Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 252145 AAA

Area Forecast Discussion
National Weather Service Portland OR
245 PM PDT Wed Apr 25 2018

.SYNOPSIS...High pressure over the will slowly shift toward the
Rockies tonight and Thu. At same time, upper low pres will slowly
approach from the southwest. So, expect a gradual increase of onshore
flow on Thu, with much cooler air and clouds across the region late
Thu and Thu night. As the upper low shifts into the region, will see
shower from time to time, with plenty of clouds and cooler air for
the weekend. Does appear that will have return to drier air with mild
temperatures early to middle of next week.

.SHORT TERM...Tonight through Saturday...Not a lot of change in
overall pattern tonight. High pres sitting over the region will
maintain dry conditions into Thu. Thermal low pres sitting just
inland from the coast. This shows up well with the widespread marine
stratus along the coast, where temperatures are some 20 to 30 deg
cooler this afternoon than as seen on Tue. Thermal trough will slowly
shift to the interior tonight, but marine layer remains shallow
enough that will have difficult time punching inland.

Upper low will slowly approach the region from the southwest on Thu.
But due to its slowness, seems the thermal trough will sit over the
interior lowlands into Thu afternoon. As such, will see westerly
onshore begin increasing trough the drainages in the Coast Range from
Lane county in the morning, progressing northward as the day
progresses. This will keep some offshore flow at Portland/Vancouver
metro into the afternoon, then will see winds at that area flip to
south or southwest towards late afternoon. With this pattern, would
expect to see wide range in temps, with upper 60s to lower 70s at
Corvallis and Eugene, to near 80 around Portland/Vancouver metro.

Thermal trough will works its way up and over the Cascades Thu night,
allowing for deepening marine layer on the west side. This will
result in much more clouds for Thu night int Fri am.

Now, other twist in our forecast will be threat of showers as the
upper low approaches. At moment, not all that impressed with the
potential, owing to fairly dry air mass and weak instability
initially. Still, with proximity of thermal trough and afternoon
surface heating, probably be enough to pop afternoon cumulus over the
Cascades. Will go with 20 pct pops over the Cascades from Santiam
Pass southward later Thu afternoon. Upper flow appears bit more south
to southeasterly for Thu night and Fri. So, as showers pop up over
southern Oregon and the Cascades, will have motion such that showers
will continue drifting northwestward in the flow. Do not think will
see thunderstorm threat on Fri, as will have fairly deep marine layer
by that time. However, would not be surprised to see a few
thunderstorms over the Cascades on Fri into Fri evening. Have not put
in forecast yet, as would like to see how the next few runs of models
shake out to get more confidence.

Still cool and showery on Saturday as the upper low shifts into
northern California and southern Oregon./Rockey.

.LONG TERM...Sat night through Tuesday...Models consistent in
showing the upper low progressing slowly across the Pac NW, augmented
by another shortwave attaching to the low Sat night and Sun. This
will keep a decent chance of showers over the region into Sun, which
combined with the low level onshore flow keeps temps a little below
seasonal normals. Sun night into Mon models in pretty good agreement
that the upper trough shifts east as a ridge of high pres presses in
from the west. This will allow temps to moderate some during the
early next week, while chances for showers diminish.

.AVIATION...Marine clouds will continue to produce areas of IFR
conditions along the coast overnight. Offshore flow may try to
develop along the south Washington and north Oregon coast, which
could keep KAST VFR overnight, but confidence is low. Farther
south at KONP, expect IFR conditions to gradually improve into
MVFR thresholds as the marine layer deepens tonight and Thursday.
There is a chance very high end IFR to MVFR stratus could work
its way into KEUG towards 12-15z Thursday, but confidence is low.
Expect marine clouds to spread across the rest of the forecast
area after 00z Friday.

KPDX AND APPROACHES...Offshore flow will result in VFR conditions
through 00z Friday. Marine clouds will likely spread over the
terminal after 00z Friday. /Neuman

.MARINE...Weak westerly winds this afternoon should return to a
more southerly direction overnight, but should remain well less
than 25 kt. Weak pressure gradients over the waters should keep
winds generally below Small Craft Advisory thresholds through the
first half of the weekend. A weak front dropping southeastward
across the waters on Sunday may result in Small Craft Advisory
level wind gusts of 25 kt across the waters Sunday and early
Monday. These winds should result in seas climbing into the 9-12
ft range during this time. High pressure returning to the
northeast Pacific should result in winds and seas relaxing early
next week. /Neuman




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