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769
FXUS66 KPQR 202350
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
449 PM PDT Tue Apr 20 2021

.SYNOPSIS...High pressure will remain over the region through
Wednesday, with daytime temperatures continuing well above normal.
Increasing onshore flow Thursday brings cooler conditions. The
pattern looks to hold through Friday followed by a change to
unsettled conditions.

&&

.SHORT TERM...Tonight through Friday...Afternoon GOES-17 water vapor
imagery revealed strong high pressure over the eastern Pacific and
extending over the the Pacific Northwest. Meanwhile, a compact upper
level low could be seen over northern California. This feature has
produced several lightning strikes, but all of them south of our
forecast area. In addition, dry air aloft was pushing south through
western Oregon. At 23Z, many inland locations observed humidity
values in mid teens to lower 20s. Finally, visible imagery indicated
areas of stratus hugging portions of the coastline, especially in the
vicinity of Capes.

The high pressure centered offshore moves closer to the coastline
tonight and Wednesday. This results in northeast flow aloft. A
thermally-induced surface trough is expected to reside over the
interior valleys overnight, but shift to the Cascades mid to late Wed
afternoon. The NAM suggests slightly stronger onshore low-level flow
developing late in the day, but this should only impact the coast and
coastal valleys. Model 850 mb temps warm another couple of degrees
Wed. Thus, interior valleys are likely to be in the mid to uppere
70s. Subtle changes begin to occur Wed evening as the upper flow
pattern backs to the northwest. This will help to induce stronger
onshore low-level flow and allow marine stratus to spread into
coastal valleys.

Models show a short-wave disturbance dropping south through central
and eastern Washington and Oregon Thu afternoon as the 500 mb ridge
re-centers itself along 132W. Expect increasing onshore flow, which
will result in several degrees of cooling inland. The upper ridge
migrates to the coastline Fri morning and then shifts east of the
Cascades by early afternoon. The leading edge of an approaching
frontal system is expected to be just offshore by 00Z Sat. Have
indicated slight chance POPS for the south Washington and north
Oregon coast mid to late Thu afternoon. The NBM 1-D viewer for KAST
suggests a couple hundredths late in the day. Weishaar

.LONG TERM...Friday night through Tuesday...By Saturday normal Pac NW
mid-spring conditions return. The GFS and ECMWF deterministic runs
both indicate a large 500 mb trough settling over the central and
eastern Pacific by 00Z Sun. Some much-needed rainfall is expected Fri
night and Sat. The ECMWF ensembles for KPDX show anywhere from 0.2 to
0.4 inches Sat, with the mean value close to one-third of an inch.
The control run is just slightly lower. Ensembles for KEUG show a
little less. The upper trough remains over the Pac NW Sat night
through Mon, although the deterministic GFS wants to shift the upper
trough east a little sooner. The overall WPC mean 500 mb cluster
valid Sun holds the upper trough near the Oregon and Washington
coast. However, nearly 80 percent of the GFS ensemble members suggest
the upper trough will be located slightly more to the east. In any
event, confidence is moderate to high that unsettled conditions will
continue Sun. Snow levels fall to just below the Cascade passes late
Sat night and Sun. The last vestiges of the upper trough move through
the region Mon, followed by a transitory ridge and improving
conditions Tue. Weishaar

&&

.AVIATION...high pressure over the region will maintain VFR
across the interior tonight through Wed. But, shallow marine
layer remains along the coast. IFR stratus mostly along the
coastline and extending offshore, as northerly winds keeping it
from moving much onshore. But, as north winds weaken this
evening, will see that stratus return and spread onshore,
affecting nearly all coastal areas. Expect widespread IFR to low
end MVFR will tonight through early Wed afternoon. In fact, hints
are that the stratus may persist through Wed night, with only
some lifting of the CIGs.

For detailed regional Pac NW aviation weather information, go
online to: https://weather.gov/zse

KPDX AND APPROACHES...High pressure and dry northerly flow aloft
will maintain VFR with clear skies tonight through Wed. /Rockey

&&

.MARINE...Rather breezy along the coast this evening, thanks to
high pressure offshore and thermal trough along the far south
Oregon coast into nw Calif. Winds will gradually ease later
tonight into Wed, but will maintain northerly component. Will
hold current Small Craft Advisory through tonight. Overall, seas
running 7 to 9 ft, with seas 9 to 10 ft with locally choppy and
steep waves where more windy areas tonight.

High pressure weakens Thursday and Friday, although, low pressure
will develop over the offshore waters. This low possibly gets
close enough to shore to bring a cold front across the coastal
waters Saturday. Seas will remain less than 10 feet through
remain rather choppy with a strong wind wave influence. /Bonkey

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory until 5 AM PDT Thursday for Coastal waters
     from Cascade Head to Florence OR out 10 NM-coastal waters
     from Cape Shoalwater WA to Florence OR from 10 to 60 NM.

     Small Craft Advisory from 11 AM to 11 PM PDT Wednesday for
     Coastal waters from Cape Shoalwater WA to Cascade Head OR
     out 10 NM.

&&


$$

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