891
FXUS66 KPDT 042228
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
228 PM PST Wed Dec 4 2024

.SHORT TERM...Tonight through Friday night...Widespread stratus
clouds continue to blanket the entire Columbia Basin down to the
Columbia Deschutes Plateau, as evident on visible satellite. The
low level inversion is quite pronounced as most elevations above
3500 feet are in the mid-30s to mid-40s and under sunny skies.
Those under the inversion and stratus clouds are near to slightly
above freezing. The Freezing Fog Advisory for central and north
central Oregon and the southern Blue Mtn Foothills was extended to
Thursday morning. Although the visibility has improved for most
of the advisory area, the dense freezing fog will (confidence 80%)
return in the evening. Fog and stratus have lifted for most of
the John Day Basin, and the advisory was allowed to expire. There
was too much uncertainty to extend that overnight.

There were reports of light drizzle, freezing drizzle, and snow
flurries this morning under fog and stratus. Even high resolution
models fail to show precipitation due to fog droplets, but it only
takes light freezing drizzle to create slick walkways and spot of
ice on the roads. This was included in the forecast for tonight
and Thursday morning, but no highlights are needed. Will address
in a weather story, as it will be patchy at best. Thursday night
is a slightly different story. An upper trough currently between
130-140W is sending the brunt of moisture over B.C. The southern
branch of the trough will split apart as it encounters the upper
ridge over WA/OR, so it does little in bringing measurable
precipitation over the forecast area. It will, however, cause the
fog and stratus to lift and increase the potential for freezing
drizzle before sunrise on Friday. Will mention a chance in the
forecast, as confidence that light freezing precipitation will
occur is 40% but the probability of measurable ice accumulation is
15%. This will depend on how quickly (if at all) the stratus
lifts. Since this shortwave trough is not particularly strong, it
is highly unlikely that stratus will lift entirely across the
Columbia Basin and surrounding valleys. Tough forecast indeed.

The persistent high pressure ridge will shift eastward and flatten
on Friday, allowing a warm front to spread precipitation across
western WA, including the WA Cascades. PoPs late Friday and Friday
night for the WA Cascade east slopes are 60-80% and 30% in the
Kittitas Valley. Warm air advection will cause snow levels to rise
to around 7000 feet Friday and 8000 feet Friday night. Therefore,
precipitation will fall as rain along the east slopes. There is a
slight possibility (10%) that some of the colder valleys will be
slow to warm above freezing and have a brief period of light
freezing rain. The NBM shows 0% for freezing rain for areas south
of the Chelan/Kittitas County line for Friday (better chances
north of the county line) so will leave out of the forecast.
Wister/85


.LONG TERM...Saturday through Wednesday...The long term remains
active initially as a system moves through our region and
clears out our stagnant conditions, but by next week we should
return to quiet ridging that will once again be conducive of fog
and low stratus.

Saturday begins with an upper low just offshore of the Canadian
coastline with a cold front beginning to move inland across the
PacNW to start the forecast period. This boundary will move in
over the course of Saturday, with the low moving inland across
Canada and weakening into an open trough that will continue across
the region through the start of the week. The main concerns as
this system moves inland will be the potential for some
temperatures a bit cooler than forecast thanks to lingering
stratus and fog, with the falling rain possibly falling as
freezing rain initially. That being said, uncertainty is high due
to the nature of the low stratus and fog layer, and confidence is
too low (<20%) to justify its inclusion as of this update.
The forecast sounding for Yakima though starts to give some worry,
with an inversion at the surface and sub-freezing temperatures
leading into a warm nose just aloft. The forecast sounding
quickly warms to above freezing at the surface though, likely
with the system helping to clear the cooler air out, so for now
will continue to keep this just out of the forecast.

Breezy winds will be likely with this system which will definitely
help to clear out the stagnant conditions we`ve seen over the last
couple of weeks. The NBM shows a probability of 24 hour maximum
wind gust for Sunday of 50-100% for the majority of our Washington
zones as well as the lower Columbia Basin and Blue Mountain
foothills in Oregon, as well as across all Cascade zones. The NBM
seems to be identifying this a bit better than this morning`s run,
so have left the raw NBM data alone to account for this breezy
wind potential.

Finally, conditions should calm back down by next week as high
pressure ridging moves back in. Ensemble clusters agree we should
fall back under ridging with good confidence until the very end of
the forecast period later Wednesday when a deepening offshore
trough could move inland, but model divergence grows significantly
here. Therefore there is overall moderate to high confidence
(60-70%) in the long term forecast. Goatley/87

&&

.AVIATION...00Z TAFs...A mix of flight categories between MVFR to
IFR continue due to low stratus across DLS/YKM/PSC/ALW/PDT. This
should continue into tomorrow with little change expected.
Meanwhile, IFR to LIFR conditions will continue to be possible
moving into the overnight hours in Central Oregon for BDN/RDM due
to fog across the area. RDM is more likely to see these
conditions, and with some light southerly flow setting up it may
help keep BDN just outside of the deep fog layer, but it cannot be
ruled out fully. Finally, some patchy drizzle could occur at most
sites, and anywhere below freezing this could become freezing
drizzle and cause some light ice buildup on elevated surfaces.
Goatley/87


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  25  32  25  37 /  10  10  10   0
ALW  27  34  27  38 /  10  10  10   0
PSC  30  34  30  35 /  10  10  10   0
YKM  27  33  26  35 /  10  10  10   0
HRI  30  36  30  37 /  10  10  10   0
ELN  24  34  25  37 /  10  10  10   0
RDM  22  44  24  53 /  10  10   0   0
LGD  28  44  28  48 /   0   0   0   0
GCD  29  50  28  53 /   0   0   0   0
DLS  32  40  30  42 /  10  10  10   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Freezing Fog Advisory until 11 AM PST Thursday for ORZ508-510-
     511.

WA...None.
&&

$$

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