Area Forecast Discussion
Issued by NWS Portland, OR
FXUS66 KPQR 242221
Area Forecast Discussion
National Weather Service Portland OR
321 PM PDT Fri Mar 24 2017
.SYNOPSIS...An upper level trough will continue to bring showers to
southwest Washington and northwest Oregon tonight, with a slight
chance of thunderstorms this evening. High pressure will cause
showers to taper off late Saturday, but the next frontal system will
likely bring more rain Sunday followed by showers Monday. Periods of
wet weather is expected to linger through the end of next week.
.SHORT TERM...(Tonight through Monday)...Water vapor satellite
imagery shows the base of the upper level trough starting to move
onshore this afternoon, with a weak shortwave disturbance embedded in
southwesterly flow aloft. This will help maintain showers across the
forecast area tonight and Saturday. Doppler radar imagery shows
scattered showers streaming across northwest Oregon and southwest
Washington this afternoon, with the strongest showers generally north
of Salem as expected.
Now, to address the elephant in the room...Will we see thunderstorms
this afternoon? The answer is maybe! Based on model parameters,
conditions remain favorable for thunderstorm development across the
forecast area through the evening. All recent model runs show surface
based CAPE around 300 to 500 J/kg with 0 to 3 km bulk shear around 25
to 30 kt. This suggests that if thunderstorms are able to develop
this afternoon/evening they have the potential to become organized.
Given our low freezing level, around 4000 ft, it would not take a lot
for any storms that do develop to produce some small hail. This is
all good news if you want to see thunder. Now for the bad
news...visible satellite imagery reveals abundant cloud cover across
the forecast area. There have been some breaks in the clouds this
afternoon, but they have been brief and not widespread enough to lead
to any significant diurnal heating. This is limiting our surface
based instability. Given the lack of clearing this afternoon decided
to remove the mention of small hail from the forecast. Will continue
to keep thunderstorms in the forecast through this evening based on
some mixed layer CAPE and terrain based winds. The threat of
thunderstorms will come to an end later this evening as we lose our
Showers look to linger through Saturday as the upper level trough
slowly moves east of the Cascades. Models then show weak high
pressure briefly building across the Pac NW by Saturday night. This
will bring an end to the current round of showers and a period of dry
weather late Saturday/early Sunday. Rain looks to return by Sunday
morning as another frontal systems moves across the forecast area.
This system looks very similar to yesterday`s system, but with more
QPF in the Cascades. As such, would not be surprised to see a Winter
Weather Advisory issued for the Cascades on Sunday with snow levels
around 4000 ft and new snow accumulations around 6 to 8 inches
possible. Rain will turn to showers by late Sunday, with showery
conditions continuing through Monday. /64
.LONG TERM...(Monday night through Friday)...12z models remain in
fairly good agreement with regard to the long wave pattern of the
upcoming work week. They show the upper trough from this weekend
departing east becoming positioned over the Northern Rockies by
early Monday evening. A flat upper ridge crosses the region Tuesday
through early Wednesday before a flat upper trough takes its place
Wednesday night. The trough deepens as it crosses the PacNW Thursday
which allows for a little higher amplitude ridge to build for
Friday. Unfortunately (depending on your preferred weather), this
low amplitude upper pattern does not lend toward any day long dry
period for the upcoming work week. As of today`s model runs, best
chance may end up being Friday as the ridge builds, however the GFS
is showing a short-wave riding over the top with rain affecting the
The overall cloudy/rainy/snowy sequence will keep temperatures a
little cooler than normal during the day and warmer at night. Snow
levels will stay around 4-5000 feet except for Wednesday and Friday
where they rise to 5-7000 feet. /JBonk
.AVIATION...Showery weather pattern will continue tonight into Sat.
VFR conditions will tend to be most widespread, but there will
remain a chance for MVFR conditions at all locations
through 21z Sat.
KPDX AND APPROACHES...Mainly VFR conditions tonight and Sat as a
showery weather pattern continues. There will likely be a couple of
temporary periods of MVFR cigs both tonight and through 21z Sat.
.MARINE...Winds were gusting up to 30 kt Fri afternoon ahead of a
weak front approaching the coast. Expect winds to settle back down
later this evening, dropping below small craft criteria as the front
moves inland and a ridge of high pres builds over the coastal waters
late tonight and Sat. Another front will move across the coastal
water Sun, with south winds increasing ahead of the front late Sat
night into Sun. Winds appear likely to reach into the low end gale
category with the front Sun. Another frontal system is poised to
move across the coastal waters Tue, with peak winds expected to
gusts close to 30 kt.
As an active weather pattern continues through this weekend and into
next week, there will be several competing swell trains. The short
period southerly swell this afternoon will subside tonight, but a
building westerly swell at the same time will tend to keep seas up
around the 10 ft mark through most of the night. Southerly winds
will rebuild steep short period seas Sun back up into the 11 to 13
PZ...Small Craft Advisory for winds until 11 PM PDT this evening for
Waters from Cape Shoalwater WA to Cascade Head OR from 10
to 60 nm.
Small Craft Advisory for hazardous seas until 5 AM PDT Saturday
for Coastal Waters from Cape Shoalwater WA to Florence OR
out 60 nm.
Small Craft Advisory for winds until 7 PM PDT this evening for
Coastal Waters from Cape Shoalwater WA to Florence OR out
10 nm.Columbia River Bar-Waters from Cascade Head to
Florence OR from 10 to 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 2 AM to
6 AM PDT Saturday.
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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.