textproduct: Pendleton

KEY MESSAGES

- Warming afternoon high temperatures, albeit with chilly morning lows for valley locations, and mostly dry through Tuesday under a building upper-level ridge.

- Active weather associated with a couple low-pressure systems is likely (80% confidence) to return late Tuesday and persist through the week.

DISCUSSION

Water vapor imagery shows a closed upper-level low departing to the east over the northern Rockies, leaving a cool northwesterly flow over the region in its wake. Meanwhile, a shortwave ridge is located just upstream, extending from offshore in the Pacific to inland over western British Columbia. This ridge will continue to slide eastward into the Pacific Northwest through Monday, resulting in dry conditions and warming daytime highs. That said, valley locations will continue to observe chilly morning temperatures owing to a dry air mass and predominantly light winds overnight under the ridge. Confidence is low (10-30%) in observing near-freezing temperatures Monday morning for cold-prone valleys of central and northeast Oregon.

Confidence is high (80%) in a return to more active weather as a couple upper-level lows influence the Pacific Northwest's weather Tuesday afternoon through the end of the week. Ensemble guidance takes the shortwave trough in the Gulf of Alaska southeast along the coast through Tuesday, resulting in amplifying southwesterly flow aloft over the forecast area. Coupled with an increase in PWATs and some weak instability, there is a slight chance (10-20%) of thunderstorms currently forecast over the Blue Mountains region Tuesday afternoon. Ensemble guidance is in loose agreement that the wave will then eject northeast across the Pacific Northwest Tuesday night through Wednesday, with continued chances of precipitation.

The main attraction, an anomalously deep, closed upper-level low continues to be advertised by all clusters of ensemble solutions late Wednesday through the remainder of the week. Ensemble systems have yet to come to a concensus on exact time of arrival, or if/when the low comes onshore; differences in location/timing among ensemble solutions accounts for approximately 40-60% of variance through the weekend. Regarding solutions, roughly half of the 00Z ensemble members take the low inland, or partially inland, by late Thursday and Friday, while the remainder keep it centered just offshore. The former solutions would be cooler with well-below-normal temperatures and widespread showers while the latter would be warmer and drier, increasingly so the farther offshore the low is centered.

It's too early to talk about exact precipitation amounts, but will note the NBM is advertising a widespread 50-80% chance of a wetting rain (0.10" or greater) for the lower elevations and 60-95% chance for the mountains between Tuesday night and Thursday night.

AVIATION /12Z TAFS THROUGH 12Z MONDAY/

VFR conditions are forecast through the period. Diurnally driven winds of predominantly 10 kts or less, with periodic higher gusts, anticipated at all sites.

Preliminary Point Temps/PoPs

PDT 76 45 86 54 / 0 0 0 0 ALW 76 51 89 59 / 0 0 0 0 PSC 79 48 88 53 / 0 0 0 0 YKM 79 53 85 57 / 0 0 0 0 HRI 78 48 88 54 / 0 0 0 0 ELN 75 47 80 53 / 0 0 0 0 RDM 78 40 86 45 / 0 0 0 0 LGD 78 43 89 51 / 0 0 0 0 GCD 79 46 91 53 / 0 0 0 0 DLS 80 55 87 61 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...None. OR...Air Quality Alert until noon PDT Monday for ORZ041-505-506- 509>511. Freeze Warning until 9 AM PDT this morning for ORZ050.