Heavy Snow in Washington, February 25 to 26, 2023

Area Forecast Discussion
National Weather Service Portland OR
245 PM PST Sat Feb 25 2023

.SYNOPSIS...Next front offshore will race across the region later
tonight into early Sunday, with another round of valley rain and
mountain snow, but also areas of valley floor level snow from
Portland north and eastward. Showery for most of Sunday, with snow
levels sitting around 1000 to 1500 feet. Unsettled weather continues
into first half of the coming week, with more valley rain and
mountain snow. But, will see continued potential of low-elevation
snow Mon into Tue, mainly Portland/Vancouver north and eastward.
&&

.SHORT TERM DISCUSSION (TONIGHT THROUGH TUESDAY)...

Key Points:

    1)	Clouds and precipitation return tonight. Snow to valley
	floor likely from Portland/Vancouver and to north and east.

    2)	Another round of low-elevation snow possibly later Sunday
	night into early Monday, but more likely to see just rain
	as temperatures may stay just above freezing.

    3)	Another round of low-elevation snow possibly Monday night
	into Tuesday, but high uncertainty on amounts if any.

Rather pleasant afternoon across the region, with temperatures well
into the 40s along the coast, and for areas to south of Salem. Still
bit cooler where snow remains on the ground, with lowlands in upper
30s to around 40. Temperatures will fall back into the 30s after this
evening.

Low pressure over the southeast Gulf of Alaska will settle into the
region offshore tonight and Sunday. As it does, will see a fast
moving front approach, with increasing clouds through this evening.
Precipitation will spread to the coast after 9 pm, with that
precipitation spreading inland overnight. Snow levels will remain
somewhat low, running at 300 to 800 feet in the Coast Range and
Willapa Hills. As such, will see snow over the higher terrain. System
is moving a fairly decent clip, so total snow amounts will be
somewhat limited. Even so, likely to see 3 to 9 inches in the the
Coast Range and Willapa Hills, enough to support current snow
advisories/warnings for that area. Similar results for the Cascades,
though will see bit heavier QPF and overall snowfall, with 4 to 8
inches of snow expected. Front will push across region early Sunday
morning, with snow levels slowly lifting upward. Bulk of accumulating
snow in the coastal mtns will end, but continues across the
Cascades and foothills with the showers. An additional 3 to 8
inches of snow likely for those areas during rest of the day. So,
current Winter Storm Warning across the south WA/north OR Cascades
looks on track, with snow advisories to the south in Lane County
where QPF is bit less and snow levels bit higher.

Now, big question remains:  Will it snow in the interior valleys?
Well, at this point in time, looks like will see precipitation start
as light rain or a rain/snow mix for areas north of Salem. But, may
be just a tad too much warm air aloft (between 3000 and 5000 ft above
surface) that will stay as rain for areas to the south. That will
also help to provide for warm temperatures for areas south of
Newport/Chehalem Mt to an Aurora/Canby line, with southerly winds
picking up tonight. Suspect temperatures will stay at or above 32 for
those areas, and as such, little if any snow expected. Now to the
north, that south wind will have not much effect, allowing for the
precipitation to transition to a period of snow. That timing looks to
be after midnight. Unlike the previous snow event, this will be more
wet snow, a half to 1 inch likely for lowlands east of the West
Hills, with 1 to 3 inches as move out across Clark County through the
Cowlitz River Valley, as well as east of I-205 in Portland towards
Sandy. But, by 5 or 6 am, will see snow level rise a bit, with any
low-elevation snow ending. But, will still have mix of rain/snow
showers in higher terrain, mostly at or above 1000 feet. As such,
will maintain current Winter Weather Advisory for areas from Portland
northward. More snow as get into the Columbia River Gorge as well,
but like other areas, will see that onshore flow turn, with snow
levels rising in the morning. Bottom line: if have somewhere to go
early Sunday morning, may want to wait just a bit, as conditions will
get better.

Again, showers on tap for rest of Sunday, with snow level high enough
to maintain rain or brief rain/snow mix at worst for lowest inland
spots. But, now new accumulations.  Showers continue into Sunday
evening, with snow across the higher terrain (above 1000 to 1500 ft).
Another front will buckle inland across the region later Sunday night
into early Monday. But, appears may be just tad too warm to see much
if any low elevation snow. That said, will maintain minor amounts
(less than an inch) for inland areas, yes, from Portland north and
east. Again, too warm to the south and along coast, where just have
rain. Expect snow at times for the higher terrain of the Coast
Range/Cascades.

The low that was just offshore will finally shift to the coast later
Monday and Monday night. Interestingly, models still having issues
resolving where the surface lows will come ashore, as that has all to
do with lowland temperatures and snow potential for Monday night into
Tuesday. The NAM models continue to trend the low coming ashore
around or just south of Astoria, while the GFS shows around Newport.
This system is still concerning, as it will bring the potential for
yet another round of low elevation snow and the potential for
mesoscale banding of heavy snow (which the GFS suggests is possible
given an overlap of strong frontogenesis, Q-vector convergence, and
conditional symmetric instability within a saturated environment).
Unfortunately, the snowfall forecast is highly uncertain for this
system as the outcome will depend on the exact strength and track of
the surface low and any mesoscale142 bands of heavier precipitation
that develop. The past few GFS/EURO runs continue to show a setup
that is remarkably similar to the one that plagued our region with
lowland snow threats this past week, with easterly winds
redeveloping and bringing in more cold air as the surface low
temporarily stalls somewhere near the mouth of the Columbia River.
Deterministic model and ensemble snow amounts are nearly all an inch
or more for the lowlands by Monday night, with some ensemble members
showing much more than that. The outcome will depend on where the
surface low tracks into the region. If the GFS is correct and the
low comes ashore around Newport, the more likely we will see low
elevation snow across much of the interior. But, if the lows comes
ashore near Astoria or northward like the EURO and NAM are now both
suggesting, then will likely see rain rather than snow. For now,
have decided to keep 1 to 3 inches snow in the forecast across
Portland/Vancouver area, and to the north through Cowlitz lowlands,
with heavier amounts to east into the Gorge/Cascades. But, higher
confidence in getting just rain for areas to south of Portland.

Ugh, still challenging forecast, with lots of unanswered questions
still. Stay tuned and keep up with latest forecasts.  /Rockey/TK

.LONG TERM...(TUESDAY THROUGH FRIDAY)...Confidence is high that
below normal temperatures will continue through the long term
forecast. The GEFS/EPS/CMC ensemble mean for 500 mb heights are all
showing a trough over the western CONUS on Tuesday before a transient
shortwave ridge pushes through the area on Wednesday. It appears this
ridge will then be followed by a period of nearly zonal flow on
Thursday before yet another trough begins pushing into the Pacific
Northwest late Friday. As a result, Wednesday appears to be the only
dry day of the week thanks to the shortwave ridge set to move in.
Aside from that, models and their ensembles are showing several more
rounds of Cascade snow and valley rain next week. The NBM continues
to suggest snow levels will get low enough to support additional
chances of lowland snow, or at least a rain/snow mix.

At this point, there is average forecast confidence that there will
be some additional snowfall during the Monday night through Tuesday
period. However, there is considerable uncertainty in how much, if
any, occurs and the favored locations. Right now, the NBM is showing
a 20-40% chance of one inch of snow or more across the interior
lowlands and a 10-20% chance along the coast. That said, these
probabilities have been too low lately so confidence is not very high
that these probabilities are truly representative. Based on 24-hour
snow amounts from the GEFS/EPS, it appears these probabilities may be
a bit too low still. Another frontal system is expected to push
through the area Thursday, but this one will have more maritime
influence with snow confined above 2000 feet. -TK

&&

.AVIATION...00z TAFs: At 2130Z satellite imagery revealed VFR SKC
conditions across the area. However, high-level clouds could be
seen along the south Washington coast with a frontal system well
offshore. Expect increasing middle and high clouds from the
northwest to southeast in the 00Z to 06Z Sunday time frame. Model
guidance shows the leading edge of the next frontal system
reaching the south Washington coast around 03Z Sunday. Cigs will
continue to lower from northwest to southeast between through the
evening, with predominant MVFR along the coast by 08Z Sunday.
Precipitation type will be somewhat challenging north of KPDX
overnight. Snow will likely be the predominant type in the
southwest Washington lowlands and in the Gorge between 08Z and 18Z
Sunday. A short period of snow, on the order of 2-5 hours, will
be possible for KPDX and areas to the northwest, to include
locations such as KSPB, Vernonia, Forest Grove, Banks, etc. Rain
should be the predominant precipitation type for those areas after
18Z. Otherwise, rain will be the dominant type for lowland areas
south of a KPDX to KTMK line through Sunday morning then
transitioning to showers.

Expect predominant MVFR, with areas of IFR, after 08Z Sunday. IFR
probabilities will be higher in areas that have snow. Minor snow
accumulations are expected for the PDX metro terminals and areas to
the north, but should be washe142d out quickly as precip turns to
rain with a switch to southerly flow. Conditions likely IFR in any
snow, but should improve to MVFR as rain becomes more dominant.

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

KPDX AND APPROACHES...VFR SKC through early evening. Expect
increasing high clouds early this evening and then mid-clouds later
in the evening. Cigs lower to around FL100 by 03Z. Precipitation
moves over the area around 08Z, likely beginning as snow. There will
be a 2-5 hour window between 08Z and 13Z, with snow as the
predominant precipitation type. An inch of snow accumulation is
possible, but it should melt quickly after precip changes to rain.
IFR develops around 08Z, then should improve to MVFR when snow
changes to rain. Frontal passage looks to be around 15Z Sunday,
becoming an MVFR/VFR mix after 18Z.		  /Weishaar

&&

.MARINE...At 1 PM water vapor satellite imagery showed weak high
pressure over the coastal waters. A frontal system could be seen
extending from Hadai Gwaii to around 45N 140W. Buoy observations
revealed benign wind and sea conditions with wind speeds generally
10 kt or less and wave heights around 5 ft. Model guidance
indicates boundary layer wind speeds increasing to 20-25 kt over
the outer waters by mid-evening. Models suggest a low-level jet
developing over the inner waters between 10 PM and 1 AM Sunday.
The cold front is expected to move through the coastal waters
between 1 AM and 4 AM Sunday. Will maintain the current gale
warnings for PZZ251 and PZZ252.

Wind speeds subside below 20 kt late Sunday afternoon or early
evening. The wind forecast becomes more challenging late Sunday
night through early Tuesday. Models show a surface low pressure
somewhere over the Washington or far northern Oregon offshore
(beyond 60 nm) waters 4 AM Monday. The location and track of this
surface will will have a major impact on the overall coastal
waters wind regime. One model, valid 10 AM Monday, indicates 15 to
20 kt boundary layer wind speeds while another model shows a
narrow band of 30 to 35 kt speeds over the central Oregon waters.
The GEFS ensembles have several individual members with surface
low centers south of buoy 46050. The EPS ensembles show a vast
majority of members with surface low centers near or north of buoy
46089. The more southern solutions would result in the lower wind
speeds. Due to the large degree of uncertainty, have gone with a
general model average for now.

Wave heights are forecast to exceed 10 ft late tonight or early
Sunday morning and peak around 15 ft over the northern waters
Sunday afternoon. Seas hold at or above 12 ft Sunday evening
through Monday afternoon, with dominant periods 11 to 12 seconds.
The current Hazardous Seas Warning is valid through early Monday
evening. May be able to convert to Small Craft Advisory a little
sooner, but will let next shift or two evaluate this possibility.
Seas build again Monday night, potentially into the upper teens
over the far southern waters. Maximum wave heights will depend on
the surface low location and track. Weishaar

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Winter Storm Warning from 8 PM this evening to noon PST Sunday
     for Coast Range of Northwest Oregon.

     Winter Weather Advisory from 8 PM this evening to noon PST
     Sunday for Central Coast Range of Western Oregon.

     Winter Weather Advisory from 8 PM this evening to 10 AM PST
     Sunday for Greater Portland Metro Area-Lower Columbia.

     Winter Storm Warning from 10 PM this evening to 1 PM PST Sunday
     for Northern Oregon Cascade Foothills-Western Columbia
     River Gorge.

     Winter Storm Warning from midnight tonight to 7 PM PST Sunday
     for Northern Oregon Cascades.

     Winter Weather Advisory from midnight tonight to noon PST Sunday

     for Cascade Foothills in Lane County.

     Winter Weather Advisory from midnight tonight to 7 PM PST Sunday

     for Cascades in Lane County.

     Winter Weather Advisory from midnight tonight to 1 PM PST Sunday

     for Upper Hood River Valley.

     Winter Storm Warning from midnight tonight to 1 PM PST Sunday
     for Central Columbia River Gorge.

WA...Winter Storm Warning from 8 PM this evening to noon PST Sunday
     for Willapa Hills.

     Winter Weather Advisory from 8 PM this evening to 10 AM PST
     Sunday for Greater Vancouver Area-I-5 Corridor in Cowlitz
     County.

     Winter Storm Warning from 10 PM this evening to 1 PM PST Sunday
     for South Washington Cascade Foothills-Western Columbia
     River Gorge.

     Winter Storm Warning from midnight tonight to 7 PM PST Sunday
     for South Washington Cascades.

     Winter Storm Warning from midnight tonight to 1 PM PST Sunday
     for Central Columbia River Gorge.

PZ...Hazardous Seas Warning from 2 AM to 6 PM PST Sunday for Coastal
     waters from Cape Shoalwater WA to Florence OR out 10
     NM-Columbia River Bar-Coastal waters from Cape Shoalwater
     WA to Florence OR from 10 to 60 NM.

     Gale Warning from 6 PM this evening to 2 AM PST Sunday for
     Coastal waters from Cape Shoalwater WA to Cape Foulweather
     OR out 10 NM.

     Small Craft Advisory from 6 PM this evening to 2 AM PST Sunday
     for Coastal waters from Cape Foulweather to Florence OR out
     10 NM-Coastal waters from Cape Shoalwater WA to Florence OR
     from 10 to 60 NM.
&&

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