Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 262149
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
242 PM PDT Thu Apr 26 2018


.SYNOPSIS...A much cooler and wetter pattern will take hold Friday
and continue through the weekend before conditions slowly warm back
up and dry out next week.


&&


.SHORT TERM...Today through Sunday...Visible satellite imagery
reveals marine stratus banked up against the Coast Range this
afternoon with sunny skies inland. A surface pressure analysis shows
the thermal low pressure lying along the Cascade foothills and
stretches back westward over the Portland metro. Temperatures are
unseasonably warm near the thermal trough with temperatures generally
in the upper 70s to low 80s...warmest for those with an east wind
such as Troutdale. Temperatures are much cooler to the north, west
and south of the thermal trough with temperatures currently near 70F
at Kelso, 65F in Eugene and between 50-55F along the coast.

The KDLS-KTTD pressure gradient and resulting east winds have been
steadily decreasing the past couple of hours and should continue to
do so this afternoon. Expect the thermal low pressure to shift east
of the Cascades this evening, which should result in the cooler
marine air over the southern half of the Willamette Valley and along
the lower stretches of the Columbia River to spread into the
Portland metro and Columbia River Gorge. Expect the marine layer and
attendant cloud deck to deepen considerably overnight and spread to
near the Cascade crest by Friday morning.

Meanwhile, water vapor imagery reveals a large upper level trough
spinning off the California and Oregon coast slowly moving toward
the region. Increasing instability ahead of this feature will lead
to showers breaking out over southern Oregon late this afternoon,
which will then spread northwestward across western Oregon. Thunder
will be possible, particularly over Lane County this evening, but it
wouldn`t surprise me if a storm or two held on long enough to
produce thunder over Linn, Benton and Lincoln Counties, but
confidence is low enough that have kept the mention of thunder more
limited in scale for now.

More importantly, a combination of cooling temperatures aloft and
lift ahead of the aforementioned upper level trough will result in a
band of showers oriented from SE to NW lifting northward across the
area late tonight and Friday. As the upper level trough moves across
the area over the weekend, expect valley high temperatures to
struggle to hit 60F with shower activity likely picking up during the
afternoon and early evening hours each day. /Neuman


.LONG TERM...Sunday night through Wednesday...Models are in general
agreement a secondary shortwave trough will drop south-southeastward
across the northeast Pacific and Pacific Northwest on Monday. This
will likely keep temperatures cool with a good chance for occasional
showers on Monday. Models then suggest shortwave ridging offshore
will shift eastward and result in a warming and drying trend across
the Pacific Northwest during the early to middle part of next week.
Subtle differences in the placement of the ridge axis leave some
uncertainty with regards to temperatures so kept the inherited
forecast largely intact. /Neuman


&&


.AVIATION...Marine stratus remains along the coast, with
conditions currently a mix of IFR and MVFR. Cigs have been
gradually lifting, and expect mostly MVFR going forward. The
stratus should remain through the day, although there is a slight
chance that KAST may break out for a couple of hours later this
afternoon. An upper level low pressure system offshore may bring
a few thunderstorms into the region from the south later this
afternoon. If any do make it into our area, expect they will
mainly remain near the Cascades and foothills, with an outside
chance of one drifting into the Willamette Valley. The best
chance for a valley storm would likely be from KEUG-KSLE.

As the low approaches the coast tonight, the marine layer will
continue to deepen. A surge of marine stratus clouds into the
interior lowlands is expected. Think that MVFR conditions will
develop from south to north through the valley, from about 06Z
Fri-12Z Fri. A band of showers will also move into the region
from the south, with the current hi-res models showing the band
reaching a line from KONP-KEUG between 15Z-18Z, then slowly
progressing northward through the region through the remainder of
the day. Expect MVFR conditions to extend well into the
afternoon, if not through all of tomorrow.

KPDX AND APPROACHES...Expect VFR conditions and mostly clear
skies through this evening. A marine stratus push is expected to
bring MVFR cigs by 09Z-12Z Fri, which will likely persist through
most of the day. There is a slight chance of a shower late this
evening. Then a band of steadier showers will likely move into
the region tomorrow afternoon. Pyle


&&


.MARINE...Conditions remain fairly quiet for the next few days. Weak
high pressure will reside over the waters through Fri, with S-SW
winds at 15 kt or less. Then a broad area of low pressure will
move in over the weekend. A weak front is expected Sat night into
Sunday. The models have been suggesting that there would be a
period of small craft advisory winds behind the front on Sun or
early Mon, but they have trended weaker since yesterday and this
does not appear to be as much of a sure thing at this time. Then
high pressure builds into the waters Mon and remains through at
least the first half of next week.

Seas generally 7 ft or less through Sun. Wave heights build to
around 9-10 ft Sun night, primarily over the outer waters. Seas
near 10 ft persist through Mon and then begin to subside Tue.
Pyle


&&


.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...None


&&


$$

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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 CWA or forecast area.



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