Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 191704
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
1000 AM PDT Tue Sep 19 2017

.SYNOPSIS...Fall may arrive on Friday, but Mother Nature decided not
to wait. Cool and unstable onshore flow maintains plenty of showers
and a few thunderstorms today, with some light snowfall in the high
Cascades. Another low will another round of more steady rain and some
additional high Cascade snow tonight into Wednesday, with possibly a
few in the northern interior. The primary upper trough will linger
over Washington and Oregon Thursday then slowly move east by the
weekend, with showers slowly decreasing. High pressure builds in over
the weekend and continues into early next week for some great late
September weather in store for the region, with cool nights with
patchy fog but mild sunny days.
&&

.SHORT TERM...Today through Thursday...Fairly widespread showers
evident on radar this morning with a few isolated thunderstorms along
the northern coast. Snow levels continue to hold around 5000 feet,
with some light accumulations possible in the highest Cascade
elevations as the showers continue. Another low pressure system will
drop in from the northwest and interact with more moisture offshore
that will initially move into southern Oregon but then spread north
into our area tonight. This moisture could form a deformation band
with heavy rain somewhere close to the Columbia River and pose a risk
of flash flooding and landslides over the Eagle Creek fire. Snow
levels will rise a bit tonight and early Wednesday.

The remainder of Wednesday will see more showers spreading over the
area in onshore flow. The models again suggest there will be some
instability Wednesday afternoon for a chance of thunderstorms.

A Flash Flood Watch remains in effect for the burn areas in the Gorge
and the Oregon Cascades through Wednesday morning, with the highest
rainfall rates expected late tonight through early Wednesday, though
heavier showers later today may be sufficient to cause some localized
issues.

For Thursday, the primary upper low will be east of the Cascades.
This will bring northerly to northeasterly upper level flow into the
region, with the potential for some showers to clip the eastern half
of the forecast area in the return flow around the upper low.
Cullen

.LONG TERM...No Changes. Previous discussion follows...
Thursday night through Sunday...Showers will be decreasing Thursday
as upper trough axis shifts more south of the region, and northerly
flow starts to bleed into our region. But, can not rule out a shower
from time to time. Will keep chance of showers in the forecast
through Fri evening, but higher threat will be on Thu over the higher
terrain.

High pressure builds over the region Fri, and remains through the
weekend. This will bring dry weather, with typical late September
weather. So, some late night/early am fog or low clouds, otherwise
partly to mostly clear with Mild days and cool nights.  Rockey.

&&

.AVIATION...Showery pattern will continue today with the typical
scenario of primarily VFR with short-lived bouts of MVFR cigs
and/or vis in heavier showers. The chance for thunderstorms will
generally be limited to coastal areas today, but there is a
chance inland north of about KSLE. Expect coverage of MVFR
conditions to increase tonight as a warm front approaches the
coast and spreads rain inland.

KPDX AND APPROACHES...Mainly VFR conditions today and tonight.
However small chance of MVFR cigs and/or vis restrictions
possible in heavier showers. Bowen

&&

.MARINE...Low pressure near WA coast with surface trough
extending south across the OR coast this morning. Winds have come
down some, with gusts to around 25 kt now, mainly well off the
northern Oregon coast. Showers and possibly thunderstorms will
make winds more erratic and gusty today. Winds ease this
afternoon. Seas have peaked at round 17 ft this morning and are
on their way down. Seas should subside below 10 ft later this
evening, but may creep above 10 ft tonight with the next storm.

Another low pressure system is forecast to track across the
south Oregon coastal waters later tonight. The track and
intensity are still somewhat still uncertain, but models are in
better agreement than 24 hours ago. Gusts above 25 kt are looking
less likely with the system than they were before. Either way,
seas will steepen with this system, with wave heights in the
range of 9 to 11 ft.

Winds dropping below 20 kt late Wed, are likely to remain on the
light side through the end of the week as a quieter weather pattern
returns. Bowen

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Flash Flood Watch through Wednesday morning for Cascades in
     Lane County-Central Columbia River Gorge-Northern Oregon
     Cascades-Western Columbia River Gorge.

WA...None.
PZ...Small Craft Advisory for winds until 1 PM PDT this afternoon
     for Coastal waters from Cape Shoalwater WA to Cascade Head
     OR out 10 NM-Coastal waters from Cascade Head to Florence
     OR out 10 NM-Waters from Cape Shoalwater WA to Cascade
     Head OR from 10 to 60 NM-Waters from Cascade Head to
     Florence OR from 10 to 60 NM.

     Small Craft Advisory for winds from 11 PM this evening to 11 AM
     PDT Wednesday for Coastal waters from Cape Shoalwater WA
     to Cascade Head OR out 10 NM-Coastal waters from Cascade
     Head to Florence OR out 10 NM-Waters from Cape Shoalwater
     WA to Cascade Head OR from 10 to 60 NM-Waters from Cascade
     Head to Florence OR from 10 to 60 NM.

     Small Craft Advisory for hazardous seas until 11 PM PDT this
     evening for Coastal waters from Cape Shoalwater WA to
     Cascade Head OR out 10 NM-Coastal waters from Cascade Head
     to Florence OR out 10 NM.

     Small Craft Advisory for hazardous seas until 3 AM PDT
     Wednesday for Waters from Cape Shoalwater WA to Cascade
     Head OR from 10 to 60 NM-Waters from Cascade Head to
     Florence OR from 10 to 60 NM.

     Small Craft Advisory for Rough Columbia River Bar until
     midnight PDT tonight.

&&

$$

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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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