Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 070549

Area Forecast Discussion
National Weather Service Pendleton OR
949 PM PST Mon Feb 6 2023

Updated Aviation Discussion

.EVENING UPDATE...Satellite imagery this evening showing
increasing upper level cloud cover over eastern WA and northeast
OR while conditions at the surface are mostly calm and mild. This
is due to a flattening upper ridge allowing a more maritime
airmass to advect into the region under westerly flow aloft.
Through tonight, the upper ridge will continue to push east in
response to an approaching upper trough in the northeastern
Pacific. As the trough approaches, westerly flow will gradually
turn to the southwest and increase the influence of the maritime
airmass across the forecast area. The increasing cloud layer will
help to limit any radiative cooling processes tonight, with lows
mainly in the mid to upper 30s in the lower elevations. Light snow
showers will also begin tonight across the WA Cascades as the
moisture in westerly flow interacts with this region. Snow levels
will generally start off around 3.5kft tonight, but as the
region comes under southwesterly flow aloft, snow levels will
briefly rise to 4kft to 5kft by tomorrow afternoon, then dropping
quickly to around 1kft to 1.5kft in the evening and overnight
tomorrow as the upper trough moves over the area. Snow showers
will mainly be confined to the WA Cascades tonight, but will
spread south and east to the other mountain zones tomorrow. Snow
accumulations of 8 to 12 inches in the WA Cascades will be
possible, with locally higher amounts near the crest. The interior
northern Blues and the higher peaks of the OR Cascade east slopes
can expect 4 to 7 inches of snow accumulations. Light rain will
also be possible in the lower elevations through early tomorrow
evening, but rain amounts will mainly be less than a tenth of an
inch. The trough passage, a low level jet max, and a frontal
passage at the surface will also result in tightening pressure
gradients with breezy westerly winds anticipated in the lower
elevations. Strongest winds are expected along the Blue Mountain
Foothills, Simcoe Highlands, and the northwest Blues in WA.

.AVIATION...06Z TAFs...VFR conditions to prevail through the
period. Weather system moving through the area will produce rain
impacts at sites DLS/PDT/RDM/BDN/ALW after 18Z, starting at site
DLS and extending to the southern and eastern sites between
23Z-01Z. Chance of rain lower at sites YKM/PSC, so have kept
PROB30 for these sites after 20Z. Periodic reductions in CIGS and
vsby to MVFR may be possible with rain impacts, but not expected
to be prevailing. Winds will be light, less than 12kts, tonight at
all sites except PSC where winds will be 12-17kts with gusts up
to 25kts possible. Tomorrow, westerly winds will increase to
12-20kts with gusts up to 30kts possible at all sites in the
afternoon and evening tomorrow. Lawhorn/82

PREV DISCUSSION... /issued 346 PM PST Mon Feb 6 2023/

.SHORT TERM...TODAY THROUGH WEDNESDAY...Current radar and visible
satellite imagery showing dry conditions under partly to mostly
cloudy skies as the upper level ridge that encroached into the
Pacific Northwest this morning, continues to shift east and
flatten into this evening. This is in response to an upper level
trough that will approach the Washington/British Columbian coasts
on Tuesday to provide mountain snowfall through the Cascades and
breezy winds through the Simcoe Highlands and the Northern Blue
Mountain Foothills of Oregon/Washington. Thus, a Winter Weather
Advisory has been issued for the east slopes of the Washington
Cascades early Tuesday morning through Wednesday morning as
amounts of 8 to 12 inches will be possible above 3500 feet. Winds
of 30 to 40 mph with gusts of up to 55 mph will be possible along
the Simcoe Highlands and the Northern Blue Mountain Foothills,
with the former beginning early Tuesday morning and the latter
beginning late Tuesday afternoon as both extend into early
Wednesday morning.

The upper level trough that brought us a dry and calm day today
will be flattening as the axis slides to the east through the
remainder of the day today and into Tuesday morning. This will
lead to flow aloft shifting from the north to the west into
Tuesday. Cloud cover will be increasing tonight as a more maritime
airmass pushes into the region, allowing for overnight lows to
only drop into the mid to upper 30s for lower elevations of the
Basin. This additional moisture, coupled with westerly flow, will
attribute to snow showers initiating along the Washington Cascades
tonight. Snow levels will be around 3500 feet early Tuesday
morning before rising to 4500 feet through the afternoon, and
then plummeting to the 500 to 1500 foot range into Wednesday
morning. The upper level flow will be rather zonal Tuesday as an
upper level trough approaches the Washington/British Columbian
coasts by the afternoon. The flow will incur a more southerly
component through the early half of Tuesday to allow temperatures
to peak into the low to mid 50s for lower elevations of the Basin.
Showers will begin to overspread into the Basin Tuesday afternoon
before continuing across the eastern mountains through the
evening and overnight period into Wednesday morning. Rain amounts
of 0.05 to 0.15 inch will be possible through the Lower Columbia
Basin, Central Oregon, and through the foothills of the Blue
Mountains, as 8 to 12 inches of snow is likely above 3500 feet
along the Washington Cascades and 4 to 8 inches of snow is
expected over the Northern Blues Tuesday into Wednesday morning.
Moderate to high confidence (60-80%) resides in these snow amounts
as the NBMv4.1 highlights a 70-90% chance of 8 inches of snow
over the east slopes of the Washington Cascades and a 30 to 40%
chance of 6 inches of snow over the Northern Blue Mountains. Thus,
there is higher confidence (80%) in snow accumulations over the
Washington Cascades than over the Northern Blue Mountains (60%).
Rain chances focus at elevation of the Cascades and Blues Tuesday
night into Wednesday morning, with chances over the mountain zones
dropping off through the early afternoon. This is in response to
an upper level ridge following closely behind the departing trough
on Wednesday to provide drier and cooler conditions as the flow
aloft incurs a more northerly component, keeping highs in the
upper 40s to low 50s.

Pressure gradients will also be tightening as the upper level
trough pushes onshore and erodes the back side of the exiting
ridge to allow for breezy winds to materialize along the Simcoe
Highlands and the Northern Blue Mountains. A jet max will also
accompany the trough as it proceeds through the region Tuesday
into Wednesday morning, which will further heighten wind gusts and
lead to additional confidence regarding expected winds. Gusts will
begin to increase through the Simcoe Highlands tonight before
reaching advisory criteria (46 mph) early Tuesday morning around 4
AM. Winds will continue to increase and expand into Bickleton by
early afternoon before slowly subsiding into Wednesday morning.
Winds over the Northern Blue Mountain Foothills will be more
delayed until the late afternoon on Tuesday, extending into early
Wednesday morning. Wind gusts of up to 55 mph out of the southwest
will be possible through this timeframe. Confidence is high (80%)
regarding these wind gust values, as the NBMv4.1 shows a 90%
chance of 55 mph wind gusts over the Simcoe Highlands and a 70%
over the Northern Blue Mountain Foothills. 75

.LONG TERM...THURSDAY THROUGH MONDAY...Models have continued
their trend of the last couple of runs of having a ridge over the
Pacific Northwest giving way to a trough Thursday night through
Saturday. A flatter ridge rebuilds Saturday night and Sunday
though another trough will affect the area Monday.

Models start out in excellent agreement in having a ridge centered
over our area Thursday. By Friday, models agree in having the ridge
move east over the Rockies while a trough approaches the Pacific
northwest coast. Differences are creeping in as the GFS favors a
faster, shallower trough while the ECMWF have a deeper trough
further offshore. On Saturday, models all have a closed low forming
in the base of the trough as it moves overhead. The ECMWF and
Canadian have the low along the central California coast while the
GFS has a weaker low near Las Vegas.  Models do keep a generally
northwest flow over our area, so the differences are not major for
our area. Sunday, models have the ridge rebuilding over the area
though with differences in the strength of the ridge as the ECMWF
and Canadian low in southern California undercut the ridge more than
the GFS low in Arizona. Additionally, the ECMWF and Canadian are
stronger and a little faster with the next trough approaching out of
the Gulf of Alaska. Even with these differences, Sunday looks dry.
By Monday, models all show the trough somewhere near the coast with
a front pushing into the area Monday afternoon. The Canadian is the
strongest and fastest and the GFS weakest and slowest. The ECMWF is
similar in strength to the Canadian with timing closer to the GFS.
Cluster phase space analysis shows a fairly normal and increasing
amount of spread among model ensemble members over time.
Deterministic runs of the GFS and ECMWF are fairly close to their
ensemble mean values through Sunday before the ECMWF becomes an
outlier on Monday. The deterministic Canadian runs had much poorer
agreement with its ensemble means and it was discounted for the most
part. Overall, forecast confidence is good.

Ridging over the area will give us a dry day Thursday with highs in
the mid 40s to lower 50s in the lower elevations and in the mid 30s
to mid 40s in the mountains. Pressure gradients will be strong along
the Blue Mountains and currently have southerly winds around 20 mph
in the Grande Ronde Valley in the afternoon. It is too soon to have
much confidence, but wind highlights might be needed then. On
Friday, the trough will send a front to the Cascades in the morning
and across the area Friday afternoon and night. Precipitation
amounts will be light and mainly in the mountains which may receive
an inch or two of snow. the lower elevations will see little if any
rain. A northwest flow in the wake of the trough departure on
Saturday will keep a chance of very light snow along the Cascade
crest and in the higher peaks of the eastern mountains. The
northwest flow will continue Sunday though any snow will be confined
to the Cascade crest. Monday will see the next trough and front
arrive with a chance for a few inches of snow in the higher
mountains and a few hundredths of an inch of rain in the Columbia
Basin with central Oregon mainly dry. Temperatures Friday through
Monday will remain in the mid 40s to lower 50s and in the mid 30s to
mid 40s in the mountains. Perry/83


PDT  34  53  33  47 /   0  30  60   0
ALW  35  54  34  49 /   0  50  70  10
PSC  38  51  36  50 /   0  30  20   0
YKM  31  50  29  48 /  10  30  10   0
HRI  35  54  36  51 /   0  20  40   0
ELN  32  44  29  43 /  10  40  20   0
RDM  31  52  25  44 /   0  20  20   0
LGD  31  42  28  40 /   0  40  80  10
GCD  29  48  24  40 /   0  20  40   0
DLS  36  50  36  50 /  10  60  30   0


OR...Wind Advisory from 4 PM Tuesday to 7 AM PST Wednesday for ORZ507.

WA...Wind Advisory from 4 PM Tuesday to 7 AM PST Wednesday for WAZ029-

     Winter Weather Advisory from 4 AM Tuesday to 10 AM PST Wednesday
     for WAZ520.

     Wind Advisory from 4 AM Tuesday to 7 AM PST Wednesday for WAZ521.



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