Winter Storm in Oregon, April 12 to 13, 2022

Area Forecast Discussion...Correction
National Weather Service Portland OR
444 AM PDT Tue Apr 12 2022

Corrected WWA section

.SYNOPSIS...An unseasonably cool weather pattern will continue
through the end of the week as an upper low remains nearly stationary
near Vancouver island. This will bring rounds of showers into the
area, with low snow levels bringing the threat of lowland snow during
the nights and early mornings, as well as the threat of afternoon
thunderstorms. Temperatures will be slowly moderating through the end
of the week, possibly reaching climatologically normal values by
Sunday.

&&

.SHORT TERM...Today through Wednesday night...An upper low locked
nearly stationary near Vancouver Island will continue allowing the
cool moist airmass to slide down from the Gulf of Alaska through the
short term. The cold air aloft along with forcing from a
strengthening jet on the backside of the trough axis and ribbons of
PVA moving into the area has allowed for the growth of some of the
showers moving into the Pac NW to become thunderstorms early this
morning. Current snow levels are around 1200 feet and expected to
continue to fall this morning to around 500-900 ft north of Salem,
and remain around 1200 ft south. However, showers will allow for
locally lower snow levels, and with current wet bulb values in the
mid 30s, it wouldn't take too much to see a few flakes lower than
the snow levels. As temperatures warm later this morning, snow
levels will rise allowing for less of a threat of impactful snow
over the Coast Range, so will keep the expiration time of 11am of
the Winter Weather Advisory. The colder temperatures aloft will keep
the threat of isolated thunderstorms across most of the area today.
The Advisory looks to remain warranted for most of the Cascade
mountains and foothills through today. The one concern is tonight
when according to the GFS, the upper low swings a shortwave down,
increasing QG lift over the area and increasing frontogenesis
especially over the Lane County Cascades. Some possible CSI is also
being advertised, so some banding of snow may push them into higher
snowfall rates. Will upgrade to a Winter Storm Warning over the
aforementioned area, and leave the rest of the Advisories as is.

With the low sinking slightly further south, colder air will allow
for lower snow levels tonight, around 300 feet and into Wednesday
morning across the rest of the area. This will bring a better chance
for lowland snow. Won't be expecting the snowfall amounts like
yesterday morning, when more moisture and dynamics where available.
However, still light amounts at valley floor, likely a trace.

The near vertically stacked low will remain off the OR/WA coast for
Wednesday to continue the cool and showery pattern. Flow aloft
slowly starts to turn slightly more westerly rather than
northwesterly, which will help moderate temperatures a bit. But with
the upper low so close, will continue the threat of thunderstorms
across most of the area. Snow levels will be rising ever so
slightly, so still expecting snow showers for the higher terrain,
enough for the advisory to keep going for the OR Cascades. Have
trimmed back the ending time of the WA Cascades though to end early
tomorrow morning. However, the better moisture will be found further
south, so the snowfall amounts will be lessening compared to
Tuesday.  /Kriederman

.LONG TERM...Thursday through Sunday...Models and their
ensembles are in good agreement the cool upper level trough over the
region to begin the week will more or less persist across the
northeast Pacific and Pacific Northwest through the end of the week.
Most scenarios do suggest temperatures will moderate a bit towards
the end of the week so instead of high temperatures ending up solidly
in the 40s like they are early this week, we should begin to see
valley high temperatures climb up into the low 50s. With 500mb
temperatures remaining -25 to -30C or colder, we should continue to
see scattered showers across the area, particularly during the
afternoon and early evening hours through the end of the week. Snow
levels will generally hover above the valley floor, but low enough
that a few of the higher hills and certainly the mountains could see
a quick dusting of snow under any heavier shower Thursday and Friday.

While operational models suggest the upper level trough will
consolidate and swing across the Pacific Northwest Saturday, ensemble
guidance suggests this is far from a given so NBM PoPs are
generally only in the 40-60% range for Saturday. There is some
suggestion that shortwave ridging will then begin to build into the
region Sunday so NBM PoPs decreasing into the 15-30% chance range
seem reasonable with temperatures moderating back closer to average.

&&

.AVIATION...12z TAFs: The area is under relatively weak westerly
flow as Monday's weather-making system continues to pull away to
the east and a weak surface low remains to the northwest. VFR
conditions are predominant as of 330 AM with ceilings
occasionally dropping in showers.

The aforementioned low pressure system will move southward
Wednesday morning and then eastward Wednesday afternoon, resulting
in backing of the low level winds through the morning Wednesday.
Under this southerly low level flow regime with abundant clouds,
temperatures outside of showers should generally remain above
freezing. However, with some of these heavier showers, deformation
of the snow level to sea level by sublimational cooling is a
distinct possibility. It's difficult to quantify the odds of this
occurring - especially given uncertainty in the presence and
persistence of heavy convective showers in any particular
location.

HREF model guidance suggests around a 40-50% chance for MVFR
ceilings in the lowlands and generally a greater than 70% chance
in the Cascades, Cascade Foothills, and Coast Range through
around 18z Monday; beyond 18z, it suggests those chances lower by
~20% across the board with sunshine "burning off" some of the
lower clouds between showers. However, other model guidance (GFS
Lamp, for example) suggests a strong likelihood that ceilings will
remain VFR through the period. Thus, uncertainty is high in
ceilings, and mention of high MVFR ceilings in the TAFs has been
confined to the period prior to 18z...until a lowering trend in
ceilings begins from south to north as the low draws nearer to the
area Tuesday evening. By the end of the TAF period at 12z
Wednesday, there is around a 60-80% chance for IFR ceilings at all
inland TAF sites, and similar probabilities for MVFR ceilings
along the coast.

The one caveat to the ceiling and visibility forecasts is the
showers, which will likely lead to oscillations in flight
categories through the day today. While low VFR/high MVFR ceilings
and VFR visibilities will be predominant, low MVFR and IFR
conditions will be possible in showers - with worse conditions
favored where rain changes to snow early Tuesday morning until
around 17z.

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

KPDX AND APPROACHES...VFR conditions prevail at PDX as of 330 AM,
but with showers both visibility and especially ceiling drops
will be possible through the day Tuesday. Uncertainty remains high
in the prospect of precipitation-type changeover this morning,
and and so emphasis remains on the possibility of snow mixing in
with rain at the terminal if a heavy enough shower can persist.
Chances for this diminish quickly after around 17z, and after that
conditions are likely to be predominantly VFR between showers.
After around 00z Wednesday, ceilings will gradually lower with IFR
ceilings likely becoming predominant by around 09z. -Bumgardner

&&

.MARINE...A weak low pressure system moves into the waters from
the north Tuesday morning, and then becomes somewhat stationary
through the evening. Under this low, numerous showers and
scattered thunderstorms will continue. Outside of showers, Small
Craft Force winds will be predominant with frequent gusts around
25 kts; within and in the immediate vicinity of showers, gusts are
likely to surpass 30 kts and could locally exceed 35. The
westerly swell of around 9 to 10 ft will continue to support Small
Craft significant wave heights, with combined seas still progged
to peak in the 13 to 15 foot range at 10 to 12 seconds Tuesday
morning through the mid afternoon.

The westerly swell should lose a bit of its luster Tuesday
evening, and with winds gradually easing combined seas will lower
as well. It will be a gradual decline in wave heights, but by late
Wednesday morning, if not sooner, seas should at last fall short
of the Small Craft category.

With another weak surface low pressure system meandering in from
the north Tuesday night and then becoming stationary over the
waters Wednesday into Thursday, scattered showers and a slight
chance of thunderstorms will continue. Between showers and
thunderstorms, winds will be light and seas around 4 to 6 ft.
Within shower activity, some gusty winds and choppier seas will
be possible. -Bumgardner

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 11 AM PDT this morning for Central
     Coast Range of Western Oregon-Coast Range of Northwest
     Oregon.

     Winter Weather Advisory until 5 AM PDT Wednesday for Cascade
     Foothills in Lane County-Northern Oregon Cascade Foothills.

     Winter Weather Advisory until 11 PM PDT Wednesday for Northern
     Oregon Cascades.

     Winter Storm Warning until 6 AM PDT Wednesday for Cascades in
     Lane County.

WA...Winter Weather Advisory until 11 AM PDT this morning for Willapa
     Hills.

     Winter Weather Advisory until midnight PDT tonight for South
     Washington Cascades.

     Winter Weather Advisory until 5 PM PDT this afternoon for South
     Washington Cascade Foothills.

PZ...Small Craft Advisory until 9 PM PDT this evening for Columbia
     River Bar.

     Small Craft Advisory until 3 AM PDT Wednesday for coastal waters
     from Cape Shoalwater WA to Florence OR out 60 nm.

&&

www.weather.gov/portland

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