Pendleton, Oregon 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

552
FXUS66 KPDT 221134
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
345 AM PST Sun Dec 22 2024

Updated Aviation Discussion

.SHORT TERM...Today through Tuesday...

Key Messages:

1. Widespread rain and mountain snow this afternoon into Monday.

2. Breezy winds this evening, returning late Monday.

3. Widespread rain and mountain snow returns late Monday.

Current radar and infrared satellite imagery showing spotty, light
returns along the west slopes of the Cascades under partly to
mostly cloudy skies. Satellite imagery along with local webcams
are also indicating areas of freezing fog across the Yakima and
Kittitas Valleys, which has warranted the issuance of Freezing
Fog Advisories across both valleys until 10 AM Sunday as
visibilities of one-quarter mile or less in freezing temperatures
are occurring. Ground observations and webcams also show some
patchy fog extending into Prosser and along SR 240 north of
Richland, which has prompted a special weather statement
highlighting the potential of abrupt changes in visibility in
these areas.

The synoptic pattern is a rather active one through the beginning
part of the week as two back-to-back systems are poised to pass
through our area, one tonight into Monday morning and the second
Monday evening through Tuesday morning. Both systems are
relatively similar in the fact they both will be approaching the
area from the southwest, and will also be accompanied by a frontal
system consisting of a warm front approaching from the south and a
cold front that will pass west-to-east through the area shortly
after the warm front. This will lead to warm air advecting into
the area behind the warm front, increasing snow levels into the
5500-6500 foot range tonight into Monday morning and Monday night
into Tuesday morning. These elevated snow levels will keep snow
at higher elevations across the Cascade and Blue Mountains, with
our Cascade Passes expected to only pick up 1-2 inches with each
system. Confidence in these snow amounts is moderate to high
(60-80%) as the NBM suggests less than a 5% chance of 1 inch or
more over Snoqualmie and Santiam Passes tonight but an 80% chance
of 1 inch or more snow across Santiam Pass Monday night into
Tuesday. Rain chances (50-90%) will be increasing west-to-east
this afternoon and evening, peaking between 4 PM and 11 PM as the
warm front passes mid-afternoon and the cold front passes later
this evening. Rain accumulations are expected to be highest along
the east slopes of the Washington Cascades and the Simcoe
Highlands as 0.15-0.30" is likely. Lower rain amounts are expected
across the Basin and foothills as 0.05-0.25" of rain is
anticipated along the northern Blue Mountain foothills and
0.01-0.10" of rain is likely across Central Oregon and lower
elevations of the Columbia Basin. Confidence in these rain amounts
is high (80%) as the NBM suggests an 80-100% chance of 0.10" or
more rainfall along the east slopes of the Washington Cascades and
the Simcoe Highlands, and a 35-65% chance of 0.05" or more
rainfall across Central Oregon and lower elevations of the
Columbia Basin.

The passing frontal system will also attribute to breezy winds
across the Grande Ronde Valley and the Blue Mountains/foothills.
The incoming cold front will allow a pressure gradient to develop
along the Blue Mountains as the GFS, NAM, and SREF all advertise a
pressure gradient of 3.5-5.5 mb between Baker City (KBKE) and
Meacham (KMEH). These gradients to hover around the normal
advisory threshold of 5 mb, but the SREF and NAM only highlight
breaking above 5 mb for a short time between 6 PM and 8 PM
tonight. Thus, gusts of up to 25 mph are expected along the Blue
Mountain foothills as gusts of 25 to 35 mph are likely across the
Grande Ronde Valley. Confidence in these wind values is moderate
to high (70-80%) as the NBM suggests a 25-35% chance of advisory-
level wind gusts (45 mph or greater) over the Grande Ronde
Valley. A transient upper level ridge between systems, along with
a present jet, will help the second system develop an even
stronger pressure gradient along the Blue Mountains Monday
evening into Tuesday as the GFS, NAM, and SREF all advertise a
4.5-6 mb gradient between Baker City (KBKE) and Meacham (KMEH).
Sustained south winds of 20-30 mph and gusts up to 50 mph will be
possible over the Grande Ronde Valley as the NBM suggests a
45-55% chance of advisory-level wind gusts and a 65-75% chance of
sustained winds of 30 mph or greater (advisory-level sustained
winds). Thus, it is likely (60-70%) that a Wind Advisory will be
necessary from 7 PM Monday through 10 AM Tuesday - but will hold
off on issuing the product until confidence increase. Gusts of
35-40 mph will also be possible along the Blue Mountain foothills
as downsloping winds are expected to occur associated with the
developing pressure gradient. Advisory-level winds look less
likely to occur across the foothills, as the NBM suggests a 40-50%
chance of gusts reaching 45 mph or greater.

As suggested above, the second system looks to be slightly
stronger than the first, leading to higher expected rain amounts
across the region. The current forecast calls for 0.35-0.45" along
the east slopes of the Cascades, John Day-Ochoco Basin, and the
Blue Mountain foothills and 0.15-0.35" over Central Oregon and
lower elevations of the Columbia Basin. Confidence in these rain
amounts is moderate (50-70%) as the NBM suggests a 50-75% chance
of 0.35" or more rainfall across the east slopes of the Cascades,
John Day-Ochoco Basin, and the Blue Mountain foothills, and a
10-30% chance over Central Oregon and lower elevations of the
Columbia Basin. As snow levels are again elevated with this
second system, the majority of snow accumulations will be at
elevations of 4500 feet or greater as 2-5 inches of snowfall is
expected. Confidence in these snow amounts is moderate (60%) as
the NBM hints at a 70-80% chance of 2 inches of snow or more over
elevations of 4500 feet across the Cascade and Blue Mountains. 75


.LONG TERM...Christmas Day to Sunday...There are several forecast
challenges in store for the long term period, affecting holiday
travel. The challenges are primarily a prolonged period of moderate
to heavy snow over the mountains as well as the locally windy
conditions. The good news is that snow levels will generally range
from 3000-4000 feet over south central WA and 4500-5500 feet
elsewhere, therefore I-90 east of Snoqualmie Summit as well as I-84
over Meacham and the heavily traveled HWY97 will have low to very
low probabilities of significant snow accumulations. However, state
routes such as HWY 12, 20, 26 and 395 south of Pilot Rock will need
to be closely monitored for periods of moderate to heavy snow. Be on
the lookout for winter highlights this coming week.

Christmas morning is looking dry across the board with the exception
of the Cascade crest. Precipitation ahead of a Pacific trough will
spread east of the Cascades during the day and will be widespread
Christmas night. It will also be breezy to windy for many areas and
very windy for the wind prone areas of the Grande Ronde Valley and
Blue Mtn Foothills. There is a strong AR associated with this
system, but mainly over western WA/OR. The ECMWF EFI has areas of
high QPF and snow along the Cascades, the Blues and Wallowas. The
upper level trough will travel across WA/OR Thursday and will exit
the region Thursday night. Reviewing the probabilities of snow
accumulations, it is highly likely (60-100% chance) that the east
slopes of the WA Cascades above 3K feet, the northern Blues above
4.5k feet, and the high elevations of the Wallowas and John Day-
Ochoco Highlands will observe 3-6" of snow and 40-70% chance of 6-
10" with this event by Thursday evening.

Models are in agreement of another trough right at the heels with
little break in precipitation.  By mid-morning Friday, areawide
precipitation is forecast.  Snow levels will be a little higher and
generally around 3.5k feet along the east slopes of the WA Cascades
and 4.5-5.5k feet elsewhere.  Once again, there is a good
probability of moderate to heavy snow.  The GFS and ECMWF AR progs
advertise a 400 kg/m/s IVT over the OR Cascades. This is also an
area with strong upper level support of the polar jet. It will also
be breezy to windy on Friday.

Models have been consistent with a strong and moist westerly flow on
Saturday with continued rain and snow along the Cascades and the
eastern mountains but less precipitation along the rain-shadowed
areas of the Columbia Basin down to Central Oregon.

Sunday (Dec 29) appears to be another wet day but one met with a
strong warm front.  That is, according to the deterministic runs of
the GFS, ECMWF and Canadian.  The AR progs do not go that far out
yet, but there are hints of a strong AR approaching the coast by
early morning. Cluster QPF analysis also show differences on the
intensity of precipitation, but there is a general agreement with
the H5 synoptic pattern. If the deterministic runs verify, then snow
levels will generally range from 5-6k feet.

Also, it should also be mentioned that hydrographs are forecasting
rises along area rivers during the week.  Nothing reaching flood
stage, but some are pushing bankfull by the end of the week.

It will be a busy holiday week, for sure. Wister/85


&&

.AVIATION...12Z TAFs...Low stratus and fog with LIFR conditions have
been observed this morning at YKM and DLS. As upper level winds
increase with more mixing over the next few hours, conditions will
likely improve to MVFR or VFR. However, rain late this afternoon and
evening will create opportunities for fog and stratus to redevelop
and back to IFR/LIFR.  All other sites are currently VFR or right at
5SM BR with variable CIGS 5-15 kft. Showers this afternoon and
evening will also result in areas of MVFR or less. Winds will be
light for most terminal airports with the exception of RDM and BDN,
as winds shift to the SW overnight with gusts 20-25 kts.  Wister/85


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  49  38  51  39 /  30  70  10  70
ALW  48  41  52  41 /  20  80  20  70
PSC  44  38  49  41 /  30  70   0  80
YKM  40  32  42  33 /  60  70   0  80
HRI  46  38  50  41 /  30  70   0  80
ELN  37  31  40  34 /  70  80  10  80
RDM  48  31  46  38 /  40  50   0  70
LGD  44  37  46  37 /  30  80  20  60
GCD  47  34  46  37 /  30  80  10  70
DLS  46  39  49  42 /  70  80  10  80

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...Freezing Fog Advisory until 10 AM PST this morning for WAZ026-
     027.

&&

$$

SHORT TERM...75
LONG TERM....85
AVIATION...85