textproduct: Pendleton

KEY MESSAGES

- Periodic showers will continue to impact the region this weekend, with best chances across the mountains

- Low chances (15-25%) of isolated thunderstorms mainly across mountainous terrain through Monday

- Mostly dry conditions with light winds and seasonable temperatures the remainder of next week

DISCUSSION

Water vapor satellite imagery tonight shows an upper level low over the PacNW, with a shortwave impulse swinging north around the low and across the forecast area. Nighttime satellite imagery shows partly to mostly cloudy conditions across the forecast area with a few areas of stratus/fog evident in portions of central OR.

The aforementioned shortwave impulse will continue to lift north across the forecast area into the morning hours, with radar showing a ling of isolated showers across portions of the Columbia Basin and a few isolated showers/thunderstorms across the eastern mountains as well.

The PacNW will continue to be under the influence of the upper closed low through Monday, with persistent rain chances (rain/snow mix above 8kft) over the mountain zones. Afternoon temperatures across the region will be nearly 10-15 degrees below average each day, with afternoon high temperatures in the mid 60s to mid 70s today and tomorrow (confidence 85-95%). While the upper closed low will remain parked over the PacNW through Sunday, the center of the low will remain west of the Cascade crest through Sunday afternoon before pivoting northeast and out of the region Sunday night through Monday afternoon. (confidence 70-85%). Today, rain shower chances will be tied to the shortwave impulse passage this morning, followed by another shortwave passage this afternoon and evening. CAMs indicate that the best chances this afternoon will be along the WA Cascade crest and the northern Blues for more organized shower activity, with a few isolated showers elsewhere. Sunday, shower activity will become more widespread as the upper low swings east and north, with the best chances in the lower elevations Sunday afternoon and evening. Isolated thunderstorms will be possible over the mountains today as weak instability develops with additional support from the cold core of the low aloft. As for precipitation amounts, the Cascades and Blues will see 0.1 to 0.25 inches of daily precip (confidence 70-90%), with locally higher amounts where thunderstorms develop. Monday, the upper level low passing over the Cascades will lead to a tightening of the cross-Cascade surface pressure gradient, resulting in breezy to locally gusty winds (15-30mph with gusts 30-45mph) through the Cascade gaps and portions of the Columbia Basin. By Monday evening, what is left of the upper level low will have pushed east of the region, with showers diminishing area wide.

Tuesday, ensemble members are in good agreement of weak upper troughing developing from the PacNW into the eastern Pacific, though the forecast area will be within an area of split flow (confidence 65-70%). Winds will remain light under this pattern, with a warming trend bringing temperatures closer to normal. By Wednesday, ensemble members depict a weak cut-off low developing with remnants of TS Marie merging into the low well west of the central CA coast while near zonal flow develops over the PacNW later in the evening. This continues to result in dry conditions across the region with near seasonable temperatures and light winds (confidence 50-60%)

Thursday through Friday, disagreement grows amongst ensemble members in the evolution of the synoptic pattern. While there is great agreement in an upper trough diving south along the BC coastline Thursday, ensemble members come into disagreement into whether the cut-off low in the eastern Pacific merges into the upper trough or not. Around three-quarters of the members agree that the cut-off low will get swallowed into the flow with the remaining energy lifting across the PacNW with showers, however disagree on timing (~55% late Thursday, remaining 15-20% sometime Friday). Regarding these solutions, there is low confidence (25-30%) in precipitation details owing to the uncertainty of remaining moisture from TS Marie. Otherwise, the remaining members favor the low remaining in the eastern Pacific, with mostly dry conditions across the forecast area during this period. Lawhorn/82

AVIATION /18Z TAFS THROUGH 18Z SUNDAY/

Showers continue to prevail through the forecast area through tomorrow with on and off chances through the period. Will see greater chances of precip along the central Oregon sites in the later afternoon to evening hours tonight. Currently don't have MVFR or lower as confidence is lower in this set-up. There's a 5 to 15 percent chance we can see moderate showers moving over some of the TAF sites, reducing visibility and/or cloud decks down to at least MVFR levels.

Preliminary Point Temps/PoPs

PDT 70 45 68 48 / 30 10 30 40 ALW 72 50 71 52 / 30 10 10 60 PSC 75 48 73 50 / 20 0 20 30 YKM 72 51 72 52 / 40 10 30 10 HRI 73 49 71 50 / 20 10 30 30 ELN 69 45 71 48 / 50 10 20 20 RDM 67 40 61 37 / 20 60 90 50 LGD 72 43 72 46 / 30 10 50 80 GCD 70 45 69 46 / 40 10 70 80 DLS 74 55 71 55 / 30 30 50 10

PDT WATCHES/WARNINGS/ADVISORIES

WA...None. OR...None.