textproduct: Pendleton

KEY MESSAGES

- Slight chance of thunderstorms overnight

- Breezy to windy conditions expected across the Columbia Plateau overnight through Wednesday

- Moderate to heavy snow expected above 4500 feet in the Blue Mountains overnight through Wednesday

- Elevated streamflow on area streams and rivers Wednesday and Thursday

UPDATE

Expanded Wind Advisories to include the lower Columbia Basin of Washington and the Yakima Valley, and included the southern Blue Mountains above 4500 feet in the Winter Weather Advisories. Made minor adjustments to timing of winter headlines based on latest 00Z NWP guidance.

DISCUSSION...Issued 157 PM PDT Tue Apr 21 2026

Current satellite imagery shows a closed off low pressure system just offshore of Oregon and California bringing a maritime airmass into the PacNW region. High cloud cover tops are currently situated over Central Oregon that will continue to build into the area as the low moves to the east. Radar currently shows very light returns near the Cascade Mountains with a light thunderstorm developing in Umatilla County.

The synoptic pattern shows that the previously mentioned low pressure offshore will continue pushing east, jumping rain chances up to 80-100 percent by the later afternoon hours today across the Blue Mountains and Foothills, with more moderate 40-60 percent chances across the rest of the CWA. Our greatest thunderstorm window will be this afternoon through the evening hours today as 72% of clusters develop 500-750 J/kg of CAPE through the Blue Mountains into the Foothills & Kittitas Valley. HREF ensembles have a cluster of >40 dBZ & greater than 50 MUCAPE paintballs rapidly developing in the Blue Mountains and Foothills by the 3-4 PM window that move eastward going into the evening hours. By the late afternoon/early evening hours, the greatest thunderstorm chances will be in the Wallowas/Northern Blue Mountains as the low pressure system jogs across the east.

Guidance is in very good agreement that as the closed low moves to the Idaho/Montana region, our weather will be more influenced by a secondary trough located in British Columbia jogging south, that will bring heavier precipitation rates on Wednesday morning through the evening hours. Guidance points that the 6-hour precipitation rate through Wednesday morning will be between 0.15 to 0.35 inches, with the more heavier rain situated along the Foothills and the Blue Mountains. NBM and cluster analysis are in good agreement that the heaviest rain will occur Wednesday morning through the late afternoon with the maxima QPF situated across the Northern Blue Mountains with at least 2 to 3 inches expected (70-80% confidence). The high confidence is backed by the EFI showing 0.9 to 1.0 values across the Northern Blues into the eastern most parts of the Columbia Basin with a shift of tails value of 2. One last thing to note about the precipitation values is that cluster analysis mostly show 0.3 to 0.5 inch 6-hour rain rates in the more impacted areas, but about 9% of members suggests stronger orographic lift from the north/northwest winds to bring up to 1.25 inches of rain in a 6-hour window along the Foothills and small parts of the Oregon Columbia Basin. Although chances are small, the possibility to see some localized flooding in exists. Tight pressure gradients from the second trough will enable breezy to locally windy conditions embedded with the rain showers across parts of the Columbia Basin through the Simcoe Highlands & Kittitas Valley. Wind gusts will be as high as 50 mph (60-80% confidence) that will bring reduced visibility along roads, especially for higher profile vehicles.

Cooler northern air from the second trough will bring snow levels down to about 4500-5500 feet across the CWA, bringing high level mountain snow into the region. Although the NBM wants to bring 20-25 inches in certain parts of the Northern Blue Mountains (mainly above 4500 feet), there's great uncertainty that will manifest with the 10 to 1 snow ratio and being in a late season system. Temperatures are just below freezing Wednesday with the highs around 30-35 that could regress some of the snow accumulation. Given that, we don't have much confidence to warrant a warning for this system, however the range indicated in the advisory depicts that of a high-end advisory.

Thursday onwards will bring a drier and warmer period thanks to a rex-style high pressure pattern. Mostly dry conditions with a few showers developing in the Wallowas and Northern Blues but no major impacts expected through the weekend.

AVIATION /06Z TAFS THROUGH 06Z THURSDAY/

A low-pressure system will bring ample precipitation and breezy to windy conditions to the forecast area over the next 24 hours. Confidence is medium-high (50-90 percent) that steady rain overnight through Wednesday afternoon at PDT/ALW/PSC will bring periods of sub-VFR CIGs/VSBYs, while less persistent rain at DLS/RDM/BDN/YKM outside of the heaviest rainfall yields lower confidence (30-60 percent) in temporary sub-VFR conditions. VFR conditions are forecast to return to all sites by Wednesday evening.

Winds are expected (80 percent confidence) to be breezy to windy, locally very windy, overnight through the remainder of the period. 86

HYDROLOGY...Issued 359 PM PDT Tue Apr 21 2026

Moderate to heavy rain over the next 36 hours is expected to lead to rising levels on area streams and rivers. Currently, several forecast points along the Naches, Walla Walla and Grande Ronde rivers are expected to rise above action stage as early as tomorrow morning. Currently, the Umatilla at Gibbon and Pendleton are forecast to reach minor flood stage. The biggest concern would be along small streams and creeks flowing off the Blue Mountains where intense periods of rain could lead to rapid rises and minor localized flooding. However, the National Water Model is not indicating any significant flooding along these small streams and creeks. 83/77

Preliminary Point Temps/PoPs

PDT 43 51 38 58 / 100 100 40 40 ALW 44 51 41 58 / 90 100 50 60 PSC 48 61 42 65 / 90 100 20 30 YKM 49 63 40 67 / 80 80 0 10 HRI 45 55 40 62 / 90 100 20 20 ELN 45 56 39 59 / 80 80 10 10 RDM 36 52 26 61 / 50 50 10 0 LGD 40 46 36 52 / 100 100 80 50 GCD 38 46 31 54 / 100 90 70 20 DLS 48 58 42 65 / 60 80 10 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...Wind Advisory from 5 AM to 11 PM PDT Wednesday for WAZ024- 027>029-521. Flood Watch through Thursday evening for WAZ029. Winter Weather Advisory from 2 AM to 8 PM PDT Wednesday for WAZ030. Wind Advisory until 11 PM PDT Wednesday for WAZ026. OR...Wind Advisory from 5 AM to 11 PM PDT Wednesday for ORZ041-044- 507-508-510. Flood Watch through Thursday evening for ORZ502-507-508. Winter Weather Advisory from 2 AM to 8 PM PDT Wednesday for ORZ502-503.