Heavy Snow in Washington, February 25, 2019

Area Forecast Discussion
National Weather Service Portland OR
1006 PM PST Sun Feb 24 2019

.SYNOPSIS...An unseasonably cold and unsettled weather pattern will
continue for much of the next week with a major snow event
increasingly likely for the southern Willamette Valley and adjacent
foothills tonight into Monday.

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.SHORT TERM...Tonight through Wednesday...Have made mostly minor
tweaks to the temperature, snow level, QPF and snow amount grids from
this evening through Monday afternoon.

The main story is that precipitation intensified along the stalled
frontal boundary over Lane and Douglas Counties this afternoon and
resulted in snow levels lowering to near the valley floor through
dynamical cooling processes. The NAM and GEM hinted at this scenario.
This resulted in several inches of snow falling around Cottage Grove,
Lowell and Lorane and 6" around Oakridge and Leaburg as of 5-6 PM.
This has also allowed snow to begin to accumulate around the Eugene
metro in the past couple of hours. This trend is expected to continue
through the overnight hours and should result in heavier snowfall
totals across the far southern Willamette Valley than initially
anticipated. As a result of the earlier onset of snow, this should
end up being quite the late season snowstorm, albeit a wet and sloppy
one for the southern Willamette Valley. The snow will likely weigh
down trees and result in at least some power outages around Eugene
and Springfield. The surrounding hills of the southern Willamette
Valley around 1000 feet and foothill river valleys to the south and
east of Eugene such as Oakridge, Lowell, Leaburg and Lorane have the
potential to experience a major to possibly even historic snowstorm
with snowfall totals ranging in the 1-2 ft range. Widespread power
outages appear increasingly likely in these areas.

Otherwise, the surface low pressure offshore appears on track to make
landfall along the southern Oregon coast near or just south of Cape
Blanco before it pushes east-northeastward towards Bend. This track
should prevent temperatures from warming up much across Lane County
and hence why we are worried about the high snow totals in the hills
and Cascade foothill valleys to the south and east of Eugene. As the
low pressure approaches the coast and tracks across southwest Oregon,
this should result in increasing easterly winds through the Columbia
River Gorge. Eventually these winds will turn northerly in the
north-central Willamette Valley and should start making their way up
the Willamette Valley towards Salem, Corvallis and Eugene. This
should help to lower snow levels to the valley floor as moisture
falls into this cooler and drier airmass at these locations. The
developing low level north-northeasterly low level flow overnight
should also upslope into the western third of the Willamette Valley
and produce locally higher amounts for places like Monroe, Philomath,
Corvallis, Adair Village, Dallas when compared to Salem, Albany and
especially those on the east side of the Willamette Valley like
Molalla, Silverton, Stayton, Lebanon & Brownsville. There are some
hints of this in the high resolution models, which leads to at least
modest confidence this general pattern will play out like similar
past events such as Feb 2014.

It appears the Portland metro will likely end up north of the
heaviest precipitation as the best low level isentropic lift and
frontogenesis will generally stay south and east of the Portland
metro. Nonetheless, a 700mb low pressure is forecast to move squarely
over the metro area Monday morning and should provide enough low to
mid level warm advection to generate an area of light snow across the
Portland metro between 4-10 AM Saturday. At this point we are
expecting at least some flurries and probably even some light
accumulations across the metro...generally a Trace to 2". The
easterly flow coming out of the Columbia River Gorge will likely
produce upslope flow into the West Hills and Coast Range that should
enable some of this snow being generated aloft to have an easier time
reaching the surface in areas like Scappoose, Forest Grove and Banks.
As a result, snowfall totals may be slightly higher in these locales
Monday morning relative to the rest of the Portland metro. Given the
late February sun angle is quite a bit higher than Dec/Jan, expect
any roads that do accumulate snow to melt rather quickly late Monday
morning/midday around the Portland metro.

It should also be noted that the NAM and GFS suggest a deformation
band around this 700mb circulation will set up shop over Columbia and
Cowlitz County area midday Monday into Monday afternoon. This could
produce a band of localized accumulations, mainly on grassy surfaces
for areas around Kelso/Longview, St Helens, Vernonia, etc. Monday
afternoon. However, the late February sun angle despite the cloud
cover should be enough to prevent few issues for area roads.

We should see temperatures recover a bit Tuesday and allow for areas
that get buried tonight to melt out a bit. However, a weak, but
unseasonably cold storm system could bring a threat for some lower
elevation, albeit light, snow on Wednesday, but haven't really had
time to look into it at this point. /Neuman

.LONG TERM...Wednesday night through Sunday...The operational GFS and
ECWMF have a weak surface low off the northern Oregon coast, which
will slowly push its way onshore. Bringing with it a continued
chance for precipitation through Thursday evening. There is a
significant difference between the GFS and ECWMF during this time
period. The GFS has the 850mb temperatures between -8C and -7C,
while the ECWMF has these temperatures slightly warmer between -7C
and -6C. I have leaned into the GFS more when considering the
potential for low level snow, but with the understanding that the
models have been changing significantly between runs in the
extended. As this low moves out of the area, a shortwave ridge is
expected to develop Friday, and looks to be the dominate feature
through Sunday. The ridge will reduce precipitation chances through
the weekend.  Overall, I have slightly lowered temperatures across
the area through the extended. It should be noted that these model
runs, especially the GFS has a clearly defined surface low coming
from the tropics towards, the start of next week. We will be keeping
an eye on this feature to see how it might impact the area. /42
&&

.AVIATION...Low pressure approaching the southern Oregon coast
will spread a band of steady precipitation from south to north
through the fcst area overnight into Mon AM. The precipitation
will remain from around KSLE southward through around 12Z, then
spread north into the northern Willamette Valley between 12Z-18Z.
This precipitation is expected to fall as mainly snow for the
inland TAF sites. Significant accumulations are expected from
KSLE south, with lighter amounts for the Portland metro sites.
The south coast, including KONP, will likely see a mix of rain
and snow, while the north coast sees little in the way of
precipitation.

Conditions will deteriorate from south to north later this
evening and overnight as the snow band lifts. Expect predominantly
MVFR within the precipitation band, with localized IFR expected
in periods of heavier snow. Precipitation will begin to taper off
after 18Z, although some snow showers will likely continue into
the afternoon. Conditions gradually improve to VFR in the
afternoon from north to south. KEUG may see low cigs linger
through much of the day.

KPDX AND APPROACHES...Easterly winds through the gorge will
continue to reach the terminal tonight and much of Mon. Expect
light snow to develop by 12Z and continue through the morning.
There is potential for light accumulation. Conditions should
deteriorate to MVFR, or possibly IFR, once the snow starts.
Precipitation starts to taper off after 18Z, but snow showers
may continue at times through the afternoon. Expect cigs to
improve to VFR in the afternoon. Pyle

&&

.MARINE...A low is expected to form along the quasi-stationary
frontal boundary stretching southwest from about Florence late
tonight into Monday. It looks like it will make landfall around
Port Orford late tonight, keeping the strongest winds to its
south and out of the local waters. To the north of the low,
winds will be offshore with some gusts to 25 kt possible, so have
issued a Small Craft Advisory for Winds from tonight through the
day Monday.

After Monday, it looks like winds and seas will stay below 20 kt
and 10 ft, respectively, through at least midweek, but
confidence in the forecast is low at this point due to numerous
possible solutions. Bowen

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Warning until midnight PST Monday night for
     Central Columbia River Gorge-Upper Hood River Valley.

     Winter Storm Warning until 7 PM PST Monday for Western Columbia
     River Gorge.

     Winter Storm Warning until 8 PM PST Monday for Northern Oregon
     Cascade Foothills.

     Winter Storm Warning until midnight PST Monday night for
     Northern Oregon Cascades.

     Winter Storm Warning until 7 PM PST Monday for Cascade
     Foothills in Lane County.

     Winter Storm Warning until 10 PM PST Monday for Cascades in
     Lane County.

     Winter Weather Advisory from 2 AM to 2 PM PST Monday for
     Greater Portland Metro Area.

     Winter Weather Advisory until 9 AM PST Monday for Central
     Oregon Coast.

     Winter Storm Warning until 1 PM PST Monday for Central Coast
     Range of Western Oregon-South Willamette Valley.

     Winter Weather Advisory until 1 PM PST Monday for Coast Range
     of Northwest Oregon.

     Winter Storm Warning until 2 PM PST Monday for Central
     Willamette Valley.

WA...Winter Storm Warning until midnight PST Monday night for
     Central Columbia River Gorge.

     Winter Storm Warning until 7 PM PST Monday for Western Columbia
     River Gorge.

     Winter Weather Advisory from 2 AM to 2 PM PST Monday for
     Greater Vancouver Area.

     Winter Weather Advisory until 6 PM PST Monday for South
     Washington Cascade Foothills-South Washington Cascades.

PZ...Small Craft Advisory for winds from 1 AM to 4 PM PST Monday for
     Coastal Waters from Cape Shoalwater WA to Florence OR out
     60 NM.

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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. This area is
commonly referred to as the forecast area.

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PORTLAND OR
747 PM PST SUN FEB 24 2019

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0700 PM     HEAVY SNOW       LEABURG                 44.12N 122.68W
02/24/2019  E6.0 INCH        LANE               OR   PUBLIC

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CN