854
FXUS66 KPDT 231753
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
953 AM PST Mon Dec 23 2024

Updated Aviation Discussion

.AVIATION...18Z TAFs...A mix of flight categories expected thanks
to incoming system that will bring another round of precipitation
including rain and some mist. Expect most sites to see a mix of
VFR to MVFR status, though lowering CIGs from BR may bring IFR or
lower skies to YKM and possibly DLS. On the note of DLS, expect
dense fog to last another 2-3 hours before burning off. Breezy
winds expected PDT/ALW/RDM/BDN with gusts 20-30 knots as the
system moves through later tonight. This may also bring some low
level wind shear to BDN/RDM between 00-06Z. PDT may also see some
brief wind shear, but expecting surface winds to catch up faster
here and for now have not included it into the TAFs. Goatley/87


&&

.PREV DISCUSSION... /issued 315 AM PST Mon Dec 23 2024/

SHORT TERM...Today through Christmas night...It`s a busy week
weatherwise with a series of back-to-back systems, and forecasters
will be on their toes with the winter weather and wind concerns.
Highlights in the short term include the issuance of a High Wind
Warning for the northern foothills of the OR Blue Mtns east-
southeast of Milton-Freewater and Pendleton for tonight through
Tuesday morning. Wind advisories will be issued for the Grande Ronde
Valley, the WA Blue Mtn Foothills from Walla Walla eastward and the
Southern Blue Mtn Foothills for the same time period. A lee-side
surface low setting up west of the Blue Mtns will cause downsloping
for gusts 45-55 mph in the advisory areas and 55-65 mph in the
warning areas. Highest confidence in the damaging winds will be near
Cabbage and Mission. Even the ECMWF EFI highlights this area for the
potential for extreme wind.

There is an exiting shortwave trough moving into ID at this time,
taking precipitation with it. Showers will end by late morning,
and the rest of the day will be rather quiet as the forecast area
falls between systems. The next system arriving will spread
precipitation to the western half of the CWA this evening then the
remainder of the region tonight. Once again, winds will be very
strong from the Grande Ronde Valley to the Blue Mtn Foothills.
Snow levels will increase to around 6-8 kft with the exception of
the east slopes of the WA Cascades where colder air will keep snow
levels down to around 3500 ft. The upper level trough will track
across WA/OR Tuesday, Christmas Eve, lowering snow levels down to
3-4 kft. Snow accumulations will not be particularly heavy with
this system, as it stretches and loses its upper level support. No
winter highlights are planned for this particular system, as
snowfall amounts will generally range 1-4" over the mountains with
higher amounts along the peaks of the Cascades and Wallowas.

Models have consistently shown Christmas morning as dry due to a
weak transitory ridge. Patchy fog and freezing fog may cause minor
traffic issues. However, weather concerns ramp up late Christmas
and into Christmas night. PoPs will be 80-100% across the board
Christmas night. There is high confidence that all mountain zones
will have 1-2 inches of snow, but there is low to moderate
confidence (20-40%) that the adjacent valleys and central OR will
receive similar accumulations. The probability decreases for most
zones for any accumulations 3" or more except along the east
slopes of the WA Cascades and the higher elevations of the
Elkhorns and Eagle Caps (50-70%). The most likely snowfall amounts
Christmas night will be 3-6" for these areas. Keep in mind these
concerns extend through Thursday onward, as discussed in the Long
Term section of this discussion. Wister/85

LONG TERM...Thursday through Monday...

Bottom Line Up Front

1. A wave train of systems will occupy the long term with widespread
   rain and high mountain snow.

2. Winds will be breezy through the period.

3. Above normal temperatures.

The long term will be characterized by multiple upper level systems
making their way across the PacNW. Models remain in relatively firm
agreement through the long term keeping confidence in the the
forecast moderate to high (50-60%). With the southwest orientation
of three out of the four system, snow levels will remain above 4000
feet through Sunday afternoon. Models show the last system to make
its way across Sunday afternoon will shift to the northwest bringing
back cooler temperatures and lower snow levels. With the wave train
of systems, hydro concerns may come to the forefront and will be
closely monitored as the timeframe draws closer. It`s going to be a
wet one.

Models show each system to bring precipitation across the region
each day. The system Thursday will bring some high mountain snow to
the Cascades, eastern mountains as well as the Blues. On Thursday,
40-60% of the ensembles show Snoqualmie Pass receiving 1-3 inches,
higher elevations of the Northern Blues seeing 1-3 inches and 1-2
inches along the eastern mountains. Higher totals along the
higher peaks. Lower elevations will see rain with 40-60% of the
raw ensembles showing upwards of 0.25 inches of rain with the
exception of Madras and Redmond area where only 20-30% show 0.25
inches. The next system will then swing across the region with a
bit stronger of a warm front and more precipitation. High
mountains snow is again expected, however, the I- 84 and I-90
corridors will see a light dusting. Low elevations however, will
see an additional 0.25-0.45 inches with 45-70% ensemble agreement.
Saturday will be much the same with yet another system bringing
in a round of precipitation with 40-60% showing 0.15- 0.25 inches
along the lower elevations and 0.04-0.06 inches in the Basin.
Last, but certainly not least, models show the system Sunday to
shift in from the northwest. This will allow for cooler
temperatures to return to the region dropping snow levels to near
2800 feet and above along the WA Cascades and 3500 feet along the
remaining portions of the forecast area. 30-50% of the ensembles
show yet another 0.25-0.50 inches of rain along the lower
elevations. 50-60% of the raw ensembles shows Snoqualmie Pass will
see 5+ inches of snow, 60-70% for 5+ inches along Santiam Pass
and 30-40% for the higher elevations of the Northern Blues
(Tollgate, Ski Bluewood). With all that said, hydro concerns may
arise with the amount of precipitation expected. Forecast models
do show an uptick in river heights on Saturday and due to this
will be closely monitored as the systems progress.

With the onslaught of systems, winds will remain breezy across the
region with a brief break Saturday. NBM shows winds to average 10-12
mph. With little variance in the models, confidence is moderate to
high (50-60%) the winds will remain breezy through the period yet
well below critical.

Lastly, EFI shows portions of the area, especially the Basin and
foothills of the Blues being above average temperatures through the
majority of the period. NBM shows temperatures to average between 50-
55 degrees across the Basin, Gorge and foothills of the Blues while
central OR is in the mid to upper 40s. Confidence again is moderate
to high as the ensembles as well as models remain in decent
agreement through the period. Bennese/90


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  51  40  46  30 /  20  80  90  10
ALW  53  41  48  35 /  30  70 100  20
PSC  50  40  48  35 /  10  90  80   0
YKM  45  34  45  27 /   0 100  40   0
HRI  50  41  48  33 /  10  80  90   0
ELN  43  35  43  28 /   0 100  40   0
RDM  48  38  44  25 /   0  90  60   0
LGD  47  38  44  27 /  50  60 100  30
GCD  47  38  46  26 /  30  60 100  20
DLS  49  43  50  35 /   0 100  60  10

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Dense Fog Advisory until 1 PM PST this afternoon for ORZ041.

     Wind Advisory from 10 PM this evening to 7 AM PST Tuesday for
     ORZ049-508.

     High Wind Warning from 10 PM this evening to 7 AM PST Tuesday
     for ORZ507.

WA...Dense Fog Advisory until 1 PM PST this afternoon for WAZ024-026.

     Wind Advisory from 10 PM this evening to 7 AM PST Tuesday for
     WAZ029.

&&

$$

SHORT TERM...85
LONG TERM....90
AVIATION...87