Heavy Snow in Oregon, March 27 to 28, 2023

Area Forecast Discussion
National Weather Service Pendleton OR
426 PM PDT Mon Mar 27 2023

Updated aviation discussion

.SHORT TERM...Tonight through Wednesday night...No changes planned
for the current winter storm warning and advisories as well as
the wind advisories. This forecast has been a bit of a challenge
in terms of snow accumulations and whether advisories should be
upgraded to warnings and whether additional zones need winter
highlights. When looking at the snow forecast graphics, it is
clear that many of the higher ridges along the Cascades and the
eastern mountains will see a significant amount of snow--10-15
inches and even higher--tonight through early Wednesday morning.
However, these amounts are above the passes and above the
populated areas and do no warrant warnings. QPF and snow amounts
along the upper slopes of the eastern Washington Cascades (zone
522) were increased for Tuesday night based on a band of moisture
rotating around a closed low enhanced by east-northeast winds
from the surface to 850mb. Again, the heaviest accumulations will
be above 5000 feet and south of I-90. Future shifts may decide to
issue a winter highlight if moderate to heavy snow is forecast
below 5000 feet and more of the forecast zone. The NBM
probabilities back up the forecast by showing 40% > 10" and 30% >
12" centered over the higher ridges south of Rimrock Lake but
lesser amounts elsewhere.

The upper low is tightening its circulation and is currently about
650 miles west of Astoria. KMAX (Medford) radar is filling with
returns as a warm front ahead of the low pushes into southwest
Oregon. By sunset this evening, precipitation will spread into the
southern Oregon Cascades. By midnight, precipitation will develop
over the remainder of the Oregon Cascades and as far east as
Seneca and John Day. By sunrise on Tuesday, most of eastern Oregon
and south central Washington will have a chance of rain and
mountain snow. Areas north of Prosser and Walla Walla will only
have a 10-20% chance of precipitation. Models are trending a
little higher on snow levels as well as QPF, and snow levels are
now expected to rise to around 3500-4000 feet. The higher QPF, of
course, has led to higher snow amounts. Once again, it's the
higher ridges that will get the significant snow. Tuesday will be
wet and breezy/windy for most of the forecast area with the low
west of Brookings. On Tuesday night and Wednesday, the low will
fill into an open trough as it tracks across the Great Basin. The
deterministic models along with the HREF are showing the band of
precipitation over south central WA that resulted in increasing
PoPs, QPF and snow amounts for Tuesday night. On Wednesday, there
will be snow showers over Wallowa County due to wrap around moisture
that will need to be monitored, but the heaviest accumulations
will be along the higher elevations. Most of the snow on Wednesday will
be scattered across the eastern mountains and valleys.

Surface gradients will set up for very windy conditions in the
Grande Ronde Valley. As a lee side surface low develops at the
base of the Oregon Blue Mountains, winds will push through the
I-84 corridor and through gaps of the Blue Mountains for very
windy conditions there as well. This will begin tonight and
continue through Tuesday afternoon before gradually decreasing.
Wister/85

.LONG TERM...Thursday through Monday...The PacNW will continue to
experience a prolonged period of low pressure, as models remain in
good agreement on broad troughing encompassing the region this
weekend through at least early next week. The resulting weather will
be more persistent chances for mountain snow, elevated winds, and
more unseasonably cold conditions for the lower elevations,
especially overnight.

Thursday will be marked by a departing low pressure system
responsible for the precipitation the forecast area is expected to
experience tonight into Tuesday. Models hint at a low chance (30%)
for high mountain precip on Thursday due to upslope flow parallel to
the mountain ranges in our area, but this synoptic setup, combined
with a quick transitory ridge moving through on Friday, will make
the Thursday-Friday period the driest in the long term. This will
change heading into the weekend as models bring in a potent, broad
low from the Gulf of Alaska through the PacNW, resulting in elevated
chances (40-60%) for lower-elevation precip, and high chances (>75%)
for mountain snow, especially as snow levels look to be in the 2500-
3500 ft range. QPF amounts are always subject to change this far
out, but ensemble guidance seems to already be suggesting headline-
worthy snow across our mountain zones. Precip is a bit more limited
across the Basin and other lower elevation zones, but as of now the
synoptic pattern is at least favorable for some type of precip to
occur this weekend across most of our population centers, even if it
is in the form of light, cold rain.

Winds will become a concern as well, starting on the Thursday-Friday
period due to the transitory nature of the synoptic pattern, before
a larger wind threat arrives with the onset of Saturday's system.
Already seeing a signal for gusts in the 25-35 mph range on
Saturday, which is concerning given long-range models generally
underestimate winds this far out. The NBM is even more aggressive,
suggesting winds approaching 40 mph within the Basin. So even while
the precip threat may be limited, winds certainly look problematic.
With the way the majority of ensemble members are pointing right
now, will gauge confidence at around 40% for the potential of
advisory-level winds during the day Saturday.

Consensus across ensemble members seems to be that this broad trough
will envelop the region with its cold sector through the end of the
period. Members differ on timing the overall progression of this
system, but most members depict the trough axis overhead even by
next Monday. This pattern favors continued precip chances for the
mountains, while lower-elevation precip is a bit more uncertain
(confidence less than 30%), due to the lack of consensus behind
where the trough axis ultimately settles. Either way, there is high
confidence (>70%) that we will continue to see unseasonably cool
conditions, with highs in the 40s and 50s over the weekend, and lows
falling into the 20s and 30s, with some possibly colder lows
whenever the trough axis lies overhead. Conditions like these
certainly make spring feel that much farther away. Evans/74

&&

.AVIATION...00z TAFs...Expecting mostly VFR conditions through
the period, with the only exception being BDN/RDM which may see
light snow beginning this evening and continuing into Tuesday. While
central Oregon snowfall is expected to be limited, gusty winds will
occur around the same time, leading to lowered vsbys. MVFR to IFR
conditions are expected, and lower conditions cannot be ruled out.
Elsewhere, clouds will increase and lower overnight along with some
light precipitation potential. Winds 5-15 kts across most sites
overnight.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  38  53  35  53 /  10  40  30  20
ALW  40  56  39  56 /  10  30  30  40
PSC  40  58  40  59 /  10  20  20  10
YKM  33  55  37  58 /  20  30  40  10
HRI  40  57  38  57 /  10  40  30  20
ELN  32  53  36  57 /  10  20  30  10
RDM  33  46  25  48 /  80  90  20  10
LGD  33  43  34  49 /  10  60  30  50
GCD  31  43  27  46 /  60 100  50  40
DLS  40  51  40  56 /  50  80  60  20

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory from 8 PM this evening to 3 PM PDT Tuesday for
     ORZ049-507.

     Winter Weather Advisory from 8 PM this evening to 11 PM PDT
     Tuesday for ORZ503-506.

     Winter Storm Warning from 5 PM this afternoon to 8 PM PDT
     Tuesday for ORZ509.

WA...None.
&&

SHORT TERM...85
LONG TERM....74
AVIATION...74