101
FXUS66 KPDT 282330
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
430 PM PDT Tue Apr 28 2026

.Updated Aviation Discussion.


.KEY MESSAGES...

- Temperatures warming to above normal through early next week

- Uncertainty late Friday through early next week in potential
  for mountain shower and thunderstorm activity

&&

.DISCUSSION...
Water vapor imagery shows a weak shortwave embedded within
northwesterly flow aloft over western OR/WA. Some cirrus has
overspread the forecast area in advance of the shortwave, and
Day Cloud Phase RGB imagery reveals some cumulus development
over the Blue and Cascade mountains. Shower activity within
their respective cumulus fields has been isolated, with radar
showing a few over the northern Blue, Wallowa, and Wenatchee
mountains. Chances of lightning are low (10 percent or less)
with the shower activity.

Wednesday and Thursday, an upper-level ridge in the northeast
Pacific will move eastward over the region, resulting in rising
heights and warming temperatures. While mostly dry conditions
are forecast, yet another shortwave will dive south-southeast
from Canada on Wednesday. Coupled with weak diurnally driven
instability (CAPE of 500 J/kg or less), this will facilitate a
slight chance (10-20 percent) of showers over the Blue
Mountains.

By Friday, ensemble mean 500-hPa heights show a couple
shortwaves moving eastward in the northeast Pacific towards the
Pacific Northwest. Ensemble cluster analysis is in good
agreement that the first shortwave will partially flatten the
upper-level ridge, but its track well north of the region into
British Columbia should keep the forecast area dry (90 percent
confidence). Moreover, heights actually continue to rise over
our area, so temperatures are forecast to continue to increase
for most of our population centers relative to Thursday.

Uncertainty in the forecast grows considerably by later Friday
through next week as ensemble cluster analysis suggests a high
chance (80 percent or higher) of the latter shortwave trough
deepening and cutting off into a closed low off the coast of
Oregon and California. Based on the 00Z and 12Z ensemble runs,
a little less than half of members keep the region warm to hot
and dry under a ridge (Rex block pattern with a closed low
offshore and ridging in the Pacific Northwest and an anomalous
high offshore north of the low), while the remaining members
place the low in a position near the OR/CA border that would
provide precipitation, somewhat cooler temperatures (though
still above normal), and potential for thunderstorms. While it
is unlikely, analysis of QPF clusters suggests a roughly 25
percent chance of a widespread wetting rain for the Oregon
Cascades and Blue Mountains as the aforementioned closed low
eventually moves inland early next week (likely Monday through
Tuesday).

While there is still uncertainty in how hot conditions will be under
the ridge this weekend into early next week (largely due to the
aforementioned uncertainty in shower and thunderstorm chances
and resultant cloud cover), chances of afternoon highs
exceeding 80 degrees across all major population centers are
high (60-99 percent) per latest NBM calibrated probabilities. By
Sunday through Tuesday, chances of reaching 90 degrees are low-
medium (20-60 percent) for the Yakima Valley and lower Columbia
Basin. Moreover, there is a low-medium (20-60 percent) chance
of HeatRisk reaching Moderate (level 2 of 4) in the Yakima
Valley and lower Columbia Basin on Monday and Tuesday. This
level of heat affects those who are sensitive to heat,
especially those without cooling/hydration. 86

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
VFR conditions currently at all sites, which will stay the course
through the period. Breezy winds between 15-25kts are occurring at
KDLS/KRDM/KBDN, and will slowly subside by early evening. 15-20kft
ceilings continue overnight before breaking up and lifting early
Wednesday morning. 75

&&

.Preliminary Point Temps/PoPs...
PDT  66  40  69  43 /   0   0   0   0
ALW  66  44  69  47 /   0   0   0   0
PSC  71  43  74  44 /   0   0   0   0
YKM  69  40  73  45 /   0   0   0   0
HRI  68  41  72  42 /   0   0   0   0
ELN  62  38  69  42 /   0   0   0   0
RDM  64  29  66  34 /   0   0   0   0
LGD  63  40  66  38 /  10   0  10   0
GCD  63  38  66  38 /   0   0  20   0
DLS  66  41  74  47 /   0   0   0   0

&&

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

&&

$$

DISCUSSION...86
AVIATION...75