000
FXUS66 KPDT 011719
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
919 AM PST Tue Dec 1 2020

.Updated Aviation Discussion.

.MORNING UPDATE...
A chilly and mostly clear morning is on tap with just a few low
clouds across the John Day Highlands and portions of Central
Oregon. Rather seasonable conditions are expected for this first
day of Meteorological Winter with highs mainly in the low to mid
40s in the basin and valleys, and mid to upper 30s in the
mountains. Generally light winds can be expected with abundant
sunshine through the day as an amplified upper ridge builds in
across the Pacific Northwest.

Not many changes to the going forecast as conditions are largely
on track. With the ridge building in, patchy to areas of freezing
fog are on target tonight through Wednesday morning, especially in
the Columbia Basin and Gorge. With clear skies in many areas, it
will be quite chilly tonight in the teens and 20s. Have a great
day! 79

.AVIATION...18Z TAFS. VFR conditions will persist through the
period across all sites. High pressure is in control and will keep
skies clear and winds below 10 kt. The only exception is DLS, where
winds could briefly reach near 11kt between 10Z and 12Z. Vsby drops
to 6SM for PDT and ALW toward the end of the period due to
developing overnight fog, which will begin to be daily issue over
the coming days. 75

&&

.PREV DISCUSSION... /issued 323 AM PST Tue Dec 1 2020/

SHORT TERM...Today through Thursday night...
The main sensible weather concerns in the short term revolve
around a favorable pattern conducive for a) increased smoke and
air pollutants that could impact air quality via stagnation and b)
fog/freezing fog at times. Locally gusty winds will also be
possible late Wednesday- Thursday, specifically in the Grande
Ronde Valley/Ladd and Pyles canyons. Latest water vapor depicts an
amplified mid-level ridge positioned offshore the West Coast
extending across British Columbia with northwest flow aloft
overhead. Latest NtMicro RGB satellite imagery shows passing
swaths of high clouds and low stratus mainly confined to over the
Blues with low ceilings noted at KMEH and KGCD.

Large scale pattern will remain highly amplified the next 72
hours as the aforementioned upper ridge moves to be over WA/OR by
Tuesday night. The upper ridge will then move slightly more over
the Northern Intermountain West Wednesday Night as a weak
shortwave trough moves to the northern CA/OR coast. This open wave
is not expected to affect the region much as it dampens moving
east. Overall ensemble and deterministic guidance in pretty fair
agreement with respect to the large scale pattern with the ridge
dominating this period. Meanwhile, strong surface high pressure
evolves in the region today and its influence will be seen through
the short term. However, the surface high will evolve temporarily
to be more east Wednesday-early Thursday. Of which, starting
Wednesday modest pressure differences are forecast with an
increasing pressure gradient as surface ridging extends into Baker
as seen via BKE-MEH differences. This will help promote potential
breezy/gusty winds in the Grande Ronde Valley. Latest NBM
guidance and ECMWF plumes show pretty consistent agreement
regarding timing with highest winds anticipated late tomorrow
through tomorrow night. Thus confidence is increasing though gusts
look to be sub-advisory presently but will continue to monitor.
Current thinking is gusts in excess of 30 mph, peaking as high as
40 mph. Meantime, conditions favorable for strong radiational
cooling develop tonight with soundings showing the development of
a strong inversion in the Basin and favorable conditions extending
into adjoining Valleys and the Gorge as well, and this feature is
expected to prevail. This will favor the development of freezing
fog-fog at times. Further, this will promote potential for air
quality concerns given the inversion and low mixing heights.

LONG TERM...Friday through Tuesday...
Dry and stagnant conditions look to continue through at least
Sunday, as global forecast models show little relief from
persistent high pressure until a weak trough passes over the area
during the day Sunday. Both the GFS and ECMWF deterministic are in
agreement in depicting this feature, although neither produce
precip beyond the Cascades. Despite this, even a slight increase
in wind speed associated with this trough could be enough to mix
us out a bit and provide relief from continuous low clouds and air
stagnation.

Monday and Tuesday look to be a return to dry conditions, before
ensembles depict precip associated with a more active pattern
beginning the middle of next work week. Still a lot of
disagreements across global models, and also too far out to make
any major commitments in the long term period. Temps will likely
see a gradual decrease over the period as low clouds persist for
most low-lying areas, as highs struggle to climb out of the 30s.
Central OR may see warming, however, as dry conditions and clear
skies allow for more robust daytime heating, with upper 40s and
low 50s possible by the end of the week. Perhaps a bit more mild
heading into next week, but no major departure from dry and stable
conditions expected through most of the long term period.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  43  21  37  19 /   0   0   0   0
ALW  45  23  39  23 /   0   0   0   0
PSC  43  25  39  23 /   0   0   0   0
YKM  44  21  42  18 /   0   0   0   0
HRI  43  25  39  21 /   0   0   0   0
ELN  43  22  39  20 /   0   0   0   0
RDM  43  18  44  17 /   0   0   0   0
LGD  39  18  40  22 /   0   0   0   0
GCD  43  21  46  22 /   0   0   0   0
DLS  45  29  41  28 /   0   0   0   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
&&

$$

SHORT TERM...79
LONG TERM....74
AVIATION...74

NWS PDT Office Area Forecast Discussion