Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 031704

Area Forecast Discussion
National Weather Service Portland OR
954 AM PDT Fri Apr 3 2020

.SYNOPSIS...A cool and unsettled showery weather pattern will persist
through Monday before a warming and drying trend takes hold next


.SHORT TERM...Today through Sunday...Water vapor satellite imagery
continues to reveal a longwave trough lingering across the Pacific
Northwest, northern Rocky Mountain states and western Canada. A
large closed upper level low pressure centered over British Columbia
and Alberta will continue to result in shortwave troughs and vort
maxes rotating south to southeastward into the Pacific Northwest and
California over the next several days.

The first storm system is currently shifting from southwest British
Columbia into eastern Washington. While the best instability and
forcing is expected to remain well north and east of the region
today, there is enough lingering low level instability and westerly
flow to allow a few showers over the northeast Pacific to continue
to shift onshore. As daytime heating ramps up over the next few
hours, expect shower coverage to increase slightly inland. However,
instability will become increasingly squashed from a lowering
subsidence inversion. This will be in response to upper level
heights temporarily building across the region in the wake of the
aforementioned shortwave trough shifting from southwest British
Columbia/eastern Washington into northern Idaho and Montana.

The next storm system of interest is a rather compact closed low
pressure currently located near 140W and 47N via satellite imagery.
Models are in general agreement it will slide southeastward and
reach the northern California and southern Oregon coast Saturday
morning. A secondary shortwave trough currently over the Gulf of
Alaska will drop southward and help to eject the aforementioned low
pressure northeastward across Oregon on Saturday. This will result
in a broad area of light rain and mountain snow spreading northward
late tonight and Sunday morning across northwest Oregon. Models have
trended a bit higher with their snow levels tonight and Sunday
morning, presumably due to an extensive mid and high level cloud
deck overspreading the region overnight. While cannot completely
rule out some light snow accumulations on grassy surfaces around 500-
1000 feet, this should not impact travel where the bulk of the
population lives across northwest Oregon.

It should be noted that there may be just enough cooling aloft and
late day clearing to allow some surface instability to generate a
few showers across Douglas County before spreading northeastward
into Lane and Linn Counties late Saturday afternoon and early
evening. While instability does not appear sufficient for
thunderstorms based on model soundings and projected high
temperatures, it could still end up somewhat close. Will examine
this more closely with the afternoon forecast package.

It appears likely weak surface low pressure over the southern Oregon
coast will shift northward into the Willamette Valley and/or the
norther Oregon coast Saturday night. This will result in showers
likely persisting across parts of the CWA Saturday night, but
exactly where the heaviest QPF will resides remains more uncertain.

Otherwise, the aforementioned shortwave trough dropping southward out
of the Gulf of Alaska today and Saturday will shift southeastward
towards northern California on Sunday. In doing so, cooler
temperatures aloft will work their way over the region, which should
result in another round of showers bubbling up across the region
Sunday afternoon and early evening. There are also hints of a band
of precipitation setting up across northwest Oregon Sunday night
that could result in an area of more persistent rain and mountain
snow, but substantial differences in the placement and intensity of
the band of precipitation exist between the models.

Models continue to suggest shortwave ridging will begin to shift over
the region on Monday. While there should be enough lingering
instability to generate another round of widely scattered showers in
the afternoon and early evening hours on Monday , it appears Monday
will be the beginning of a warming and drying trend. /Neuman

.LONG TERM...Monday night through Thursday...The upper low continues
to drift south off the west coast through Thursday.  By late Monday
it should be far enough south for any wrap around moisture to bring
pcpn to our area. A ridge of high pressure should build in Tuesday
through Thursday for a dry period. Ensemble models currently in
agreement with this occurring. However with recent model variability
will take a wait and see approach.


.AVIATION...Cool northwesterly flow aloft will continue today as
another weak disturbance drifts inland. This will bring spotty
showers through this afternoon with occasional MVFR possible.
Expect remaining low stratus in the central and south valley to
lift and scatter over the next couple of hours with mainly VFR
conditions through tonight under scattered to broken mid and high
clouds. Another disturbance then looks to approach from the south
tonight resulting in an increasing chance for rain and MVFR
conditions late tonight for far southern portions of the forecast
area and spread north through Saturday morning.

PDX AND APPROACHES...Spotty light showers could result in brief
periods of MVFR conditions through this afternoon. Otherwise,
expect VFR conditions through tonight with scattered to broken
mid and high clouds. Expect to see an increasing chance for rain
and MVFR conditions Saturday morning as another disturbance moves
across the region from the south. /64


.MARINE...Rather tranquil conditions expected across the waters
for the next several days, with north to northeasterly winds
less than 15 kt and seas in the 3 to 5 ft range. Low pressure
well offshore will approach the south Oregon coast Sat night into
Sun, then lift northward along the coast, before shifting inland
over northwest Oregon on Sun. May see uptick in southerly winds
along the coast south of Cascade Head, but confidence not all
that good yet. Even so, winds may get up to 20 kt for a time
early Sun for areas close to shore south of Newport.

High pressure will remain offshore to start next week, with weak
lower pressure inland over the Pac NW. This will bring west to
northwest winds 10 to 15 kt, with seas 4 to 6 ft.     /Rockey





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This discussion is for Northwest oregon and Southwest Washington
from the Cascade crest to 60 nm offshore. This 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.