Heavy Snow in Washington, February 25 to 29, 2024

Area Forecast Discussion...UPDATE
National Weather Service Portland OR
815 PM PST Sun Feb 25 2024

Updated aviation discussion

.SYNOPSIS...The next weather system is pushing into the area,
with rain and mountain snow spreading south throughout the rest of
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 NIGHT THROUGH SATURDAY NIGHT...The leading edge
of the next system is beginning to move in from the north as of
Saturday afternoon, bringing light rain showers to the northern
Willamette Valley and SW Washington. The surface front, however,
isn't expected to move south through the area until late tonight into
early Monday morning. In the latter half of tonight, as the front
moves through, snow levels will fall rapidly 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. 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 at areas with exposed terrain, with gusts
up to 40-50 mph possible. Low visibility will be expected at times
due to gusty winds and heavy snow. NAM and GFS guidance continue to
indicate CAPE values 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 until around the middle of Monday
night. We continue to be in the SPC's thunder outlook for Monday, and
the NBM suggests a 15-25% probability of thunderstorms with higher
probabilities along the coast.

Ensemble guidance continues to trend towards lower snow levels Monday
night into early Tuesday, with snow levels of 200-500 feet expected.
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 70%
probability that snow levels will fall to at least 500 ft for the
Portland metro, and 25-40% probability in the southern Willamette
Valley near Eugene. 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 continues to show 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 remain in near universal agreement 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 60-75% 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. The potential for lowland snow exists Thursday night and
Friday night, but currently uncertainty remains high. Currently NBM
has a 15-30% probability of lowland snow on Thursday night and 20-40%
on Friday night, though less than 10% probability of 1+
inch of accumulation each night.

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/JLiu

&&

.AVIATION...A cold front will continue to slide south across NW
Oregon tonight with increasing westerly flow aloft. Steady rain
will gradually taper off as the front moves through overnight.
Post-frontal showers are expected to increase through the day on
Monday as the upper level trough digs southward over the region.
Colder air aloft will will bring lowering snow levels as well as a
15-25% chance of thunderstorms, with highest chances near the
northern OR coast. Snow may be mixed with rain in heavier showers,
especially later Monday which may reduce vsbys at times.

As of 03Z Monday, observations show widespread IFR conditions
along the north to central Oregon coast, while a mix of VFR and
MVFR CIGS continues inland. In fact, MVFR CIGS have been more
prevalent across the central and southern Willamette Valley, while
CIGS have lifted to around FL050 where the frontal band and
associated rain is occurring. Latest HREF probs suggest chances
for widespread MVFR are highest between 04-08Z this evening, then
begin decreasing below 50% from north to south, with most
terminals at around a 60-70% chance of improving to VFR after 14Z
Monday. Expect mountains and higher terrain to be obscured
through Monday. Southwest winds will continue ahead of the front
with gusts up to 20 kt. Expect winds to shift west- northwest at
the coast behind the front.

*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...Predominately VFR continues to persist this
evening with light rain as the frontal band continues to across
the area. Frontal passage likely around 05-07Z this evening as
southwest winds around 8-10 kt shift more westerly behind the
front. May see periods of MVFR through midnight, but chances for
VFR increase after 08Z to around 60-70%. Post frontal showers
increase through Monday afternoon and evening, with a 15-20%
chance for thunderstorms. /DH

&&

.MARINE...A system dropping southeast from the British Columbia
coast is currently approaching the Pacific Northwest. The cold
front associated with this system will push across the coastal
waters this evening and turn winds northwesterly. Observations at
buoy 46050 as of 7 PM PST show west-southwest winds gusting around
25 kt. The front should pass through the southern waters between
Cape Foulweather and Florence, OR by around 9 to 10 PM PST. This
system will also bring in a northwesterly swell from north to south
which will build seas to around 14 to 16 ft Monday. Therefore, a
Hazardous Seas Warning has been issued for the northern and central
outer waters (beyond 10 NM) beginning early Monday and for the rest
of the waters beginning early Monday afternoon.

Winds briefly weaken and seas begin to slowly fall Monday night
into early Tuesday morning. Another system on Tuesday will turn
winds southerly and return at least small craft advisory gusts.
Active weather continues over the waters through the remainder of
the week. NBM guidance suggests a greater than 80% chance of Gale
force gusts returning across the waters on Wednesday.				-Alviz

&&

.BEACH HAZARDS...An approaching storm system will push a
northwesterly swell across the waters with swell heights of 9 to
11 ft at 15 seconds through tonight. This long period swell will
result in an elevated sneaker wave threat. Therefore, a Beach
Hazards Statement remains in effect for the Pacific County, WA to
Lane County, OR coastline through early Monday morning. Use extra
caution when in or near the surf zone. Always keep an eye on the
waves and be watchful of children. 					-Alviz

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Winter Weather Advisory until 1 AM PST Tuesday for Coast Range
     of Northwest Oregon.

     Winter Weather Advisory until 1 AM PST Tuesday for Central Coast
     Range of Western Oregon.

     Winter Storm Warning until 1 AM PST Tuesday for Northern Oregon
     Cascade Foothills-Northern Oregon Cascades-Upper Hood River
     Valley.

     Winter Storm Warning until 1 AM PST Tuesday for Cascade
     Foothills in Lane County-Cascades in Lane County.

WA...Winter Weather Advisory until 1 AM PST Tuesday for Willapa Hills.

     Winter Storm Warning until 1 AM PST Tuesday for South Washington
     Cascade Foothills-South Washington Cascades.

PZ...Small Craft Advisory until 1 PM PST Monday for Coastal waters
     from Cape Shoalwater WA to Florence OR out 10 NM-Columbia
     River Bar-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 Shoalwater WA to Florence OR out
     10 NM-Columbia River Bar-Waters from Cape Foulweather to
     Florence OR from 10 to 60 NM.

     Small Craft Advisory until 4 AM PST Monday for Coastal waters
     from Cape Shoalwater WA 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 from 10 to 60 NM.

&&

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