482
FXUS66 KPDT 272207
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
307 PM PDT Mon Apr 27 2026

.KEY MESSAGES...
- Isolated to scattered showers in the Cascade and Blue
  mountains this afternoon

- Isolated thunderstorms this afternoon in the Blue Mountains

- Temperatures gradually warming to above normal through the
  remainder of the week

&&

.DISCUSSION...
Water vapor imagery shows broad troughing over the western CONUS
this afternoon. A shortwave embedded within northwesterly flow
aloft is evident just offshore of the Pacific Northwest, now
approaching the OR/CA border. Similar to yesterday,
predominantly clear to partly skies are present across the lower
elevations, while the mountains have seen numerous cumulus
develop. Scattered showers are evident within the cumulus field,
mainly over the Blue Mountains. Anticipating convection will
remain mostly shallow today given the lack of vertical extent of
CAPE profiles in forecast soundings (EL around 12-18 kft) from
HREF and REFS NWP guidance. However, modest CAPE (up to 500 J/kg
per forecast soundings from the 12Z HREF/REFS) coupled with steep
low-level lapse rates and the cool air mass aloft have enabled
isolated lightning. For the remainder of the afternoon and
evening the best chances for lightning (10-20 percent) will
remain over the Blue Mountains, while other areas have very low
chances (10 percent or less).

Looking ahead, northwesterly to northerly flow aloft will
continue for Tuesday and Wednesday, albeit with rising 500-hPa
heights. NWP guidance advertises yet more weak shortwaves and
weak diurnally driven instability (CAPE of 500 J/kg or less per
latest CAMs) which will facilitate low (5-15 percent) chances of
showers, mainly over the Blue Mountains. Temperatures will warm
several degrees each day through Wednesday as a result of the
aforementioned rising heights.

Thursday through Friday, ensemble cluster analysis shows above-
normal 500-hPa heights over the Pacific Northwest and vicinity.
Afternoon high temperatures are forecast to rise to generally
around 10 degrees above normal. It`s still worthy noting all
ensemble clusters show hints of a trough or closed low moving
over southern Canada or the Northern Tier sometime between late
Thursday and late Friday, but most (75 percent) members keep dry
conditions region-wide through the period.

By next weekend, uncertainty in the 500-hPa height field
burgeons as ensemble guidance struggles to resolve the
evolution of an upper-level closed low and upper-level ridge in
the vicinity of the Pacific Northwest. Based on the 00Z
ensemble runs, over half of members keep the region warm 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, cooler temperatures, potential for thunderstorms.

By early next week, ensemble guidance shows very good agreement
(about 80 percent of members) in a Rex block pattern, which
would induce well-above-normal temperatures. Chances of
afternoon highs exceeding 80 degrees across all major population
centers are high (60-90 percent) per latest NBM calibrated
probabilities, and chances of reaching 90 degrees are low-medium
(10-40 percent) for the Yakima Valley and lower Columbia Basin.
Moreover, there is a low-medium (20-50 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 WEDNESDAY/...
VFR conditions are expected through the TAF period.
Afternoon winds have become gusty in the 15 to 20 kt range at
PDT, BDN and RDM and 20 to 25 kt range at DLS. Expect these
winds to continue into the early evening before decreasing to 10
kts or less. Everywhere else winds should remain 10 kts or less
through the night.

Winds should increase again on Tuesday and gust to around 20 kts
at all sites except YKM and PSC, and gusts can`t be ruled out at
those locations either, just don`t have enough confidence to
include at this time.

&&

.Preliminary Point Temps/PoPs...
PDT  41  66  40  69 /  10   0   0   0
ALW  45  67  44  70 /  10   0   0   0
PSC  43  72  43  74 /   0   0   0   0
YKM  40  70  41  73 /  10   0   0   0
HRI  43  69  41  73 /   0   0   0   0
ELN  40  62  38  69 /  10   0   0   0
RDM  29  64  30  67 /   0   0   0   0
LGD  35  64  40  67 /  10  10   0  10
GCD  32  64  38  68 /  10  10   0   0
DLS  44  66  42  74 /   0   0   0   0

&&

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

&&

$$

DISCUSSION...86
AVIATION...77