344 
FXUS66 KPDT 132132
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
232 PM PDT Mon Sep 13 2021

.SHORT TERM...Tonight through Wednesday night...A cool northwest 
flow over the region today will transition into a ridging pattern 
on Tuesday. This will result in warming temperatures on Tuesday 
compared to today. Afternoon highs will rise to the upper 70s to 
lower 80s across the lower elevations and 60s to 70s in the 
mountains. After that a mostly dry cold front will move across the
area on Wednesday with an increasing westerly flow at the base of
a very broad upper trough over western Canada. This will result 
in increasing westerly winds, especially through the Cascade gaps,
passes and the eastern Columbia Gorge with breezy to windy 
conditions by Wednesday late afternoon and evening. It will also 
cool down again on Wednesday with highs only in the lower to mid 
70s in the lower elevations and 50s to 60s in the mountains. There
will be some light rain showers possible in the WA southern and
central Cascades/east slopes with this front. Elsewhere will
remain dry. The winds will diminish later Wednesday evening and 
overnight. Temperatures Wednesday night will also be substantially
cooler with lows in the 40s lower elevations and upper 20s to 30s
in the mountains. Many areas will see patchy to areas of frost in
the higher elevations, especially in open meadows and fields. 
There will not be any fire weather concerns in the short term 
period due to any low RH's and windy conditions not coinciding to 
create critical fire conditions, and no thunderstorms expected. 
88

.LONG TERM...Thursday through Monday...Dry conditions to start the 
period as a weak ridge traverses the PacNW Thursday, though highs 
will be slightly below seasonal. Sensible weather concerns arrive 
Friday as a deep, anomalously strong upper-level low slides down 
from the Gulf of Alaska, weakening to an open wave by the time it 
reaches the PacNW Saturday through Sunday. The event looks to have a 
weak-moderate atmospheric river associated with it, albeit some 
uncertainty still remains regarding the exact location of the 
moisture plume. That said, our entire forecast area could see 
significant precipitation, especially across the higher terrain 
where moderate-locally heavy rainfall may occur. Otherwise, the 
ECMWF EFI tool hints that Friday and Saturday will be breezy to 
locally windy, especially across central Oregon.

ECMWF EPS, GEFS, and Canadian GEPS ensemble means are all consistent 
in bringing an anomalously deep upper-level low down from the Gulf 
of Alaska into the PacNW Friday through Sunday. However, timing and 
amplitude differences exist. Further examining ensemble means, the 
12Z GEPS and 12Z EPS are somewhat slower than the more progressive 
solution of the 12Z GEFS. Moreover, flow aloft appears more zonal in 
the EPS and GEPS, whereas the GEFS has the region entrenched in 
southwesterly flow aloft by Friday afternoon. By Saturday afternoon, 
all three ensembles have southwesterly flow over the PacNW and into 
the Inland NW, though the GEFS remains more progressive with a 
deeper trough than the EPS and GEPS. Solutions start to diverge 
considerably come Sunday afternoon with the GEPS and EPS keeping the 
upper low in place offshore of British Columbia or northwest 
Washington, while the GEFS is far more progressive with the trough 
sliding across our CWA by that time. By Monday afternoon, the EPS 
and GEFS start to align as the upper low continues to slide east 
with a ridge taking its place over the PacNW. The GEPS does show 
this trend as well, though it is about 12 hours slower and not as 
aggressive with the ridge's amplitude.

To gauge ensemble spread, we can examine clusters of like-patterned 
members. The total ensemble mean is zonal for the Inland NW for 
Friday afternoon, with the primary source of variance being the 
amplitude of 500 mb heights over the PacNW. A secondary source of 
variance, though smaller, is heights across the Northern Rockies. 
Ensemble mean 500 mb heights lower for Saturday afternoon and flow 
turns more southwesterly aloft. The primary source of variance is 
still the amplitude of the heights across the PacNW, though now we 
see a secondary source of variance creep in with the timing of the 
pattern. Cluster phase space shows the EPS favoring higher-than-mean 
heights across the PacNW compared with lower-than-mean heights for 
the GEFS, while the GEPS is somewhere in the middle with solutions 
more evenly spread across clusters. Perhaps as expected, cluster 
mean QPF indicates a wetter-than-mean solution for the GEFS, and a 
drier-than-mean solution for the EPS for Saturday for our CWA. The 
upper trough across the PacNW persists in the ensemble mean through 
Sunday afternoon, with QPF remaining in the ensemble mean for our 
forecast area. However, there is considerable uncertainty in the 
progression of the pattern. Timing of the disturbance remains the 
primary source of variance Sunday through Monday, while the 
amplitude of the feature remains a secondary source of variance. 
Precipitation looks to taper off Sunday night through Monday.

Deterministic and ensemble guidance both suggest a ridge building 
back over the PacNW Monday night through Tuesday as the upper low 
exits to the east. Plunkett/86

&&

.AVIATION...18Z TAFs...VFR conditions expected for all TAF 
sites through the period as skies continue to clear into the 
afternoon. Current gustiness at KALW should subside through this 
morning. Winds of 5-15 kts then expected for the afternoon and 
evening at all TAF sites, with gusts of 15-25 kts for the Gorge and 
Kittitas Valley. Light winds expected overnight, though KYKM and the 
Kittitas Valley may gust around 15 kts through midnight. Plunkett/86

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  43  79  56  72 /   0   0   0   0 
ALW  48  81  58  75 /   0   0   0   0 
PSC  50  81  61  77 /   0   0   0   0 
YKM  44  80  52  73 /   0   0  10   0 
HRI  48  82  59  76 /   0   0   0   0 
ELN  46  79  54  67 /   0   0  10   0 
RDM  41  79  46  72 /   0   0   0   0 
LGD  41  79  54  72 /   0   0   0   0 
GCD  41  82  48  77 /   0   0   0   0 
DLS  52  84  61  74 /   0   0   0   0 

&&

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

&&

$$

SHORT TERM...88
LONG TERM....86
AVIATION...86