766
FXUS66 KPDT 181705
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1005 AM PDT Thu Jul 18 2024

Updated Aviation Discussion

.AVIATION...18z TAFs...VFR conditions expected, with clear skies and
light, terrain driven winds prevailing, except for BDN, RDM, and DLS
which will see breezy conditions this afternoon with gusts up to 25
kts out of the NW. Haze will continue to obscure the horizon for
many sites across the forecast area. Evans/74

&&

.PREV DISCUSSION... /issued 351 AM PDT Thu Jul 18 2024/

SHORT TERM...Today through Saturday night...The circulation that
brought thunderstorms to the forecast area yesterday has exited
to the northeast leaving dry conditions in its wake. Lightning
associated with the storms initiated numerous fires across the
region, and several "hot spots" are still present early this
morning on satellite. Notably, the pre-existing Lone Rock and
Falls fires remain active. Smoke from these fires has settled
across the region, so have added haze and smoke to the forecast
across much of Oregon and south-central and southeast Washington
for the next 48 hours.

Today, hot temperatures will continue across the region as a mid-
to upper-level ridge of high pressure re-establishes itself over
the PacNW. As a result, widespread Moderate to Major HeatRisk is
forecast for the lower elevations. Have opted to downgrade the
Excessive Heat Warning for the Yakima and Kittitas valleys to a
Heat Advisory as forecast temperatures are no longer supportive of
the warning. Elsewhere in the Columbia Basin and along the
northern Blue Mountain foothills, afternoon highs are likely to
approach or eclipse 100 degrees (75-100% zone-wide chances of
exceeding 98 degrees per NBM probabilities).

While Friday will still be warm, late July is climatologically
the warmest time of year across the region and forecast
temperatures are not quite supportive of Heat Advisories; thus,
have opted to leave the expiration time of existing highlights.

A weak shortwave is forecast to track southwest to northeast over
northwest Oregon and Washington during the day Friday. Model-to-
model spread in the amplitude of the shortwave and available mid-
level moisture has precluded mention of thunderstorms across the
Blue Mountains region at this time, but it I did increase PoPs
across the mountains to convey a very slight chance of
precipitation (5-10%). 00Z HREF output is advertising modest
instability (MUCAPE of generally <500 J/kg except 500-1000 J/kg
for the Wallowas), and coupled with negative mid-level theta-e
lapse rates and modest upper-level Q-vector convergence, this
should support at least some cumulus build-ups if not isolated
showers and thunderstorms provided enough moisture is present.

As the weekend rolls around, the mid- to upper-level ridge is
expected to further amplify overhead, and confidence continues to
increase that the weekend will be very hot. To avoid confusion
with ongoing advisories, no Excessive Heat Watches have been
issued, but there is high confidence (70-100% chance) in reaching
or exceeding 100 degree afternoon temperatures across all of our
low-elevation zones, and a 30-60% chance of at least 105 degrees.
Plunkett/86

LONG TERM...Sunday through Thursday...A high amplitude ridge of
high pressure will continue over the region on Sunday with a closed
upper level low in the Gulf of Alaska and its trough extending south
into the eastern Pacific. This will boost temperatures Sunday into
the 105 to 110 range around the Columbia Basin with a 30-50%
probability of some areas seeing over 110 degrees. This will place
high temperatures within reach of record highs. The good news is
that this will be the culminating event of this long heat wave.
The upper level trough off the coast will begin to move inland
Sunday night and Monday before slowly passing over the region
during the week. This will lower high temperatures Monday by
around 10 degrees followed by another 10 degrees of some on
Tuesday with highs in the 80s to mid 90s of closer to normal for
this time of year. These cooler, or normal temperatures, will
continue Wednesday and Thursday as the upper level trough remains
over the region.

Precipitation wise this system keeps most of the moisture further
north in Canada where the center of the lower pressure remains.
There could be a low end 10-20% chance of some convection ahead of
the approaching trough. This would occur over central Oregon Sunday
afternoon and then shift to the eastern mountains on Monday. After
Monday the flow over the region will be westerly pushing the
convection east into Idaho and the northern Rockies.

Wind will become an issue from a fire weather perspective as
increasing westerly winds could cause fire spread concerns on new
and existing fires from Sunday onward. This will begin Sunday as
westerly winds increase along the east slopes of the Cascades in the
late afternoon and evening coinciding with the very hot temperatures
and low humidities.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  98  63  96  61 /   0   0   0   0
ALW 102  67 101  65 /   0   0   0   0
PSC 103  68 102  65 /   0   0   0   0
YKM 101  62 100  61 /   0   0   0   0
HRI 103  66 101  63 /   0   0   0   0
ELN 101  65  98  63 /   0   0   0   0
RDM  96  53  94  54 /   0   0   0   0
LGD  96  61  95  61 /   0   0  10   0
GCD  99  58  98  60 /   0   0  10   0
DLS  95  64  96  65 /   0   0   0   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Heat Advisory until 10 PM PDT this evening for ORZ044.

WA...Heat Advisory until 10 PM PDT this evening for WAZ026>028.

&&

$$

SHORT TERM...86
LONG TERM....91
AVIATION...74