Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 270351

Area Forecast Discussion
National Weather Service Portland OR
851 PM PDT Wed Apr 26 2017

.SYNOPSIS...Strong NW flow aloft will bring continued showers tonight
through Thu. Snow levels will remain near or just below the Cascade
passes during this time. There will be a break in the weather Fri and
Sat, with dry and milder conditions. A weakening cold front will
bring the next round of rain Sat night into Sun. The trend is then
toward drier and warmer weather as an upper level ridge strengthens
over western North America. With any luck, it may actually feel like
spring next week.


.SHORT TERM...(Tonight through Sat)...Inherited forecast looks to be
in good shape this evening, and only minor changes were made. Cool
onshore flow and shallow instability will keep showers in the
forecast through Thursday, with showers becoming increasingly
confined to the higher terrain Thu night into Fri. Main changes this
evening centered around increasing the QPF and Cascade snow totals a
little tonight through Thursday, due to the continued favorable
orographic flow and presence of a 130-150 kt jet stream over or just
south of the forecast area. As mentioned in the afternoon discussion
(below), several inches of snow are expected in the high Cascades.
However, the strong late April sun will likely melt most snow
quickly off any road surfaces during the daylight hours...especially
at pass level and below. Opted against issuing another Winter
Weather Advisory, as impacts are expected to be minimal.  Weagle

Previous short/long term discussions from 252 PM Wed Apr 26 2017

A cold front brought a period of steady precip to SW WA and NW OR
last night into this morning. The front has pushed east of the
Cascades and the region is now in a cool and showery post-frontal
airmass. Showers are widespread this afternoon as temps have warmed
enough for some surface-based convection to develop. However, showers
should become increasingly driven by orographics this evening and
overnight as surface instability drops off with diurnal cooling.
Decided to drop the Winter Weather Advisory for the Cascades this
afternoon, as observations and webcams are just not showing any
notable accumulations.

There will be little change through Thu.  We will remain on the
poleward side of a strong Pacific jet stream that will bring
persistent NW flow aloft and keep decent orographic flow into the
Cascades. Thus, expect the best shower activity to remain over the
Cascades and the higher terrain of the Coast Range. Snow levels will
hover around 3000-4000 ft and several inches of snow are expected
near the higher Cascade passes. However, road temps should be warm
enough to prevent any significant travel impacts. Expect shower
activity to increase over the lowlands again Thu afternoon as the
fcst model soundings indicate temps again warm up enough to support
surface-based convection. It does not appear that convection will be
deep enough to support thunderstorm development, but would not be
totally surprised if there are a couple of isolated lightning strikes
detected in the afternoon, particularly if any sunbreaks are able to
develop during the morning hours.

An upper level ridge over the NE Pac will begin to strengthen over
the Pac NW Thu night into Fri. This will lead to gradually decreasing
shower activity Thu night. There may be a few weak showers lingering
over the higher terrain into Fri morning. However, think most
locations will be completely dry through the day. There will likely
be quite a bit of cloud cover to start, but expect a mix of sun and
clouds during the afternoon. Temps should recover back to near 60 for
the interior lowlands. The upper ridge axis will move over the region
on Sat, bringing another dry and even milder day. Think afternoon
highs will reach into the mid 60s for the lowlands, which believe it
or not, is right around average for this time of year. There should
be a sunny start to the day, but the fcst models are suggesting that
there will be increasing high clouds later in the day ahead of the
next approaching frontal system. Pyle

.LONG TERM...Sat night through Wed...A low pres system is modeled to
move into British Columbia late Sat, bringing a weakening cold front
onshore into Washington and Oregon Sat night. The fcst models
currently suggesting light rain develops on the coast Sat evening,
then moves through the interior overnight into Sun morning. Scattered
post-frontal showers likely lingering through much of Sun. Then the
upper ridge over the NE Pac is modeled to gradually strengthen again
over the Pac NW for the first half of next week. The ridge may not be
quite strong enough to prevent a few showers around the area Mon or
Tue, so will keep low chance PoPs in the fcst for now. By Wed, it
appears that the ridge strengthens considerably over the region. Most
of our fcst models now suggesting highs reaching into the 70s over
the lowlands Wed afternoon. Hopefully we have not jinxed it, but we
have gone with 70s in the fcst. Pyle


.AVIATION...Current TAF`s seem on track with showers going to
VCSH at 04Z. Only concern is that enough showers may persist
after 08Z that VCSH may need to be continued. Otherwise, a few
areas of lower MVFR ceilings tomorrow morning and then increasing
showers during the day tomorrow due to daytime heating and an
approaching wave.

KPDX AND APPROACHES...Mostly VFR conditions with a few occasional
MVFR cigs developing after midnight. Showers increase tomorrow
and conditions will remain mostly VFR except during heavier
showers. /Bentley


.MARINE...Benign winds and seas in the short term period. Seas
will hover in the 8 to 9 foot range which will be enough for
temporary 10ft conditions during the ebbs on the Columbia River
Bar. Broader high pressure builds over the NE Pacific through
Friday, maintaining a relative lull in winds. Seas may reach
10-11 ft late Thursday through early Friday as the southern end
of a swell train clips the northern waters. A stronger front may
brush the northern half of the waters Saturday night, with
advisory level gusts possible. Cullen/Bentley


PZ...Small Craft Advisory for Rough Columbia River Bar from 3 AM to
     8 AM PDT Thursday.


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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. is the U.S. government's official web portal to all federal, state and local government web resources and services.