Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 182203
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
254 PM PDT Wed Oct 18 2017

.SYNOPSIS...A series of storm systems will bring periods of valley
rain that may be heavy at times, mountain snow and coastal winds
through the weekend.


&&


.SHORT TERM...Today through Saturday...Water vapor satellite imagery
reveals a large shortwave trough moving eastward towards the Pacific
Northwest. A warm front lifted northeastward across the region this
morning and led to widespread light rain to the north of a line
extending between Pacific City and Hood River. An atmospheric river
streaming northeastward ahead of a cold front is currently taking
aim at western Washington. Rainfall rates are generally peaking
between 0.35-0.50" per hour across the Olympic Peninsula. This
atmospheric river will gradually shift southeastward into southwest
Washington and northwest Oregon tonight and early Thursday. As the
upper level trough slides south of the region, this front will
become oriented a little more north-south and weaken over the next
24 hours. This should result in hourly rainfall rates decreasing
with time as it drops southeastward across the region. As a result,
few flooding issues exist in the short term although we will
certainly need to watch rainfall rates over the burn scars.

Meanwhile, southerly winds have pushed onto our coast this
afternoon. So far, winds have remained in check, but these should
increase over the next couple of hours and at least flirt with high
wind warning criteria. Despite this being a somewhat marginal event,
will keep headlines intact, particularly given it`s the first real
wind of the season along the coast.

A colder and more unstable airmass will spread into the region late
Thursday afternoon and evening. This should turn precipitation more
showery with a few thunderstorms possible over the coastal waters
Thursday night. Models are hinting that a weak surface low could
develop over the far northeast Pacific Thursday night, which could
create a wind field a little more conducive to rotating storms than
current model output suggests so this will be worth monitoring. In
addition, snow levels will lower to near the Cascade passes by late
Thursday night and Friday as colder air aloft filters into the
region. Moderately strong onshore flow coupled with the unstable
atmosphere should produce some decent snow accumulations in the
Cascades above 4-5kft Friday with travel impacts certainly possible
at pass level, particularly at Santiam and Willamette Passes.

Any break in the wet weather will be short lived Friday night as
models are in good agreement the next warm front will spread light
to moderate rain into northwest Oregon and southwest Washington by
late Friday night with rain continuing for much of the day Saturday.
Subtle, but significant model differences exist in exactly on how
long and where an atmospheric river will then take aim at the
region, but it has the potential to bring hydrologic issues. /Neuman


.LONG TERM...Saturday night through Wednesday....Strong moist zonal
flow will set up over the region Saturday night through Sunday. Snow
levels will rise above 8,000 feet. Still some uncertainty as to
whether or not the heaviest rain will set up over western Oregon or
western Washington. Have kept PoPs high, but forecast details such
as amount, intensity, and duration are unclear.

Expect to see some rises on area rivers, especially in the coastal
basins, and if the heavy rain sets up over urban areas, some
localized urban flooding issues could develop. Also, locations with
significant burn scars should be aware of possible flash
flooding and debris flows.

Models are suggesting, short wave ridging will develop over the
Pacific NW early next week, which should bring at least a day or two
of dry weather. The Euro model suggests a weak front could push into
the region during the middle of next week while the GFS remains dry
and mild. Given the uncertainty, have trended PoPs towards
climatology. Bishop


&&


.AVIATION...Generally a mix of VFR and MVFR conditions over the
interior this afternoon and evening, expected to go to predominantly
MVFR conditions later tonight after 07z as a cold front pushes rain
inland. MVFR conditions appear to be the most likely flight category
over the interior TAF sites through the day Thu. Along the coast
expect MVFR, and locally IFR conditions to continue tonight. As a
cold front moves onshore Thu, improvement to VFR category expected
along the north Oregon coast after 12z, while further south MVFR to
IFR conditions are expected to persist through most of the day.

KPDX AND APPROACHES...Mainly VFR conditions through 08z tonight,
although there is a chance for seeing some MVFR cigs anytime before
08z. After 08z expect MVFR conditions to become predominant as ran
associated with a cold front moves in.


&&


.MARINE...Southerly gales continue this evening ahead of a cold
front that is expected to move se across the waters tonight. The
front is expected to reach the vicinity of Cape Disappointment
around 10 pm tonight, and Newport around 6 am Thu. There is a likely
to be a coastal jet influence ahead of the front, where winds are
expected to gust as high as 45 kt within about 10 nm of the coast
ahead of the front. Seas will build in response to the local winds,
peaking on the order of 16 ft tonight.

Winds will likely drop into range of 15 to 25 kt behind the front,
which should allow seas to drop a few feet early Thu morning. A
large long period swell will arrive later in the day Thu from the
west. The swell will peak between 20 and 23 ft Thu evening, then
slowly subside Fri and Fri night. These wave heights coupled with
the long period, will necessitate a High Surf from Thu afternoon
through Fri morning. Another storm system capable of producing at
least Gale Force wind gusts appears on track for Friday night and
Saturday.


&&


.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning until 11 PM PDT this evening for Coast Range
     of Northwest Oregon-North Oregon Coast.

     High Surf Advisory from 1 PM Thursday to 10 AM PDT Friday for
     Central Oregon Coast-North Oregon Coast.

WA...High Wind Warning until 11 PM PDT this evening for South
     Washington Coast-Willapa Hills.

     High Surf Advisory from 1 PM Thursday to 10 AM PDT Friday for
     South Washington Coast.

PZ...Gale Warning until 11 PM PDT this evening for Coastal waters
     from Cape Shoalwater WA to Cascade Head OR out 10 NM-
     Waters from Cape Shoalwater WA to Cascade Head OR from 10
     to 60 NM.

     Gale Warning until 5 AM PDT Thursday for Coastal waters from
     Cascade Head to Florence OR out 10 NM-Waters from Cascade
     Head to Florence OR from 10 to 60 NM.

     Small Craft Advisory for Rough Columbia River Bar until 6 PM
     PDT Thursday.


&&


$$

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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 forecast area.


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