Heavy Snow in Oregon, February 27 to 28, 2023

Area Forecast Discussion..UPDATED
National Weather Service Portland OR
435 AM PST Mon Feb 27 2023

.SYNOPSIS...Unsettled and unseasonably chilly weather will persist
through at least Tuesday, with low snow levels and plenty of showers.
Snow levels will vary between sea level and 1000-1500 feet through
Tuesday, tending to be lowest in and near heavier showers. Elevations
below 500 feet can expect spotty, occasional snow accumulations of up
to 2 inches, with mesoscale banding potentially bringing more to a
highly localized area that is impossible to determine at this point.
Above 500 feet, several inches of snow are expected, with the
heaviest amounts in the Coast Range. A break in the weather comes
Wednesday, but additional frontal systems will keep our weather cool
and active Thursday through next weekend.

&&

.SHORT TERM...Another complex low pressure system is parked off the
Oregon Coast, with a broad upper level circulation that extends
offshore beyond 130W. The parent low has stalled near 46N/129W, with
a central pressure of roughly 993 mb and an associated occluding
frontal system approaching the Pac NW coast. It appears a secondary,
mesoscale low of around 999 mb has developed along the front near
Buoy 46050, and is lifting northward as the frontal zone inches
toward the coast. With the parent low stalling, it is becoming
vertically stacked, but a series of shortwaves pinwheeling around the
broader upper level circulation will occasionally recharge shower
activity as the disturbances move onshore. These shortwaves and their
associated enhanced showers will move onshore into Oregon then pivot
northeastward and eventually northward before wrapping back into the
broader circulation of the complex low.

With the vertical profile so close to supporting snow to the lowest
levels, it is a bit concerning to see these shortwaves approaching
from the west-southwest. A similar setup was responsible for the
heavy snow in Portland Wednesday, but with one important difference.
For last Wednesday's event, the low pressure was jammed near the
mouth of the Columbia by a stout arctic air mass pushing toward our
CWA from the north and east. Rather than being forced to stall near
any one location, these disturbances should be able to progress more
freely northeastward, thus limiting the danger of one particular
location being hammered with mesoscale banding as the Portland metro
was last Wednesday. That said, there is once again excellent forcing
for lift and our QPF reflects this with considerably higher QPF than
was suggested by the NBM. Many of the NBM's component members are
likely suffering in their analysis from the lack of radar coverage
along the Oregon coast, and we reflected this suspicion in our QPF,
with most locations forecast to receive a half inch or more of QPF.
Locations in the Coast Range could easily see more than 1 inch of QPF
as waves of showers move through. With snow levels generally in the
500-1500 ft range in the Coast Range, this means the Coast Range
passes and above have a good chance of seeing more snow - upwards of
a foot - than the Cascade passes through Tuesday.

The big question on most everyone's minds is how much snow the
lowlands can expect. As is often the case - and has certainly been
the case the pst week or two - confidence is low on amounts for any
one given location due to the mesoscale nature of the features at
play. At this point it appears the areas most favored for sustained
shower activity are roughly along a line from Mt St Helens to
Tillamook, and we have upgraded even the lower elevation zones in
this area to Winter Storm Warnings. The more sustained and heavier
the showers are, the lower the snow levels will tend to be. But this
pattern will persist well into Tuesday, so just about everyone should
at least see snowflakes in the air between now and then. At the
moment all our land zones have some sort of winter headline - so
anyone with travel plans across SW Washington and NW Oregon should be
prepared for winter travel conditions.

We are keeping a very close eye on the band of convection and related
debris approaching the Oregon coast, as it will likely dictate
whether or not we need to upgrade more zones to Winter Storm Warnings
- or whether or not we were too cavalier in issuing warnings for the
S WA/N OR coast and the lower Columbia/Cowlitz. At this point it
appears that if anything, precip is coming in a little weaker than
anticipated given its satellite presentation and WPC estimates of 0.1
to 0.2"/hr rain rates. Still, this is an example of how touch and go
this situation is, a heavy shower producing 0.2" of QPF could easily
put down a quick 1 inch of wet snow on the lowlands.

Models generally try to encourage some movement of this broader area
of low pressure at the surface and aloft beginning Tuesday night, as
stronger and more zonal jet energy aims toward the NE Pacific. This
should allow a drying trend going into Wednesday, but this would be
brief. Given the residual chilly air mass, temperatures will take
some time to moderate and we will likely continue to deal with the
problematic freeze/thaw/freeze cycle each day, which should result in
some slick roads each night and morning.  Weagle

.LONG TERM...WEDNESDAY THROUGH SUNDAY...A transient upper level
ridge expected over the region Wednesday with generally dry
conditions. However that will change late Wednesday into Thursday as
the upper ridge is replace by intensifying zonal jet across the
northern Pacific. WPC 500 mb clusters are in good agreement in the
overall change. A front arriving Thursday will bring area wide
precipitation with abundant mountain snow and the possibility of
lowland snow for the interior valley. NBM indicating less than 1" for
the valley, with probabilities for an inch below 10 percent. With the
cool air behind the cold front lowland snow likely continue into
Friday.

Friday into Saturday the jet should buckle with a digging upper
trough off the coast of the Pacific Northwest Saturday into Sunday.
This will set up a surface low somewhere off the WA/OR coast. Again
with the cold air in place still have a threat of low elevation
snow, but where the low eventually ends up with determine the amounts
of snow. NBM probabilities of 1 inch snow for the low lands,
including the coast around 10 percent Saturday and Sunday. -MH

&&

.AVIATION...12Z TAFs: Currently, VFR conditions. However, a front
will push inland through 00Z Tuesday. This will bring increasing
MVFR conditions and precipitation through the remainder of the
TAF period. Low snow levels will result in snow this morning, then
a rain/snow mix around 18Z Monday for the majority of the
forecast area. However, warmer temperatures to the south will keep
precip type at lowest elevations as rain for areas south of a
Newport to Albany line. Cannot rule the possibility for brief
periods of IFR that will likely be associated with any
precipitation. Expect terrain obscured in the higher terrain due
to clouds and precipitation through the morning. Slight chance of
thunderstorms along the coast, mainly after 18Z Mon.

After 06Z Tuesday, expect snow to become the dominant precipitation
type for inland locations, with the coast likely maintaining a
rain/snow mix. This will last through the remainder of the TAF
period.

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

KPDX AND APPROACHES...Currently, VFR conditions. However, a front
will push inland through 00Z Tuesday. This will bring increasing
MVFR conditions and precipitation through the remainder of the
TAF period. Snow this morning, then a rain/snow mix around 18Z
Monday. Cannot rule the possibility for brief periods of IFR that
will likely be associated with any precipitation. After 06Z
Tuesday, expect snow to become the dominant precipitation type.
This will last through the remainder of the TAF period. /42

&&

.MARINE...Via satellite, the center of the surface low is
is around 46N/129W and will likely continue a slow drift towards
the NE. However, models continue to exhibit discrepancies in the
exact track, but am leaning towards a landfall for the low,
between Grays Harbor and Cape Elizabeth by this afternoon. In
addition, convection can be seen on the south side of the low,
which points towards the development of multiple smaller scale low
pressure centers within the broad surface low circulation. These
difference will have a significant impact on wind speeds over the
waters. This landfall location would keep the strongest wind
speeds likely over the southern and central Oregon waters.
Observations at Buoy 46050 are showing gusts up to 40 kt.
Therefore will issue a Gale Warning for zones PZZ253 and PZZ273
through at least this afternoon. Also, will maintain a Hazardous
Seas Warning for all waters through Tuesday morning with the
exception of zones PZZ253 and PZZ273 as they will join the
Hazardous Seas warning Monday afternoon. Also, given the
convective nature of the system isolated gusts up to 35 kt
possible for all other zones.

Southerly winds will shift to the northwest by Monday night/
Tuesday morning, with wind speeds falling to 15 kt or less Tuesday
morning, with winds becoming northerly by Tuesday night/
Wednesday. Longer range models show another frontal system
impacting the waters Thursday morning with wind speeds looking to
remain below Gale Force at this time. A stronger system is
expected Friday, with the high probabilities for moderate Gales
across all waters.

Wave heights will remain in the 10 to 13 ft ranger 10 feet
through this evening. Then seas build towards 12 to 17 ft by late
tonight into Tuesday morning, with the highest waves expected in
PZZ272, PZZ273, PZZ252 and PZZ253. Seas subside to 8 to 10 ft
Tuesday night through Thursday evening. The active pattern late
in the week will result in wave heights rising to the low to mid-
teens through the weekend. /42

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Winter Storm Warning until 10 AM PST Tuesday for Central Coast
     Range of Western Oregon-Coast Range of Northwest Oregon-
     Lower Columbia-North Oregon Coast-Northern Oregon Cascade
     Foothills-Western Columbia River Gorge.

     Winter Weather Advisory until noon PST today for Central Oregon
     Coast-South Willamette Valley.

     Winter Weather Advisory until 6 AM PST Tuesday for Greater
     Portland Metro Area.

     Winter Weather Advisory until 10 PM PST this evening for Central
     Willamette Valley.

     Winter Weather Advisory from 7 AM this morning to 10 AM PST
     Tuesday for Cascades in Lane County-Northern Oregon
     Cascades.

     Winter Storm Warning until 10 AM PST Tuesday for Cascade
     Foothills in Lane County.

     Winter Weather Advisory until 2 AM PST Tuesday for Central
     Columbia River Gorge-Upper Hood River Valley.

WA...Winter Storm Warning until 10 AM PST Tuesday for I-5 Corridor in
     Cowlitz County-South Washington Cascade Foothills-South
     Washington Coast-Western Columbia River Gorge-Willapa Hills.

     Winter Weather Advisory until 6 AM PST Tuesday for Greater
     Vancouver Area.

     Winter Weather Advisory until 2 AM PST Tuesday for Central
     Columbia River Gorge.

     Winter Weather Advisory until 10 AM PST Tuesday for South
     Washington Cascades.

PZ...Hazardous Seas Warning until 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.

     Gale Warning until 3 PM PST this afternoon 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 3 PM this afternoon 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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