925
FXUS66 KPQR 242253
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
253 PM PST Sun Nov 24 2024

.SYNOPSIS...Cool and showery weather persists through Monday
but it does appear the culprit low pressure offshore will drift
southward before progressing inland Tuesday/Tuesday night
followed by high pressure building across the Pacific NW. This
will bring drier, calmer, and cooler weather mid-week
potentially lasting through the weekend.

&&

.SHORT TERM...Today through Tuesday night...Current satellite
observations still show our pesky upper-level low pressure
disturbance loitering off the WA/OR coastline with a cool and
unstable airmass overhead. Showers today have been rather
frequent for portions of the area, particularly along the coast
range into SW WA with a more stratiform band currently centered
along the I-5 corridor from roughly Salem into the
Portland/Vancouver metro and Clark County. A weak stationary
boundary and an accompanying area isentropic lift helping to
drive this activity pivots northward this evening and dissipates
but a weak upper-level impulse rotating around the larger
parent low off the coast then swings into the region overnight
into Monday morning increasing showers yet again.

Going forward, the upper-level low pressure offshore will
gradually drift south to southeastward Monday and Monday night,
eventually shifting inland over northern California on Tuesday.
So, will maintain showers through Monday, with snow levels
still at 3500 to 4000 ft. The latest 12z guidance like the
ECMWF, EPS, and HREF are all now showing a bullseye of higher
QPF values over the central Oregon Cascades which will may lead
to higher snowfall amounts approaching 3-7 inches along the crest-
line and at Santiam/Willamette pass into Tuesday morning. These
would totals approach advisory level, however, given the large
run to run QPF changes due to models struggling with shower
placement have held off on issuing any highlights - too much
forecast uncertainty. At the very least if you plan on
traversing over the mountain passes prepare for winter driving
conditions. As Tuesday progresses it does appear the low will be
far enough away as it moves into northern California that
showers will be not be as frequent and generally lighter,
especially during the afternoon hours. Beyond a chance(15-35%)
for some light lingering showers Tuesday night over the higher
terrain of the Cascades and along the coast, drier and calmer
weather will finally begin to take hold. -Schuldt


.LONG TERM...Wednesday through Sunday...Overall, models
continue to maintain a good chance(70%) for drier weather mid to
late week as high pressure builds over the Pacific NW. Still, a
few ensemble members(~30%) within the large ensemble space
including the 12z deterministic GFS depict a chance of showers,
particularly on Friday, due to them either under-amplifying the
ridge or keeping it further off the coast to our west. But,
feel that the trend to building ridge will win out as shown by
the majority of EPS members and the ECMWF. It will be cooler
through the period, primarily at night. With clearer skies, and
light offshore flow, good bet will have freezing to near
freezing temperatures (upper 20s to mid 30s) across the inland
valleys. With all the wet ground and winds being light, will
likely to have to deal with plenty of overnight/morning fog as
well. Guidance then begins to show the potential break down of
the ridge late weekend into early next week although there is
significant uncertainty as to the exact timing of this transition.
-Schuldt/Rockey


&&

.AVIATION...Radar and surface weather observations from Sunday
afternoon depicted a nearly stationary band of stratiform rain
over northwest Oregon and southwest Washington, bringing rain to
all TAF sites except for KEUG. KSLE was on the edge of this band
of rain with periods of dry weather mixed in from time to time.
Surface visibilities under this area of rain were generally at or
above 6 SM. Cigs and visibilities were generally VFR, aside from
KAST where MVFR cigs around 2500 ft were occurring. Suspect cigs
will likely bounce between MVFR and VFR thresholds through 07z
Monday at KAST and through at least 03z at KONP. Trending back
towards predominately VFR at the coast tonight before cigs lower
down to MVFR thresholds again Monday morning (>50% chance), at
least temporarily.

Expect the aforementioned band of stratiform rain to remain in
place with minimal movement through 06z Monday. After 06z, this
band of rain will quickly lift northward into northwest
Washington. Meanwhile, rain showers will begin spreading northward
behind this band of rain, impacting northwest Oregon Monday
morning. With these showers, there is a 15-30% chance for MVFR
cigs at any given time Monday morning for inland TAF sites.

PDX APPROACHES...East to southeast winds around 10 kt will
continue throughout the TAF period. Expect cigs to remain VFR,
aside from brief periods of MVFR cigs Monday morning with passing
rain showers.  -TK

&&

.MARINE...At 2 PM Sunday, marginal gale force wind gusts up to 35
kt were continuing to be observed at buoy 050, right on target
with the forecast. Meanwhile, gusts at buoy 029 were to 25 kt. The
slightly stronger winds occurring over the southern coastal
waters are in response to a small-scale surface low that is
currently weakening. Still expecting gale force winds to end for
the southern waters around 4 PM Sunday. After 4 PM, expect small
craft advisory level wind gusts between 20-30 kt and seas around
8-10 ft for all marine zones through Monday afternoon. Given
dominant wave periods around 8 seconds, seas will be particularly
steep.

Seas fall to 3-4 ft Monday night into Tuesday as light easterly
winds develop over the waters (<10 kt). Winds back to the north on
Wednesday and then remain out of the north through Friday with
seas remaining below 5 ft. Overall very benign conditions for this
time of year with a prolonged period of sub small-craft advisory
conditions likely to occur.  -TK

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory until 4 PM PST this afternoon for PZZ210.

     Small Craft Advisory until 4 PM PST Monday for PZZ251-271.

     Gale Warning until 4 PM PST this afternoon for PZZ252-253-272-
     273.

     Small Craft Advisory from 4 PM this afternoon to 4 PM PST Monday
     for PZZ252-253-272-273.
&&

$$

www.weather.gov/portland

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