Area Forecast Discussion


from: NWS Pendleton, OR

000
FXUS66 KPDT 091810
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1010 AM PST Fri Dec 9 2022

.Updated Aviation Discussion.

MORNING UPDATE...Most of the day today will represent the calm
before the storm, before a second, more potent winter weather
system arrives with precip by around nightfall for much of the
forecast area. Until then, freezing fog will remain a concern
across the low-lying areas in the Basin, extending into the
adjacent valleys of central WA. Went ahead and extended the
Freezing Fog Advisory until the late afternoon today, including
the Kittitas Valley as well, as given the amount of moisture
dumped on the surface from yesterday`s system, doubt there will be
much of a limiting factor in terms of the fog`s persistence until
the next system starts to move through. Perhaps a little bit of
an aggressive timing window, but feel confident that at least low
clouds and slick conditions could prevail, as temps look to remain
below freezing, with not much working to evaporate the moisture
already on the ground with the fog deck overhead. Elsewhere,
should see clouds slowly start to build in ahead of the oncoming
system, before precip begins around roughly 4PM for our Cascades
zones, spreading eastward through the night.

Main changes to forecast this morning were to increase the
coverage and duration of the freezing fog until the mid/late
afternoon, and to lower highs in said areas, as not expecting too
much robust daytime heating for areas under the fog deck. Evans/74

AVIATION...18Z TAFs...A winter weather system is forecast to bring
another round of impacts to all TAF sites through the next 24
hours. Onset of precipitation for DLS/BDN/RDM/YKM is expected
this afternoon after 00Z with precipitation spreading east to
PSC/PDT/ALW by 04-05Z. -FZRA is a concern for PSC due to lingering
cold air trapped in the Columbia Basin, especially if morning
FZFG fails to lift before onset of precipitation. Elsewhere,
forecast soundings would indicate precipitation at YKM should
remain SN through the event with a mix of -RA and -SN for other
sites.

Other concerns are downslope winds for ALW where HREF guidance
suggests 80-100% probabilities of exceeding 40 kt gusts 05Z
onward. PDT has a more modest 60-100% chance of exceeding 30 kt
gusts during the same period. RDM/BDN also have moderate to high
probabilities of gusts 30-40 kts, though there is a bit more
uncertainty in guidance (run-to-run, and model-to-model) as to
how much these winds will come off the Cascades and mix down into
central Oregon. Plunkett/86


PREV DISCUSSION... /issued 327 AM PST Fri Dec 9 2022/

SHORT TERM...Today through Saturday night...A winter storm is on
the way and will bring a variety of weather concerns and forecast
challenges. For now, the focus is on a weak cold front moving out
of the area and will bring light snow from the Blue Mountains and
John Day Basin east to the ID border early this morning before
its departure. Surface observations and webcams are not showing
anything significant, and ODOT reports indicate around 1-3 inches
in the areas currently under an advisory. Will let the advisory
for the southern Blue Mtns, John Day Basin, and John Day-Ochoco
Highlands expire at 4 AM.

The next front will be much stronger with snow, freezing rain,
gusty winds and blowing snow in the forecast. A few of the Winter
Weather Advisories will be upgraded to Winter Storm Warnings--the
east slopes of the Washington Cascades, the Kittitas Valley, and
the Simcoe Highlands. The combination of easterly low level winds
and a moist southwest flow aloft will lead to high QPF amounts,
and there is enough deep cold air for precipitation to primarily
fall as snow. The Kittitas Valley will be hit particularly hard
with snowfall, and the NBM backs this up with 70 pct chance of
greater than 4 inches and 35 pct chance greater than 6 inches. It
will be high end advisory for the Yakima Valley of 3-4 inches, and
the NBM indicates about a 30 pct chance greater than 4 inches.
Will leave the other advisories as is but will also add the Blue
Mountains in the mix with snowfall amounts 5-8 inches in the
forecast. There will be warm air advection with the front but not
a significant warm up. However, the cold air trapped in some of
the valleys will result in freezing rain. NBM looks reasonable
showing localized freezing rain along the east slopes of the WA/OR
Cascades but seems overdone with the freezing rain potential in
the Kittitas and Yakima Valleys. Forecast soundings from the
deterministic models are strictly snow profiles, but I will have
a chance in the forecast with only about a 30 pct certainty.

Another concern is the winds tonight through Saturday, as there
will be a tight southerly pressure gradient along with a 50kt LLJ
at 850 mb. The southerly winds will aid in warming temperatures
across the Columbia Basin and Blue Mtn Foothills into the 40s on
Saturday. Wind advisories will be issued for the Grande Ronde
Valley as well as the base of the Oregon Blue Mtn Foothills
tonight through Saturday where there is a high confidence (80 pct)
that winds will gusts to 45-55 mph. Many of the higher elevations
of central and northeast OR and far southeast WA will observe
breezy to windy condition that will contribute to patchy blowing
snow.

Precipitation will decrease in coverage and intensity on Saturday
night, but the PacNW will be located on the cold side of the polar
jet and under a cyclonic flow aloft. All of the forecast area
will have a chance of snow as snow levels lower back down to the
Columbia Basin floor. Most accumulations will be light but cannot
rule out a few embedded snow showers that may briefly provide an
inch or two of new snow. Wister/85

LONG TERM...Sunday through Friday...The beginning of the
long term period is characterized by a deep low with accompanying
winter precipitation. Higher elevations will see a  of snow (>30%)
while the lower elevations will see (>30%) of a rain snow mix. The
SREF/GFS/ECMWF/NAM are is solid agreement with the just onshore but
vary with the location of the axis. The low will continue to deepen
and move to the south of the CWA with some lingering precipitation
through Monday as the low traverses eastward.

By Tuesday the low will moved mostly to the east of the CWA with the
leading edge of a ridge pushing in. There is a slight chance (>20%)
that there could be some wrap around precipitation occurring along the
eastern portion of the CWA, but confidence is low in this occurring.
Clusters are showing a decent variance in the ensembles as the when,
where and how steep the incoming ridge will be.

Overall, the GFS/ECMWF do how a ridge to be building in after
Tuesday and persisting through at least Wednesday. After Wednesday
the GFS and ECMWF as well as the ensembles begin to completely
diverge on the incoming next system. GFS shows a weak shortwave
moves through the ridge while the ECMWF shows another system slip in
right behind the ridge so confidence is low (>30%) regarding the
midweek pattern change. Regardless, ensembles and NBM show mostly
dry and very cold conditions associated with the next pattern change
where high temperatures will not reach above freezing throughout the
CWA and through the workweek. Bennese/90

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  39  33  44  27 /  10  80  50  40
ALW  40  33  45  29 /  10  90  60  30
PSC  31  29  40  30 /  10  90  60  30
YKM  31  25  37  24 /  20 100  70  30
HRI  35  32  43  27 /   0  80  50  40
ELN  30  23  36  22 /  20 100  70  30
RDM  38  33  45  23 /  30  70  50  40
LGD  35  28  39  28 /  20  90  70  40
GCD  35  29  42  27 /  10  80  60  50
DLS  37  34  43  31 /  50 100  80  50

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Freezing Fog Advisory until 4 PM PST this afternoon for ORZ044.

     Wind Advisory from 7 PM this evening to 4 PM PST Saturday for
     ORZ049-507.

     Winter Weather Advisory from 8 PM this evening to 7 PM PST
     Saturday for ORZ502-503-506.

     Winter Weather Advisory from 4 PM this afternoon to 7 PM PST
     Saturday for ORZ509.

WA...Freezing Fog Advisory until 4 PM PST this afternoon for
     WAZ026>028.

     Winter Weather Advisory from 8 PM this evening to 7 PM PST
     Saturday for WAZ030.

     Winter Storm Warning from 4 PM this afternoon to 4 PM PST
     Saturday for WAZ026-521.

     Winter Weather Advisory from 4 PM this afternoon to 4 PM PST
     Saturday for WAZ027.

     Winter Storm Warning from 4 PM this afternoon to 7 PM PST
     Saturday for WAZ520.

&&

$$

MORNING UPDATE...74
SHORT TERM...85
LONG TERM....90
AVIATION...86

NWS PDT Office Area Forecast Discussion