834
FXUS66 KPQR 212216
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
230 PM PDT Fri Mar 21 2025


.SYNOPSIS...Valley rain and mountain snow this weekend will give way
to drier weather and the warmest temperatures of the year so far
early next week before wetter and cooler weather return mid to late
next week.


&&


.DISCUSSION...Water vapor satellite imagery shows a fairly
sharp shortwave trough approaching the Pacific Northwest. Ample
large scale lift ahead of the shortwave trough is helping to drive
rapidly cooling cloud tops over far northwest Oregon and southwest
Washington this afternoon. This has helped result in widespread
rainfall with hourly rainfall totals generally in the 0.1-0.2"
range. However, a narrow cold frontal rain band has also
materialized and currently stretches from near Waldport Oregon to
Sandy Oregon. It has so far produced hourly rainfall totals of 0.20-
0.30" across portions of Yamhill, Multnomah and Clackamas Counties.
Given model guidance matches satellite trends, this narrow cold
frontal rain band should continue to move southeastward quickly
enough to prevent widespread issues. Nonetheless, for areas south of
the aforementioned line, a notable 5-30 minute period of heavy rain
will likely materialize over the next couple of hours as the narrow
cold frontal rain band moves past those locations. Will continue to
monitor rainfall rates and reports to see if any flood advisories
are needed. It should be noted that travel conditions over the
Cascades should deteriorate rapidly over the next few hours as this
band of intense precipitation moves into the Cascades and cooler air
filters into the region.

Ensembles are in general agreement low amplitude shortwave ridging
will shift into the Pacific Northwest tonight into Saturday. This
should bring decreasing rain and allow some breaks in the clouds,
which should be enough to produce at least some patchy fog Saturday
morning. A weak front will push towards the region Saturday into
Saturday night, but there remains quite a bit of uncertainty in
whether it will hold together enough to bring widespread light rain.
As the shortwave ridge amplifies, an atmospheric river will take aim
at southwest British Columbia before shifting southward into
Washington. It appears it should shift far enough south that
additional rain will fall across the area Saturday night into
Sunday. The highest rainfall totals will almost certainly be across
southwest Washington with considerably less rain farther south
towards Lane County. There is a ~10-20% chance that southwest
Washington is clipped by enough rain (~3" in 24hrs) to result in the
Grays River reaching minor flood stage Sunday or Sunday night.

Ensembles are in good agreement shortwave ridging will result in the
atmospheric river shifting back northward Sunday night into Monday.
This will result in warming temperatures and decreasing rain chances
from south to north on Monday. Ensemble guidance is in good
agreement shortwave ridging will result in the warmest day of the
year so far across southwest Washington and northwest Oregon on
Tuesday. For example, the NBM suggests there is at least an 80-90%
chance of temperatures rising into the 70s along the I-5 corridor
and a little less than 50% chance that sites like Astoria and
Tillamook hit 70F on Tuesday. For those wanting more than 70F, the
NBM suggests around a 50% chance for temperatures to rise into
the mid to uppers 70s on Tuesday for place like KPDX.

WPC clusters and ensemble guidance provide strong support that a
cooler and wetter pattern will return mid to late next week. Details
in the timing and exact evolution of the upper level trough (is it
going to dig and come in negatively tilted vs. stay slightly farther
north and be more progressive) impacting the region are where
uncertainty in the ensemble guidance lie at this time. Regardless,
there is high confidence that temperatures will drop back to near or
below average values for this time of the year and valley rain
and mountain snow will return.


&&


.AVIATION...Westerly winds aloft continue, becoming northwest
after 03-06z Saturday behind the passage of a low pressure system.
As of 21z Friday, the cold front that has been lingering over the
airspace is moving inland, producing widespread stratiform rain,
moderate to heavy at times. Stratiform rain will move east of the
Cascades between 00-03z Saturday, with precipitation transitioning
to scattered to isolated showers which should end around 12-15z
Saturday. Conditions should remain in the MVFR to low- end VFR
range with MVFR conditions being most predominant. Isolated
pockets of IFR cigs and vsbys are possible at all TAF sites
(10-30% chance), except at KONP where LIFR to IFR conditions will
likely continue through at least 00z Saturday (>90% chance). Cigs
should begin gradually lifting towards VFR thresholds between
00-06z Saturday. There`s a 30-40% chance of MVFR cigs returning
around 08-15z Saturday, mainly for coastal terminals, with
predominately VFR conditions returning by 15-18z Saturday.

Southerly breezy winds are expected along and ahead of the front
with gusts up to 20-25 kts with a brief period of winds shifting
westerly as the front passes. Behind the front, winds are
decreasing rapidly below 10 kts, becoming westerly.

PDX AND APPROACHES...Steady rain will continue through 00-03z
Saturday before transitioning back to off-and-on rain showers.
Conditions should generally remain MVFR through 00-03z Saturday
before gradually lifting to VFR thereafter, though brief periods
of IFR conditions are possible in heavier rain. Calmer westerly
winds less than 10 kts expected, shifting southerly after 12z
Saturday. -TK/HEC


&&


.MARINE...The surface front has moved east of the waters on Friday
afternoon, which buoy observations indicate has allowed winds to
become westerly and decrease to 10-15 kt with gusts below 20 kts.
Seas have fallen to 9-12 feet at 12-15 seconds, and a westerly
swell will keep waves around this level through Sunday before
falling to 7 to 9 ft early next week. Another frontal system is
forecast to move through the waters Saturday into Sunday with
winds becoming southerly, peaking in strength on Sunday with wind
gusts up to at least 30 kt. There is a 60-80% chance for slightly
stronger gale force winds gusts up to 35 kt, mainly for the north
Oregon and south Washington coastal waters. That said, it appears
wind gusts of this magnitude would only last 3-6 hours or less.
Given how marginal conditions are, will hold off on issuing a Gale
Watch.

Beyond this system, high pressure will build over the region for
early next week. This will bring decreasing winds and seas, most
likely below small craft advisory criteria. -TK/HEC


&&


.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Winter Weather Advisory until 5 AM PDT Saturday for ORZ126>128.

WA...Winter Weather Advisory until 5 AM PDT Saturday for WAZ211.

PZ...Small Craft Advisory until 5 PM PDT this afternoon for PZZ210.

     Small Craft Advisory until 11 PM PDT this evening for
PZZ251>253-
     273.

     Small Craft Advisory until 5 AM PDT Saturday for PZZ271-272.
&&


$$

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