textproduct: Pendleton

KEY MESSAGES

- Above-normal temperatures Thursday into next week

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

DISCUSSION

Day Cloud Phase RGB imagery reveals few to scattered cumulus over the east slopes of the Washington Cascades and adjacent ridges that extend eastward through Kittitas, Yakima, Klickitat counties. Farther east, few to broken cumulus are present over the Blue Mountains. Aside from isolated showers over the higher sub-ranges of the Blue Mountains, dry conditions are forecast region-wide this afternoon.

Zooming out to examine the broader picture, a Rex block look- alike pattern is setting up across the region. An amplified upper-level ridge in the northeast Pacific extends from off the coast of the Pacific Northwest pole-ward through British Columbia. Meanwhile, a closed low is visible well off the coast of southern California (ensemble guidance takes it inland across the Southwest later Thursday through Friday, so will argue this is not a true blocking pattern). A couple shortwaves are evident upstream of the ridge, the latter of which will be the main feature of interest Friday through early next week.

Thursday, temperatures will warm to above normal (largely in the 70s for major population centers) as the aforementioned upper-level ridge slides overhead. The first shortwave noted earlier will almost certainly (95 percent confidence) track northeast of the forecast area into British Columbia and not significantly influence our weather.

By Friday afternoon, the second shortwave will approach the Pacific Northwest, deepening into a closed low or more amplified open-wave trough. Ensemble and deterministic guidance show some spread with regard to its location, which will modulate convective potential across the Cascade crest and western half of our forecast area. Solutions that are closer to the coast present a better chance of convection. Currently, have a very low chance (5-15 percent) of showers and thunderstorms for the Washington Cascade crest and areas east of the Cascade crest, while the Cascade crest of Oregon has a 15-25 percent chance of showers and 10-15 percent chance of thunderstorms.

All clusters of ensemble solutions take the upper-level closed low south along the coast of Oregon and California over the weekend. As a result, a slight chance to chance (10-40 percent) of showers and slight chance (10-20 percent) of thunderstorms remains in the forecast for the Cascades and Blues each day.

By Sunday, the upper-level pattern resembles more of a true Rex block with the closed low along the coast of California and an amplifying ridge pole-ward in the northeast Pacific, Pacific Northwest, and Canadian Rockies. This will lead to continued above-normal temperatures across the forecast area, especially north of central Oregon and the southern Blue Mountains.

Monday and Tuesday, all ensemble clusters take the closed low inland across California, the Great Basin, and the Southwest. As it begins its inland trek on Monday, the best chances (15-50 percent) of showers and thunderstorms (10-25 percent) will be over the Oregon Cascades and Blue Mountains. Analysis of QPF clusters suggests a roughly 35 percent chance of a widespread wetting rain for the Oregon Cascades and Blue Mountains while the remaining members show a more patchy precipitation signal.

To touch on forecast temperatures, while there is still uncertainty in how hot conditions will be under the ridge Friday into early next week (partly 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 medium to very high (50-99 percent) each day per latest NBM calibrated probabilities. Sunday through Tuesday, chances of reaching 90 degrees are low-medium (20-60 percent) for the Yakima Valley and lower Columbia Basin. Moreover, on Monday and Tuesday, there is a low-medium (20-60 percent) chance of HeatRisk reaching Moderate (level 2 of 4) in the Yakima Valley, lower Columbia Basin, and heat-prone portions of the eastern Columbia River Gorge as well as central to north-central Oregon. For context, this level of heat affects those who are sensitive to heat, especially those without cooling/hydration. 86

AVIATION /00Z TAFS THROUGH 00Z FRIDAY/

VFR conditions will persist through the forecast period. Winds will be light below 10 kts and variable. 90

Preliminary Point Temps/PoPs

PDT 43 74 45 80 / 0 0 0 0 ALW 46 75 49 79 / 0 0 0 0 PSC 43 79 44 84 / 0 0 0 0 YKM 45 77 47 80 / 0 0 0 0 HRI 42 78 44 82 / 0 0 0 0 ELN 42 73 44 76 / 0 0 0 0 RDM 33 72 37 78 / 0 0 0 0 LGD 38 70 41 75 / 0 0 0 0 GCD 37 71 42 76 / 0 0 0 0 DLS 47 79 50 82 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

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