Area Forecast Discussion
Issued by NWS Portland, OR

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323
FXUS66 KPQR 242345 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
445 PM PDT Mon Apr 24 2017

.SYNOPSIS...Expect periods of rain through midweek with decreasing
showers Thursday and Friday. Drier weather looks to return Saturday,
but is looking short lived with additional wet weather possible
Sunday or Monday.

&&

.UPDATE...Isolated thunderstorms have sprouted in the Cascade
foothills this afternoon near Estacada and Table Rock as colder air
aloft (around -28 degC) is beginning to move over the area. Satellite
imagery shows lines of convective showers forming across the
Willamette Valley into the foothills that indicate additional
isolated thunderstorms are possible through early this evening. Have
therefore updated the forecast to add a slight chance for
thunderstorms for the Willamette Valley and the Cascade foothills.
Besides lightning and thunder, heavy rain can be expected with these
storms. A Cocorahs report in Marion county reported a heavy downpour
of rain with one of these storms that resulted in brief minor
flooding. ~TJ

PREVIOUS SHORT TERM (from 256 PM)...Today through Thursday...A
surface low pressure is currently moving across the PDX metro and
into the Columbia River Gorge and helping to spread the widespread
rain across the area this morning to the east of the Cascades. The
airmass is beginning to destabilize as -28C air at 500mb moves into
northwest Oregon and southwest Washington. Precipitation has turned
increasingly convective with a couple small cells producing locally
heavy rain across portions of the Willamette Valley. Expect
additional showers to develop in the wake of the current activity.
Snowfall amounts have been notable above 5000` today with 7-11" so
far. Several more inches of snow will be possible so will keep the
Winter Wx Advisory going until it expires at 8pm.

Shortwave ridging building into the Pacific Northwest should bring
decreasing showers overnight. However, the next occluding frontal
storm system will push more rain into the area Tuesday afternoon,
which will likely continue into early Wednesday. Our CWA will
continue to remain on the polar side of a strong jet so showers
should continue into Wednesday and Thursday, but will become more
tied to the terrain especially at night. /Neuman


.LONG TERM...Thursday night through Monday...The strong jet aimed
over the Pacific Northwest during the middle of the week will slowly
become more north-south oriented and shift east of the region
Thursday night and Friday. This should result in decreasing showers
during this period. Models are in decent agreement that shortwave
ridging will push over the Pacific Northwest Friday night and
Saturday and will bring some sunshine and mild temperatures. This
pleasant weather could last into Sunday, but it could certainly be
short lived based on the latest operational and ensemble model
guidance. As a result, trended PoPs into at least mentionable
thresholds and closer to climo for Sunday and Monday. /Neuman


&&


.AVIATION...Surface low is currently over KPDX and will continue
slowly moving ENE. Have seen most winds start to favor a westerly
onshore direction. This will bring continued MVFR cigs across
most terminals through at least 20/03z. Exceptions are KEUG where
it is showing VFR and expect KSLE to improve over the next
couple hours. Most guidance shows the remainder of the TAF
terminals to remain under MVFR Cigs for the next 24 hours. Not
too sure how likely that will be as a front will approach
starting late tonight and start to mix the lower levels a bit.
Inland terminals will likely start picking up an MVFR Cig 020-030
after 25/12z and lasting until at least 25/18z.

KPDX AND APPROACHES...Expect MVFR cigs to continue improving
behind the surface low moving off to the east. Feel Cigs will
remain sub 040 through 03-04z before lifting up to around 040
for several hours. Another MVFR deck arrives around 12z with Cigs
most likely around 025 but possibly as low as 010-015. That lower
deck will likely remain through the morning arrivals before
lifting 16-18z Tuesday. /JBonk


&&


.MARINE...Lower end SCA wind gusts 20-25 kt will continue this
afternoon then diminish by early this evening. Seas around 10 ft
should subside shortly after the winds diminish this afternoon.

Active weather pattern continues as there is another system on
Tuesday that will likely bring high-end small craft advisory wind
and possibly brief low-end gales. Seas look to remain around 7
to 9 ft but may spike to 10-11 feet for a couple hours if winds
overachieve much. Surface high pressure then begins to build
offshore shunting the surface lows mainly north of the waters and
keeping a relative lull in winds through Friday. Seas 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. /JBonk


&&


.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 8 PM PDT this evening for
     Cascades in Lane County-Northern Oregon Cascades.

WA...Winter Weather Advisory until 8 PM PDT this evening for South
     Washington Cascades.

PZ...Small Craft Advisory for winds until 4 PM PDT this afternoon
     for Coastal Waters from Cascade Head OR to Florence OR out
     60 nm.

     Small Craft Advisory for hazardous seas until 5 PM PDT this
     afternoon for Coastal Waters from Cascade Head OR to
     Florence OR out 60 nm.

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

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

&&


$$

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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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