Heavy Snow in Oregon, February 13 to 14, 2023

Area Forecast Discussion
National Weather Service Portland OR
356 AM PST Mon Feb 13 2023

.SYNOPSIS...A fast moving cold front will sweep across the region
this morning, followed by cooler temperatures and rapidly lowering
snow levels. Snow will be heavy at times in the Cascades and Coast
Range later today through tonight. Post-frontal showers will persist
through early Tuesday, eventually clearing from north to south as
Tuesday progresses. By early Tuesday morning snow levels will be near
sea level, and light accumulations are possible for the lowlands.
Enhanced instability may lead to a few thunderstorms, especially near
the coast. High pressure will lead to a clear and cold night Tuesday
night, with mostly dry weather likely mid to late week.

&&

.SHORT TERM...Today through Wednesday...Latest GOES-West infrared and
water vapor imagery show a fairly narrow cold frontal band moving
onshore along the Washington coast, about to reach Astoria shortly
after 3 AM. While the cold front is looking impressive in its thermal
structure, moisture is lacking. Latest convection- allowing models
(CAMs) show the band of precipitation moving quickly southeast with
the front, moving through Eugene by 9 AM and across the Lane County
Cascades by noon. Based on the looks of the frontal band on this
morning's CAMs, the precip associated with the front itself will
probably last less than an hour for most of the forecast area.

The drop in temperature and snow level behind the front will be
noticeable, with snow levels lowering into the foothill elevations as
850 mb temps fall to around -5 deg C this afternoon. A reinforcing
shot of cold air this evening will push 850 mb temps and snow levels
down further tonight; the 06z NAM and GFS deterministic runs both
show 850 mb temps down to around -8 deg C by midnight tonight, which
is borderline cold enough for snow in onshore flow. This is when
models have been suggesting the next shortwave and weak surface low
move onshore, raising the potential for lowland snow late tonight
into Tuesday morning. Much will depend on the track of this weak low,
and models appear to be northward with its track. For example, the
12/00z GFS run was bringing the low onshore near Tillamook, tracking
it across Eugene before moving into the Cascades. The 13/06z GFS
appears to bring this low onshore near Hoquiam late tonight/early Tue
morning, tracking it across the Portland metro area. The 13/00z ECMWF
deterministic is even further north with the low than the GFS. This
would appear to open much of the forecast area up to southerly flow
ahead of this weak low, which could lead to a rain/snow mix or even
just rain for elevations below 500 to 1000 feet. Based on this, our
forecast is generally on the lower end of guidance for lowland snow
accumulations, with most areas below 500 ft receiving less than 1
inch. Latest NBM probabilistic guidance supports this, with most
areas below 500 feet showing a 30-40% chance of receiving 1 inch or
more of snow by midday Tuesday.

The existing suite of winter watches, warnings, and advisories look
to be in very good shape. Only very minor changes were made, mainly
to start the warnings/advisories a few hours earlier in the high
terrain of SW Washington and the north Oregon Coast Range. Although
the cold front itself will probably only bring an inch or two of snow
this morning, increasing upslope westerly flow and marginal
instability will cause post-frontal showers to quickly fill back by
this afternoon. These showers will be capable of producing small hail
in the lowlands and very heavy snowfall rates of up to 2 in/hr in the
Cascades. Combined with gusty winds, visibility will likely be poor
and travel likely very difficult at times in the Cascades through
Tuesday morning. The Oregon Cascades stand to see 12 to 20 inches of
snow by the time snow showers diminish Tuesday, with 6 to 14 inches
likely for the Coast Range and Cascade foothills above 1500 feet, and
accumulations trending down to 1 or 2 inches at 500 feet. As
mentioned above, most of the lowlands should see less than 1 inch,
but heavier showers could lead to localized snow accums of more than
1 inch.

One more aspect of this system is the instability due to impressively
cold temps aloft. The 06z GFS shows a pocket of -35 deg C temps at
500 mb moving in around midday today. Provided sufficient sunbreaks,
this could lead to deeper convection this afternoon. Heavier showers
will be capable of producing gusty winds, lightning, small hail or
graupel, and could temporarily lower the snow level. Inland areas
should stabilize a bit after sunset with the loss of solar heating,
but relatively warm sea surface temps will maintain the potential for
heavier showers and thunderstorms along the coast through Tuesday
morning. Given the cold air mass, heavier showers could lead to
spotty snow accumulations of more than 1 inch even down to sea level,
but this appears to be the exception rather than the rule...
especially considering how tonight's weak low has been trending
farther north (and thus slightly milder) in recent guidance.

Any lowland areas that accumulate snow Tuesday morning should see it
melt rapidly Tuesday as clouds decrease and temperatures quickly warm
well above freezing. Shaded areas could hold on to snow or ice a
little longer. Refreezing of roads may be an issue Tuesday night if
roads fail to dry out during the day Tuesday, as most areas are
expected to fall well into the 20s. High pressure will keep it dry
Wednesday, though light winds and strong inversions will likely keep
temps below normal for highs in the valleys while higher elevations
moderate.  Weagle

.LONG TERM...WEDNESDAY NIGHT THROUGH SATURDAY...
Upper-level ridging will bring dry weather across the region
Wednesday night into early Thursday morning. Deterministic and
ensemble guidance shows the ridge exiting by Thursday afternoon,
making way for the next system that will be coming towards the West
Coast. Currently, the majority of ensemble members are showing a
deep trough developing off- shore over the northeast Pacific
Thursday. Model guidance suggests that this trough will elongate far
enough south that it will split into a cut-off low off the
California coast along with a shortwave trough northeast of our
region. Essentially, we would be in between these two features and
trend towards less active weather. Moisture associated with the cut-
off low will be too far south to warrant any concerns for
significant precipitation, but am still waiting on the blended
guidance remove all QPF before pulling most PoPs from our forecast.
Similarly, ensembles are placing the shortwave trough axis over the
Canadian Rockies which would also keep precip away. Also, the GFS
ensemble has the shortwave trough axis extending the closest to us
compared to the Euro and Canadian ensembles, which would potentially
bring showers through Friday afternoon. Overall, chance for precip,
possibly with snow mixed in at times, remains 30% or lower for
Thursday and Friday.

As we head into next weekend, all clusters agree that we'll begin to
have northwesterly flow aloft and hint at potential ridging.
However, there is uncertainty with whether or not we will receive
precipitation. PoPs are currently 50/50 by Sunday afternoon, and
this could be due to the lingering influence of the exiting trough.
If any precip falls, expect it to be more stratiform than
convective. High temperatures are expected to be near seasonal
values, but lows will stay near or below freezing at least through
the early part of the long-term period. /Alviz

&&

.AVIATION...12Z TAFs: Initial warm front moved to the coast around
07Z at Astoria, and now appear to be moving into the north
Willamette Valley at 11Z. Will see breezy west to northwest winds
today as cool unstable air mass spreads across the area today. May
see some thunderstorms across the coastal areas later this
morning, then spreading inland for the afternoon and evening,
ending up back to coastal areas overnight. Coastal areas lifted to
a mix of VFR and MVFR. Appears the LIFR at KONP has ended as
northwest winds in more mixed air mass has reached there. Also
expect the areas of MVFR at inland valleys to improve to mostly
VFR 12-14Z Mon and last through about 06Z Tue with a return of
MVFR per HREF and other HiRes models. Snow chances increase after
06Z Tue becoming predominant by 09Z Tue so should see some
visibility reductions by that time. IFR conditions coming into
play after 09Z as well. Precipitation begins to taper off after
12Z Tue from north to south.

Breezy west winds expected along the coast as well as over the
higher terrain which will intensify through the day. Towards the
midday, expecting winds to become gusty along the coast. Strongest
wind gusts (up to 40 kt) expected later this afternoon and
evening.

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

KPDX AND APPROACHES...MVFR with cigs around 1500-3000 ft at 11Z
Mon in the area is expected to improve to VFR 12-14Z (with brief
MVFR possible through the day.) Showers continue today with a
slight chance for thunderstorms 18Z Mon to 03Z Tue. Cigs should
trend back toward MVFR after 06Z Tue, with snow levels lowering
to valley after 06-09Z Tue. Light snow accumulations expected
after 09Z along with possible IFR cigs/visibilities developing.
NBM 6-hr probabilities have about and 80% chance for at least a
tenth of an inch of snow between 06Z and 12Z Tue (the period of
highest chances for snowfall.)

&&

.MARINE...Another front will push across the region today bringing
solid west to northwesterly gales for later this morning through
early Tue morning. Gusts 35 to 40 kt expected and 45 kt is
possible. Strongest winds expected from this afternoon through
tonight. Also with the unstable air mass will feed the showers
and isolated thunderstorms, a locally brief stronger gust is not
out of the question.

With all the winds, seas will build as well, with rather choppy
chaotic seas expected on later into Tue. Seas will gradually
build, running 15 to 20 ft by midday Mon, and then 20 to 23 ft for
the afternoon into tonight. Seas will slowly subside Tue. Seas
will gradually decrease from north to south on Tue, but remaining
high enough, near 20 ft in the morning subsiding to 15 to 18 ft in
the afternoon. As such will hoist a Hazardous Seas Warning
following the Gales.

High pressure builds over the region from north to south later
Tue and Wed, with winds dropping back under 20 kt, and subsiding
seas, generally back to 10 to 13 ft for Wed.	/mh    /Rockey

&&

.BEACH HAZARDS...Increasing high energy westerly swells will
increase the chances for high surf conditions along the coast
this afternoon into Tuesday morning. A persistent westerly wind
will compound with the swell bringing waves to around 20 ft
later this afternoon. Seas appear to lower a bit sooner on Tues,
so have ended the High Surf Advisory early Tue afternoon.

A High Surf Advisory continues for the area beaches. These waves,
while they may look exciting, can be very dangerous. Destructive
waves may wash over beaches, jetties, and other structures
unexpectedly. Stay tuned to the forecast before heading onto area
beaches.
/mh -Muessle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...High Surf Advisory from noon today to noon PST Tuesday for North
     Oregon Coast.

     Winter Weather Advisory from 10 PM this evening to 10 AM PST
     Tuesday for Central Willamette Valley-Greater Portland
     Metro Area-Lower Columbia-North Oregon Coast-South
     Willamette Valley-Western Columbia River Gorge.

     High Surf Advisory from 4 PM this afternoon to noon PST Tuesday
     for Central Oregon Coast.

     Winter Storm Warning from 7 AM this morning to 1 PM PST Tuesday
     for Coast Range of Northwest Oregon.

     Winter Storm Warning from 10 AM this morning to 1 PM PST
     Tuesday for Cascade Foothills in Lane County-Central Coast
     Range of Western Oregon-Northern Oregon Cascade Foothills.

     Winter Storm Warning until 1 PM PST Tuesday for Cascades in
     Lane County-Northern Oregon Cascades.

WA...Winter Weather Advisory from 10 PM this evening to 10 AM PST
     Tuesday for Greater Vancouver Area-I-5 Corridor in Cowlitz
     County-South Washington Coast-Western Columbia River Gorge.

     Winter Weather Advisory from 7 AM this morning to 10 AM PST
     Tuesday for South Washington Cascades-Willapa Hills.

     High Surf Advisory from noon today to 4 AM PST Tuesday for
     South Washington Coast.

     Winter Weather Advisory from 10 AM this morning to 10 AM PST
     Tuesday for South Washington Cascade Foothills.

PZ...Small Craft Advisory until 1 PM PST this afternoon for Columbia
     River Bar.

     Gale Warning from 1 PM this afternoon to 10 PM PST this evening
     for Columbia River Bar.

     Hazardous Seas Warning from 10 PM this evening to 4 PM PST
     Tuesday for Columbia River Bar.

     Small Craft Advisory until noon PST today for Coastal waters
     from Cape Shoalwater WA to Florence OR out 10 NM.

     Gale Warning from noon today to 7 AM PST Tuesday for Coastal
     waters from Cape Shoalwater WA to Florence OR out 10 NM.

     Hazardous Seas Warning from 7 AM to 7 PM PST Tuesday for Coastal
     waters from Cape Shoalwater WA to Florence OR out 10 NM.

     Small Craft Advisory until 7 AM PST this morning for Coastal
     waters from Cape Shoalwater WA to Florence OR from 10 to 60
     NM.

     Gale Warning from 7 AM this morning to 10 AM PST Tuesday for
     Coastal waters from Cape Shoalwater WA to Florence OR from
     10 to 60 NM.

     Hazardous Seas Warning from 10 AM to 7 PM PST Tuesday for
     Coastal waters from Cape Shoalwater WA to Florence OR from
     10 to 60 NM.
&&

www.weather.gov/portland

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