NWS Pendleton, OR wrote its last area forecast discussion before the first event at Dec 27, 11:01 AM PST, 59 minutes earlier.
Area forecast discussion, Dec 27, 11:01 AM PST, 59 minutes before
Area Forecast Discussion
National Weather Service Pendleton OR
1101 AM PST Tue Dec 27 2022
.MORNING UPDATE...Updates to the forecast this morning revolved
around current warnings and advisories due to expected strong
winds this morning through this evening. No changes were made to
the Winter Weather Advisories.
Based on latest observations, raw 12Z HREF probabilities, and
calibrated NBM guidance, upgraded the foothills of the northern
Blue Mountains of Oregon and Washington to High Wind Warnings, and
added the Southern Blue Mountains of Oregon. HREF 850 mb ensemble
mean winds suggest the current 50-65 mph jet across central and
eastern Oregon and south-central Washington will increase this
afternoon to 60 to 70 mph with locally 80 mph. Forecast soundings
indicate a well-mixed boundary layer will develop this afternoon
up to around 810-770 mb for the Columbia Basin and foothills of
the Blue Mountains. This should allow strong winds from aloft to
mix down to the surface creating widespread windy conditions
through the evening. Plunkett/86
&&
.PREV DISCUSSION... /issued 949 AM PST Tue Dec 27 2022/
Updated for Aviation Discussion...
PREV DISCUSSION... /issued 401 AM PST Tue Dec 27 2022/
SHORT TERM...Today through Thursday...A convoluted forecast is in
store for the region over the next 24 hours thanks to a deep low
pressure system offshore WA this morning. Currently, the initial
bands of precipitation that saw light freezing rain transition
over to rain in some lower elevation areas is still draped across
far northeast OR and southeast WA, while light showery bands of
precipitation push across the Cascade east slopes in OR and WA.
Today, precipitation chances in the lower elevations will
decrease as a deep low continues to push into WA this morning. In
the mountains, rain showers will continue through this morning,
but as the low pressure system pushes inland, snow levels are
expected to drop to the surface, resulting in a transition over to
snow. Heaviest snow amounts are expected to occur with the more
showery activity this afternoon and evening in the WA and OR
Cascade east slopes, then turning over to a stratiform snow
tonight through Wednesday morning. In the WA Cascades and the
Northern Blues, the continued rain showers through this morning
will result in a rain-on-snow runoff that will likely continue
through this evening, even though precipitation changes over to
snow. The resultant run-off will see area rivers rise to at least
bankfull, but with snow levels dropping fairly quickly this
evening, there is low confidence in reaching minor flood stage.
Also, with moderate confidence that temperatures and snow levels
will lower again across the mountains, hydrological concerns are
expected to decrease as well. As for the change over to snow, snow
accumulations of 6-10 inches are expected in the east slopes of
the WA Cascades, while the OR Cascades may see snow accumulations
of 5-8 inches. These zones will also see gusty winds during this
time period, likely resulting in some areas of blowing and
drifting snow.
Otherwise, the strong surface low associated with the incoming
system is expected to produce breezy to strong winds across much
of the forecast area, which have already been noted this morning
from central OR to the Blue Mountain Foothills. Winds are only
expected to increase across the area today, with the strongest
winds expected along the Simcoe highlands, the foothills of the
Southern Blues, and along the exposed ridges in peaks. For the
aforementioned areas, wind advisories were upgraded to warnings as
short-term high-res guidance in conjunction with the HREF show
moderate to high confidence (60-80%) of strong gusts up to 65 mph,
especially this afternoon and evening. As for the rest of the
lower elevation zones, wind gusts up to 55 mph are expected,
though higher exposed ridges in the Basin to central OR as well as
isolated areas along the Blue Mountain foothills could reach 60
mph periodically this afternoon.
Tomorrow, the forecast area will see somewhat of a reprieve from
the impactful weather as the low moves off to our east and
northwest flow aloft briefly setups over the region. Light upslope
snow will still continue across the Cascades and the eastern
mountains through the day Wednesday, though any additional
accumulations will generally be less than an inch or two. As for
the lower elevations, winds are expected to weaken at least below
advisory criteria, though locally breezy to breezy winds will
continue through the morning and early afternoon.
Thursday, a weakening shortwave and surface frontal boundary will
bring renewed chances of mountain snow, with snow accumulations of
2 to 6 inches possible. As for the lower elevations, up to an inch
of snow may be possible in the morning as the system initially
pushes in to the region and snow levels are below or near the
surface. But through the late morning into the evening, snow
levels are expected to rise to 4.5kft to 5kft, resulting in a
rain/snow mix through the late morning then light rain in the
afternoon and evening.
LONG TERM...Friday through Monday...On Friday, a weak system
will move across the Pacific Northwest and bring precipitation,
mainly rain in the lower elevations, and snow in the mountains.
This system should move out, later Friday before another trough
moves approaches the coast of southern Oregon and northern
California early in the day Saturday and then moves onshore. This
trough is further south then most have been recently. While there
is still some uncertainty as to how far north the northern extent
of any precipitation will be, most of the precipitation will be in
California and southern Oregon, with low chance precipitation
further north, and limited pops in the Basin. It is possible
chance pops could be too high in some of these areas as well. By
Saturday evening, the trough moves to the east and a ridge builds
in and precipitation will come to an end and Sunday should be dry.
Another weak trough and quick shout of precipitation will move in
on Monday.
Ensemble clusters show good agreement on Friday and Saturday. By
Sunday, more variability is introduced, mainly with respect to the
trough in the southwest. which has little impact on our sensible
weather. The GFS has a stronger trough and the ensemble clusters
are about 50/50 on the stronger trough. The EOF1 500 mb pattern
shows considerable variability as well with 37% variability. By
Monday, both the ECMWF and GFS have a fairly strong trough near the
central plains, though the position is a bit different and the ECMWF
is closed off and the GFS is not but neither has much of an effect
on the weather of the Pacific Northwest, as the trough moving
onshore is fairly well handled, though some more variability occurs
there by Tuesday.
High temperatures will mainly be in the 40s on Friday, the upper 30s
to lower 40s on Saturday, a couple degrees cooler on Sunday and
mainly in the 30s on Monday. Overnight lows will generally be in
the 20s and 30s Friday and in the 20s the rest of the period.
AVIATION...18Z TAFS... Expect generally VFR conditions through
the forecast period with IFR/MVFR conditions currently at YKM and
DLS improving by 19-21z. Temperatures continue to sit above
freezing at all sites so any sporadic precipitation that falls
through the day will be in the form of rain. Can’t rule out a
brief dip to MVFR for any rain shower activity this afternoon and
evening. The primary concern will be developing strong SW winds
across the region this afternoon. High confidence for gusts in the
35-45knot range at all sites with PDT and ALW potentially seeing
max gusts closer to 55-60 knots. Wind should decrease overnight
into Wednesday morning but remain breezy. 99
&&
.PRELIMINARY POINT TEMPS/POPS...
PDT 57 37 44 30 / 30 30 10 10
ALW 60 38 45 32 / 50 60 20 10
PSC 56 37 43 26 / 20 20 0 0
YKM 48 31 42 23 / 60 30 0 10
HRI 59 38 46 29 / 20 20 10 10
ELN 44 30 35 22 / 60 60 10 10
RDM 53 32 43 29 / 60 60 20 30
LGD 50 33 38 26 / 50 60 30 10
GCD 49 30 37 25 / 50 30 20 10
DLS 53 38 46 33 / 90 80 30 30
&&
.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory until 1 AM PST Wednesday for ORZ041-044-050-505-
511.
High Wind Warning until 1 AM PST Wednesday for ORZ503-507-508-
510.
Winter Weather Advisory until 1 PM PST Wednesday for ORZ509.
WA...Wind Advisory until 1 AM PST Wednesday for WAZ024-027-028.
High Wind Warning until 1 AM PST Wednesday for WAZ029-521.
Winter Weather Advisory until 10 AM PST Wednesday for WAZ520.
&&
MORNING UPDATE...86
SHORT TERM...82
LONG TERM....77
AVIATION...99