Heavy Snow in Oregon, February 25 to 29, 2024

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1017 AM PST Sun Feb 25 2024

.SYNOPSIS...The next weather system is pushing into the area,
with rain and mountain snow spreading south throughout the day
today. Heavier band of rain and mountain snow tonight with snow
levels lowering to 1000-2000 ft by Monday morning. Cooler on
Monday with precipitation turning to showers with a chance of
thunderstorms in the afternoon and evening hours. Low
probabilities of light lowland snow Monday night into Tuesday
morning. Cool unsettled weather continues Wednesday into the
weekend, with additional rounds of moderate to heavy
precipitation and mountain snow at times.

&&

.DISCUSSION...SUNDAY THROUGH SATURDAY...The next weather system
is imminent as radar imagery shows showers are spreading south
into SW Washington early this morning. An upper trough currently
near the Alaska/Canada border will slowly deepen along the
western coast of Canada and into the Pacific NW today into
Monday. Showers will very slowly spread south through NW Oregon
throughout today ahead of the surface front, which is still
slated to move south through the area late tonight into early
Monday morning. Snow levels will be around 4500-5500 ft today,
falling rapidly with the frontal passage to 1000-2000 ft. A band
of moderate to heavy precipitation along the front will bring
0.25-0.5 inches of rain to the lowlands and moderate to heavy
snow to the Cascades and Coast Range. Convection builds behind
the front with widespread showers producing moderate to heavy
rain and snow continuing into Monday night. The Winter Storm
Warning for the Cascades and Hood River Valley above 1000 ft
and the Winter Weather Advisory for the Coast Range above 1000
ft have both been extended through late Monday night to account
for this. Snow amounts of 1-2 feet are expected for the Cascades
above 2000 ft with anywhere from 2 inches to 1 foot down to 1000
ft. For the Coast Range, 2-10 inches is possible above 1000 ft,
heaviest above 2000 ft. Gusty winds are also expected over the
mountains with gusts up to 40-50 mph possible over exposed
ridges, bringing low visibility at times due to gusty winds and
heavy snow. Additionally, NAM nest and GFS soundings indicate
CAPE of 200-500 J/kg with 0-6 km shear of 20-40 kts for
locations generally below 2000 ft, which produces a chance of
thunderstorms with the capability of lightning, heavy rain,
graupel, and small hail Monday late morning through late Monday
night. NBM suggests a 15-25% probability of thunderstorms with
higher probabilities along the coast. SPC also has put our
region in a general thunder outlook for Monday.

Guidance continues to indicate snow levels will lower below 1000
ft Monday night into early Tuesday. Ensembles indicate 850 mb
temperatures will fall to around -6 to -8 deg C, colder farther
north. There's some uncertainty on exactly how far south this
cold air will push, but NBM indicates a 40-60% probability that
snow levels will fall to at least 500 ft. For the lowlands below
500 ft, ensembles indicate -8 deg C 850 mb temperatures (the
general threshold for at least a snow/rain mix) could push at
least to the Portland metro area. NBM indicates a 15-25%
probability of any snow for the majority of the lowlands, except
for a 20-40% probability for the Portland metro area and the
lowlands of Cowlitz and Columbia Counties. However,
accumulations are expected to be limited for elevations below
1000 ft due to lighter precipitation during this time and warm
ground temperatures. NBM indicates a 10-15% chance of 1+ inch of
snow down to 500 ft and less than 10% chance below 500 ft.

The upper trough moves east into the Intermountain West on
Tuesday. Snow levels rise back to 1000-2000 ft by late Tuesday
morning. Light scattered showers will continue, mainly for the
mountains and the coast. A Pacific-moisture fueled warm front
associated with a parent low pressure system centered in the
Gulf of Alaska is expected to move through the PacNW Tuesday
night, raising snow levels back to 3000-4000 ft by Wednesday
morning and bringing another round of widespread precipitation.
This could cause some minor flooding concerns for flood prone
coastal rivers due to moderate rain on top of existing snow
causing increased snow melt. Will need keep an eye on this
concern.

Ensembles are in good agreement (90+% of the members in the 00z
WPC cluster analysis) that a strong upper trough will deepen
over the eastern Pacific Wednesday into Saturday, spinning
multiple shortwaves and surface fronts into the PacNW. At this
point, ensembles indicate that a strong cold front with
moderate Pacific moisture will push into the PacNW sometime
late Wednesday night into Thursday. This will bring a round of
heavier rain to the coast and interior lowlands. NBM indicates
a 50-75% probability of 1+ inch of rain for the interior
lowlands and 60-80% for the coast, but there's only a 5-20%
probability of 2+ inches for the interior lowlands and 10-25%
for the coast. Snow levels are also expected to fall again with
the frontal passage, bringing the potential for another round of
moderate to heavy snow to the Cascades and Coast Range below
2000 ft. Also can't rule out the potential for lowland snow
Thursday night into Friday. Currently NBM has a 20-40%
probability of any snow, though less than 10% probability of 1+
inch.

Beyond this, periods of rain and mountain snow are likely to
continue into the weekend with the trough deepening and slowly
approaching the coast. However, uncertainty remains in the
ensembles on exactly how far south the trough will deepen and
when it will finally move east through the Western US, all of
which will dictate specific impacts to SW Washington and NW
Oregon. Keep an eye out for additional details as the forecast
progresses. -HEC

&&

.AVIATION...Terminal observations as of 18z Sun show a mix of
MVFR/low-end VFR cigs across most terminals, with LIFR/IFR cigs in
the central and southern Willamette Valley. LIFR/IFR stratus in the
central/south Valley should lift to MVFR as more low level mixing
occurs late morning to early afternoon Sunday.

Current radar imagery shows a band of steady rain moving into the
coast of southwest Washington and far northwest Oregon. As the
frontal system progresses southward today, expect predominately
IFR/MVFR cigs along the coast and MVFR cigs inland by 21z Sun.
Guidance suggests a 15-25% chance of IFR cigs in the Willamette
Valley through this evening. Most terminals will transition into a
post-frontal environment with high-end MVFR/low-end VFR cigs and
showers after 06-09z Mon. Expect mainly southwesterly winds today
with widespread gusts up to 20-25 kt along the coast. Gusts up to 20
kt inland will be more brief and isolated.

*NOTE: The observation out of KONP is generally limited to wind and
pressure at this time. Periodically cloud cover is reported but is
unreliable.

PDX AND APPROACHES...Low-end VFR cigs until 21z Sun-00z Mon, when
cigs drop to predominately MVFR with the frontal passage. Widespread
rain returns late afternoon Sunday and lasts through the evening.
Post-frontal showers return between 08-10z Mon. Southwesterly winds
will be around 08-13 kt today. 					      -Alviz

&&

.MARINE...The next main system is expected to impact the waters
tonight into Monday. A trough dropping southeast along the
British Columbia coast will amplify as it enters the Pacific
Northwest. Sustained westerly/northwesterly winds will begin
increasing late Sunday morning and should peak Monday morning with
gusts up to 28-33 kt, with highest gusts expected to the north.
This system will also bring in a northwesterly swell, which will
build seas to around 9 to 12 ft Sunday and 14 to 16 ft by Monday.
Winds will begin to slowly ease Monday night into Tuesday;
however, expect seas to remain elevated. As a result, a Hazardous
Seas Warning has been issued for the northern and central waters
beginning early Monday morning and for the southern waters
beginning early Monday afternoon. Active weather continues over
the waters through the remainder of next week. -JH/Alviz

&&

.BEACH HAZARDS...The next storm system approaching the Pacific
Northwest will produce a northwesterly swell today with swell
heights up to 9 to 11 ft at 15 seconds. This long period swell
will result in an elevated sneaker wave threat. Therefore, a Beach
Hazards Statement has been issued for the Pacific County, WA to
Lane County, OR coastline starting early this morning through
tonight. Use extra caution when in or near the surf zone. Always
keep an eye on the waves and be watchful of children. -JH/Alviz

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Winter Weather Advisory from 7 PM this evening to 1 AM PST
     Tuesday for Coast Range of Northwest Oregon.

     Winter Weather Advisory from 10 PM this evening to 1 AM PST
     Tuesday for Central Coast Range of Western Oregon.

     Winter Storm Warning from 4 PM this afternoon to 1 AM PST
     Tuesday for Northern Oregon Cascade Foothills-Northern
     Oregon Cascades-Upper Hood River Valley.

     Winter Storm Warning from 10 PM this evening to 1 AM PST Tuesday
     for Cascade Foothills in Lane County-Cascades in Lane
     County.

WA...Winter Weather Advisory from 7 PM this evening to 1 AM PST
     Tuesday for Willapa Hills.

     Winter Storm Warning from 10 AM this morning to 1 AM PST Tuesday
     for South Washington Cascade Foothills-South Washington
     Cascades.

PZ...Small Craft Advisory from 10 AM this morning to 4 AM PST Monday
     for Coastal waters from Cape Falcon to Cape Foulweather OR
     out 10 NM-Columbia River Bar-Waters from Cape Falcon to
     Cape Foulweather OR from 10 to 60 NM.

     Hazardous Seas Warning from 4 AM Monday to 4 AM PST Tuesday for
     Coastal waters from Cape Shoalwater WA to Cape Foulweather
     OR out 10 NM-Columbia River Bar-Coastal waters from Cape
     Shoalwater WA to Cape Foulweather OR from 10 to 60 NM.

     Small Craft Advisory until 4 AM PST Monday for Coastal waters
     from Cape Shoalwater WA to Cape Falcon OR out 60 NM.

     Small Craft Advisory from 4 PM this afternoon to 1 PM PST Monday
     for Coastal waters from Cape Foulweather to Florence OR out
     10 NM-Waters from Cape Foulweather to Florence OR from 10
     to 60 NM.

     Hazardous Seas Warning from 1 PM Monday to 4 AM PST Tuesday for
     Coastal waters from Cape Foulweather to Florence OR out 10
     NM-Waters from Cape Foulweather to Florence OR from 10 to
     60 NM.
&&

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