Cold in Oregon, January 29, 2023

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
847 PM PST Sat Jan 28 2023

Added update section

.UPDATE...Precipitation is shutting off very quickly behind the cold
front this evening. Additionally, temperatures across the area have
not been as quick to cool as was forecasted. As of 830 PM, most
lower elevation temps remain in the upper 30s to lower 40s. As such,
have updated the forecast to slightly speed up the the end of precip
from north to south this evening and to slightly delay the cooling.

This means most lower elevations will not see snow mixed in with the
rain tonight. However, the damage has been done with regard to
moisture on the roads. Expect most areas with untreated surfaces,
will see the remaining liquid freeze as temperatures fall to and
below freezing overnight tonight. This will make for potentially
hazardous travel until the ice can melt.

Melting will occur as early as tomorrow mid-morning under sunny
skies for many areas while some areas nearer the Columbia River may
actually see the winds dry the water and ice tonight. Areas under
shade tomorrow, will see that ice hold on longer into the day.
Lastly, the reinforcing cold air pouring through the Gorge will
likely hold air temperatures below freezing and result in the icy
conditions continuing into Monday, especially on the south side of
the Gorge.

Updates have been sent.
/JBonk

&&

.SYNOPSIS...A cold front will usher in colder temperatures and
result in rain transitioning to spotty snow showers this evening.
Strong high pressure behind the front will bring a surge of east
winds through the Columbia Gorge and spilling over the Cascades
tonight through Sunday, with blustery conditions throughout the
forecast area. Sunday through Monday will be dry and cold with
slowly moderating temperatures expected later in the week.

&&

.SHORT TERM...THROUGH MONDAY NIGHT...Weak onshore flow is
supporting light, isolated rain shower activity across
northwest Oregon and southwest Washington as of early Saturday
afternoon. However, an upper level shortwave trough dropping
rapidly southward down the coast from BC later this afternoon
will signal an abrupt shift to a much colder air mass and a
brief burst of possible winter weather concerns this evening. A
backdoor arctic cold front currently analyzed across central WA
will move into the area from the north and east by later this
afternoon, bringing with it colder temperatures, gusty offshore
winds, and a brief window of heavier precipitation focused along
the front.

Temperatures currently running in the mid 40s across much of
the area will allow precipitation to begin as rain later this
afternoon, but temps rapidly dropping into the low to mid 30s
will likely allow some snow flakes to start to mix in by 6-7 PM
in the Portland metro and sooner in parts of the Gorge and the
Hood River Valley. Already seeing some evidence of this at Hood
River where the temperature dropped from 45 to 34 degrees
between 12:30 and 2:30 PM with a switch from west to east winds.
Precipitation will wind down rapidly from north to south later
this evening, exiting the Portland metro by around 9-10 PM and
exiting the south Willamette Valley by midnight. This timeline
should serve to limit and potential for any significant
accumulation in the lower elevations, especially given
relatively warm road surfaces and minimal QPF amounts. This is
well supported by NBM probabilities continuing to cap chances
for greater than 1 inch of snow at around 5 percent in any given
location. Still, certainly could see a dusting of snow on
grassy surfaces in some locations before precipitation comes to
an end later this evening. A quicker transition to snow in the
Gorge and the Hood River Valley may allow for some modest 1-3
inch accumulations in those locations, where a Winter Weather
Advisory remains in effect until 8 PM above 500 feet. Could also
see some minor accumulations aided by orographic ascent on the
east slopes of the Coast Range, but opted to hold off on an
advisory there as the forecast calls for 2-3 inches at most in
the highest points in the northern Coast Range. Cascade
locations should also stay safely below advisory criteria, with
2-4 inches expected around the passes through this evening.

Attention quickly turns to frigid temperatures tonight through
Monday morning as the front pushes through and fosters rapid
clearing overnight from north to south. Expect temps to bottom
out in the mid 20s in the valley locations and the upper 20s at
the coast for Sunday morning, with gusty offshore winds
contributing to wind chill values in the mid teens in many
locations. Meanwhile, wind chills could drop to dangerous values
well below zero in parts of the Cascades, prompting a Wind
Chill Advisory for those locations through Monday morning.
Temperatures will only manage to warm into the mid to upper 30s
in most locations on Sunday as 850 mb temps fall to around -8 C,
with gusty offshore winds holding wind chill values in the 20s
through the day in the lower elevations. Winds will slowly
diminish through Sunday night as high pressure builds back
overhead. Sunday night should be the coldest night as
temperatures fall into the low 20s in the Willamette Valley and
the mid 20s at the coast, supporting another night of winds
chill values in the teens even as winds start to drop off.
Temperatures will begin to slowly moderate by Monday, climbing
back to around 40 degrees Monday afternoon but still dropping
into the 20s in most locations away from the coast for Monday
night. /CB

.LONG TERM...TUESDAY THROUGH SATURDAY...Will see a slow
moderating trend in temperatures continue in mid next week as
high pressure remains over the region into Wednesday. The next
chance for precipitation looks to come on Tuesday as several
deterministic solutions depict a weak shortwave moving over the
top of the ridge and clipping the northern extent of the area.
The best chance for precipitation with this feature would be
along northern coastal areas where temperatures should be warm
enough to support rain. Capped PoPs at slight chance for now in
the Willamette Valley as a majority of individual ensemble
members keep PDX and other points completely dry through mid
week. Will continue to monitor as temperatures would likely
still be cold enough to support some snow potential if
precipitation were to come to fruition. Precipitation chances
increase late in the week as WPC ensemble clusters have begun to
favor upper level troughing over the coast in the Thursday
/Friday timeframe. However, chances that any precip would fall
as rain in the lower elevations also increases late in the week
given several days of moderating and weak onshore flow on the
northern periphery of the ridge helping to scour out remaining
cold air east of the Cascades. Ensemble guidance is more split
on the large scale pattern heading into the weekend, with
around 30 percent of the clusters depicting more upper level
troughing while around 70 percent suggest upper level ridging
becoming re-established over the West Coast. /CB

&&

.AVIATION...06Z TAFs: KRTX radar imagery as of 04Z shows wrap
around showers within the low tracking south quickly being scoured
out across the Upper Hood River valley and through the Columbia
River gorge. There could still be a brief period with a rain snow
mix but temperatures remain in the upper 30s, low 40s across the
Willamette Valley. Any light snow that does fall will likely melt
fairly quickly. Very cold overnight temperatures are still expected,
any precipitation that lands on elevated or untreated surfaces
will likely turn into ice. Strong winds aloft will make for
easterly speed shear between 40-50kt at 2000 ft. Expect clearing
skies in offshore flow with widespread VFR by 12Z Sunday.

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

KPDX AND APPROACHES...Chances for mix precipitation dropping
dramatically as offshore flow aloft dries out wrap around moisture
from the departing low. Gusty east winds through the night with
strong east winds aloft up to 50 kt. Very cold overnight
temperatures will lead to ice on elevated or untreated surfaces.
-BMuhlestein

&&

.MARINE...Seas are around 8 to 10 ft with mix of longer period
west swell and shorter period northwest swell. Models are a
little lower than observations. After adjustment to the forecast
will have steep choppy seas around 9 to 10 ft at 9 to 10 seconds,
so will issue a Small Craft Advisory for seas.

Models are in good agreement showing offshore flow developing this
early this evening over the north waters then spreading over the
remaining areas during the evening. Gusts to 30 kt are likely
night through early Monday, with isolated gusts up to 35 kt
possible in zones PZZ272 and PZZ252. At this time not expecting
widespread Gale Force winds, but will continue to monitor and
adjust the forecast as needed. Wind speeds begin to ease in the
north zones Sunday spreading to the south zones Sunday night.

Temperatures will be cold over the waters, but freezing spray not
expected to be an issue. However, ice accumulation for vessels
docked in harbor is possible. /mh /42

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Wind Chill Advisory until 11 AM PST Monday for Northern Oregon
     Cascades.

     Wind Chill Advisory from midnight tonight to 11 AM PST Monday
     for Cascades in Lane County.

WA...Wind Chill Advisory until 11 AM PST Monday for South Washington
     Cascades.

PZ...Small Craft Advisory until 10 AM PST Sunday for coastal waters
     from Cape Shoalwater WA to Cape Falcon OR out 60 nm.

     Small Craft Advisory until 4 PM PST Sunday for coastal waters
     from Cape Falcon OR to Cape Foulweather OR out 60 nm.

     Small Craft Advisory until 10 PM PST Sunday for Coastal waters
     from Cape Foulweather to Florence OR out 10 NM.

     Small Craft Advisory until 4 AM PST Monday for Waters from Cape
     Foulweather to Florence OR from 10 to 60 NM.
&&

&&

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