Funnel Cloud in Oregon, February 20, 2021

Area Forecast Discussion
National Weather Service Portland OR
259 PM PST Sat Feb 20 2021

.SYNOPSIS...A rather seasonal upper level northwesterly flow pattern
will bring periods of steadier rain followed by showery weather every
24-48 hours for much of the upcoming week. Most of Wednesday appears
likely to remain dry.

&&

.SHORT TERM...Tonight through Tuesday...Water vapor satellite imagery
this afternoon reveals a shortwave trough swinging southeastward
across the Great Basin. Low level instability under northwesterly
flow aloft in its wake has led to scattered light showers streaming
southeastward across the CWA today. Model soundings indicate
these showers will become increasingly shallow and should gradually
wane this evening. However, the break in the wet weather will likely
only be a few hours.

Water vapor satellite imagery also reveals a broad and low amplitude
shortwave ridge over the eastern Pacific. Meanwhile, a broad
shortwave trough is centered over the central Pacific with an
atmospheric river streaming northeastward towards Haida Gwaii and the
Alaska Panhandle. Models remain in good agreement this shortwave
trough will slowly shift eastward and allow a warm front to bring
steadier rain to the region later tonight into Sunday, particularly
along the coast and foothills. The first vestiges of this are already
being detected off the coast by KRTX and KLGX radars as the
atmosphere moistens from the top down. Six hourly rainfall amounts in
the Coast Range and Willapa Hills should generally remain under
3/4-1", which will limit hydrology concerns tonight into Sunday.

Snow levels will more or less hover around 1500 to 2500 feet this
evening before rising late tonight into Sunday morning to near 4000
feet. Given forecasted QPF and snow amounts expected with the warm
front, the inherited Winter Wx Advisory for the south Washington
Cascades looks on track and opted to expand it to include the north
Oregon Cascades as well.

As the main baroclinic zone tied to the upper level shortwave trough
sags over the region, IVT values will near the moderate atmospheric
river category of 500 kg/ms along our north coast. While the front
will not stall, it will still take some time to move southward
across the area. This will likely produce 1.25-2.5" of rain across
the Willapa Hills and north Oregon Coast Range Sunday night into
Monday morning. Given NOHRSC analysis indicates 1-3" of SWE remains
across a decent portion of the upper Grays River drainage and the
high end of the forecasted QPF values push the Grays River within
inches of minor flood stage per an internal hydrology model that did
not consider snowmelt, opted to issue a Flood Watch for Sunday night
into Monday for the Grays River at Rosburg. If flooding were to
occur Sunday night into Monday morning along the Grays River at
Rosburg, it would be minor in nature and certainly nothing like the
flooding already observed earlier this year in early-mid January.

Otherwise, Monday is shaping up to be a damp day across the entire
CWA. Modest 925-850mb westerly flow should result in considerably
less QPF across the western Willamette Valley than compared to
points near the coast and farther east towards the Cascade
foothills, though. Post frontal showers will continue to linger
Monday night into Tuesday with snow levels generally hovering around
2000 feet. /Neuman

.LONG TERM...Tuesday night through Friday...Models and their
ensembles are in relatively good agreement shortwave ridging
offshore will temporarily bend over the Pacific Northwest late
Tuesday into Wednesday. This should bring roughly 24 hours of dry
weather to the region on Wednesday. However, the break in the wet
weather appears short lived as model ensembles generally agree
shortwave ridging will build northward over the eastern Pacific and
will allow a shortwave trough to drop southeastward into the Pacific
Northwest late Wednesday into Thursday. Timing differences between
ensemble members suggest there is a small chance valley rain and
mountain snow could arrive as early as late Wednesday afternoon or
evening, but that it will more likely hold off until late Wednesday
night. A decently strong cold front should result in snow levels
lowering back down into the Cascade foothills Wednesday with another
shot of advisory level accumulations across the Cascades.

Model ensembles generally suggest additional shortwave troughs
dropping southeastward out of the Gulf of Alaska appear likely to
clip the region. This should generally keep temperatures a bit below
average with additional rounds of valley rain and mountain/foothill
snow. Confidence is low in trying to time and determine the strength
of these additional storm system(s) late next week given subtle
changes in their amplitude will result in rather large changes in
their ability to produce mountain snow. /Neuman

&&

.AVIATION...Radar and satellite show the warm front slowly
approaching the coastline this afternoon. While much of the area
is VFR, coastal terminals should quickly turn to MVFR by 02Z
Sunday. Expecting showers to turn over to more stratiform rain
with the warm front after 05Z Sunday. Winds will remain onshore
with the initial onset of the front but then will transition to a
more southerly regime. With the southerly turn, expecting winds
to ramp up becoming gusty which will especially be the case along
the coast, and through the central and southern Willamette
Valley. Inland areas will see MVFR conditions after 05Z Sunday,
although could be a bit sooner as the front appears to be moving
quite quickly. Once the front moves inland, conditions should
remain fairly stable with southerly winds and rain. Could see a
period of lower MVFR CIGs (1200-1800 ft) after 08Z Sunday along
the coast and 12Z Sunday inland but trending more towards the
2000-2300 ft CIGs. Rain should begin to taper off around 18Z
Sunday but could still see some lingering showers. One thing to
be aware of is the potential for some stronger winds aloft. As
is common with a warm front, there will be an ease in speed as
you go up in the atmosphere, but forecast soundings are showing
winds around 35 kt at 1000-2000 ft. Not enough for wind shear to
be added to the TAF, but something to be aware of.

NOTE: KUAO ASOS continues to have commercial power supply
issues. Altimeter setting remains available.

For detailed regional Pac NW aviation weather information, go
online to: https://weather.gov/zse

KPDX AND APPROACHES...No concerns for the next 24 hours. VFR
with showers around the area through this afternoon. Warm front
approaches in the evening bringing more persistent rain and MVFR
CIGs. Wind shift late in the period but no wind shear expected.
Winds aloft will be around 30-35 kt from the southwest so could
see some mixing down for periodic gusts.    -Muessle

&&

.MARINE...The next several days is highlighted by a prolonged
period of broad low pressure over the Gulf of Alaska, with
several fronts expected to push across the waters. Each front
will bring increasing winds and seas, with the next front
expected this evening. The warm front is quickly approaching
however winds will not begin to ramp up until the overnight
hours when they transition to a southerly direction. Likely will
see winds around 15 to 20 kt with gusts to 25 kt however cannot
rule out gusts to 30 kt, but likely will stay in the outer
waters west of 10 NM. Seas will slightly increase tonight around
11 to 13 ft at 13 seconds.

Given the persistent nature of the overall weather pattern, not
expecting many major changes in conditions. The next front
arrives on Sunday evening and, while models are suggesting it
slightly stronger than the one occurring tonight, it is still not
overly impressive. Will likely see winds ramp up to near gale
force speeds, but not expected to surpass that threshold. The
westerly swell will be influenced by the wind waves building to
near 12 to 14 ft at 13 seconds by Monday. Building seas will be
supported by yet another incoming disturbance on Monday night.
Expecting a rinse and repeat pattern through Wednesday, however,
there is a chance for more elevated steep seas on Thursday. While
some models are suggesting seas greater than 18 ft, especially
in the northern waters, recent model runs have trended lower than
previous outputs. Because of the lack of certainty, decided to
keep seas below 18 ft over all of the waters.	 -Muessle/TK

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 10 PM this evening to 4 PM PST
     Sunday for Northern Oregon Cascades.

WA...Flood Watch from Sunday evening through Monday afternoon for
     South Washington Coast.

     Winter Weather Advisory from 7 PM this evening to 4 PM PST
     Sunday for South Washington Cascades.

PZ...Small Craft Advisory until 4 PM PST Monday for coastal waters
     from Cape Shoalwater WA to Florence OR out 60 NM.

     Small Craft Advisory until 5 AM PST Monday for Columbia River
     Bar.

&&

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