115
FXUS66 KPDT 300516
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1016 PM PDT Sun Sep 29 2024

.AVIATION...06Z TAFs...VFR conditions are forecast through the
period, consisting of predominantly SKC or FEW250 accompanied by
diurnally and terrain-driven winds once winds diminish late this
evening. Plunkett/86

&&

.PREV DISCUSSION... /issued 824 PM PDT Sun Sep 29 2024/

EVENING UPDATE...A cool, dry air mass in tandem with diminishing
winds in the wake of today`s shortwave trough will facilitate cool
nighttime lows across the region. Confidence has increased that
a significant portion of central Oregon will see sub-freezing
lows overnight into Monday morning, so have expanded the existing
Freeze Warning to cover that zone. That said, confidence in
widespread sub-freezing lows within Bend is low (<30% chance),
while Redmond and Prineville have better odds (70-90% chance).
Similarly, in the Grande Ronde Valley, cold-prone areas of the
valley have high (>70%) odds of freezing, while La Grande itself
has significantly lower odds (20-30%). Elsewhere, low-lying and
cold-prone areas along the foothills of the Blue Mountains, Yakima
Valley, and Columbia Basin have a 10-20% chance of dipping to
freezing. Some locations within the John Day Basin, especially
towards the eastern edge in the vicinity of John Day have a 50-80%
chance of freezing, but areal coverage of the zone is too low
(<50%) to issue a Freeze Warning. Plunkett/86

PREV DISCUSSION... /issued 422 PM PDT Sun Sep 29 2024/

AFTERNOON UPDATE...Satellite imagery indicates an area of blowing
dust has developed in northern Franklin, Walla Walla, and Columbia
counties in the Columbia Basin. While observations of surface
visibility in the region are scant to nonexistent, the
presentation on Dust RGB imagery suggests at least some areas are
seeing visibilities reduced to around one mile so have issued a
Blowing Dust Advisory, valid until 8PM PDT. Plunkett/86

PREV DISCUSSION... /issued 151 PM PDT Sun Sep 29 2024/

SHORT TERM...Tonight through Tuesday...Water vapor satellite
imagery this afternoon shows a cold front pushing across the
northern Rockies, while an upper shortwave trough axis pushes
across the Cascades. A notable pocket of dry air is also pushing
across eastern WA and northeast OR at this time, with surface
observations showing RHs approaching the teens in portions of the
Columbia Basin.

The main concerns going through this evening will revolve around
critical fire weather conditions being met across much of the
Columbia Basin. As breezy winds continue through the next several
hours, the dry airmass ahead of the shortwave trough will mix
down, and will result in RHs at the surface dipping into the teens
and lower 20s in the Basin. Elsewhere, winds are expected to
remain weak enough or RHs high enough that critical fire weather
conditions will not be met. Otherwise, breezy to gusty conditions
will also continue through the Kittitas valley through this
evening.

Tonight through Tuesday, conditions across the region will quiet
down as a dry, near zonal flow develops over the PacNW with broad
high pressure ridging extending from the eastern Pacific to the
western CONUS. The only notable concerns will be a cool airmass
filtering into the region tonight, resulting in overnight
temperatures dipping into the 30s to lower 40s. In fact, portions
of the Grande Ronde/Wallowa valleys are expected to cool as low as
the mid 20s overnight, with the NBM showing a 70-85% chance of
below freezing temperatures tonight. Local portions between
Redmond and Prineville are also expected to see morning
temperatures near freezing tonight, though NBM chances of at or
below freezing temperatures is only 50-65% in this area. For now,
have kept the freeze warnings as is, with only confidence not high
enough to expand the warning to central OR. Tuesday, the region
will see a 7 to 12 degree jump in afternoon temperatures as the
region comes under the influence of the broad ridging to the
south. Lawhorn/82

LONG TERM...Wednesday through Sunday...Ensembles remain in poor
agreement starting next weekend, as solutions diverge with regards
to how a band of zonal flow will evolve beyond the midweek. This
band will swing a dry cold front across the forecast area
Wednesday, making for some breezy winds, namely through the
Cascade Gaps, before ridging builds in on Thursday. Forecast
currently calls for RHs above critical thresholds, and the wind
forecast seems to have trended downward as well. After Thursday,
however, is when forecast confidence starts to decrease due to
model discrepancies.

There is some agreement on a trough forming off the Pacific coast
early Friday, but deterministic guidance seems to carry the axis
of this system north and west of us, leaving our area dry
outside of the central WA Cascades, and even then, PoPs are low at
15-20%. Looking at ensemble member clustering, some solutions
keep ridging in place overhead, which would leave the entirety of
our CWA dry and warm. The Euro deterministic seems to favor this
at least heading into the weekend, while the GFS deterministic
depicts a progressive westerly flow pattern over the PacNW.
Ensembles do seem to lean towards ridging, so will keep the
forecast dry for the period outside of the aforementioned trough
potentially clipping the WA Cascades with some light orographic
showers. Should note that overall forecast confidence is on the
lower end, however (20-30%). Should the majority of ensemble
solutions verify, the rest of the period looks dry with above-
average temperatures climbing into the upper 70s and even low 80s
by the tail end of the weekend. Evans/74

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  36  65  38  76 /   0   0   0   0
ALW  38  68  41  77 /   0   0   0   0
PSC  41  68  42  75 /   0   0   0   0
YKM  34  66  39  75 /   0   0   0   0
HRI  39  69  40  78 /   0   0   0   0
ELN  34  65  40  75 /   0   0   0   0
RDM  29  68  34  83 /   0   0   0   0
LGD  32  65  37  78 /   0   0   0   0
GCD  33  68  40  82 /   0   0   0   0
DLS  41  72  44  82 /   0   0   0   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Freeze Warning until 9 AM PDT Monday for ORZ049-050-511.

WA...None.
&&

$$

EVENING UPDATE...86
SHORT TERM...82
LONG TERM....74
AVIATION...86