
15-20 kt Wednesday. Seas running 7 to 9 ft with short period
tonight which will create some rough conditions. A weak surface
low passes south of the waters Wednesday while broader high
pressure builds over the NEPac keeping a relative lull in winds
through Friday. Seas may next reach 10-11 ft Thursday through
early Friday as the southern end of a swell train clips the
northern waters from a surface low crossing the Alaskan
Panhandle. tw

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from midnight tonight to noon PDT
     Wednesday for Cascades in Lane County-Northern Oregon
     Cascades.

WA...None.
PZ...Small Craft Advisory for winds until 11 PM PDT this evening for
     Coastal Waters from Cape Shoalwater WA to Florence OR out
     60 nm.

     Small Craft Advisory for hazardous seas until 5 AM PDT
     Wednesday for Coastal Waters from Cape Shoalwater WA to
     Florence OR out 60 nm.

     Small Craft Advisory for Rough Columbia River Bar from 3 AM to
     7 AM PDT Wednesday.

     Small Craft Advisory for Rough Columbia River Bar from 4 PM to
     8 PM PDT Wednesday.

&&

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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.



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