Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 191032
AFDPQR

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

.SYNOPSIS...Fall may arrive on Friday, but Mother Nature decided not
to wait. One low was moving onshore this morning cool unstable
onshore flow spreading plenty of showers and a few thunderstorms
onshore with snow in the higher Cascades. This will continue through
today. Another low will bring more rain or showers and higher Cascade
snow levels to the forecast area tonight into Wednesday, with a few
thunderstorms near the coast which will spread inland Wednesday
mainly in the north. The parent upper low 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...As previously advertised, a
low was moving onshore into this morning, with plenty of cool
unstable air spreading showers and a few thunderstorms with small
hail onshore. This will continue through the day today. Snow levels
will between 5000 and 6000 feet, with the best chance of decent
accumulating snow above about 6000 feet.

Another low will drop in from the northwest and interact with more
layered 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.

Have maintained the Flash Flood Watch for the burn areas in the Gorge
and the Oregon Cascades through Wednesday morning. It is looking like
the greatest risk is tonight and Wednesday morning, though we cannot
rule some issues out for today as well.

The parent upper low will be centered inland over Washington and
Oregon with northerly flow aloft over our area. We will see some
backdoor showers over our area though with less coverage. Tolleson

.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 some MVFR cigs and/or vis restrictions possible in heavier
showers.
&&

.MARINE...Low pres off the WA coast with surface trough extending
south across the OR waters early this morning. Small craft advisory
winds are gusting to near 30 kt at buoy 46089 - about 80 NM offshore,
but winds are more lower closer to shore. Winds ease later this
afternoon as the low weakens. Organized showers or thunderstorms will
make winds more erratic and gusty today. Winds ease this afternoon.
Seas have climbed to 15 to 17 ft with a 14 to 16 second period and is
probably a few hours away from peaking another foot or two higher.
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. At this point, it still looks likely to produce
wind gusts to 30 kt, with the possibility of some gale force gusts
late tonight and Wed morning, especially over the southern part of
the waters. Seas will steepen with this system, with wave heights in
the range of 10 to 12 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.
&&

.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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