textproduct: Pendleton

KEY MESSAGES

- Mostly dry, mild, and locally breezy today.

- Region-wide precipitation chances return Monday, then pivot to the Washington Cascades on Tuesday.

- Mostly dry, warm, and locally breezy conditions east of the Cascades Tuesday through Thursday.

DISCUSSION

GOES-18 imagery shows a quasi-zonal jet over the forecast area this afternoon. Radar reveals some isolated showers across the mountains, mainly the Cascades, with otherwise dry conditions apparent within the lowlands. Upstream, troughing is evident in the northeast Pacific, and the shortwave in the vicinity of 38N, 146W is the main feature of interest for our forecast area as we move forward in time. The aforementioned shortwave is expected (95% confidence) to track east-northeast across the Pacific and move onshore Monday. The wave will facilitate widespread precipitation chances through the day. Otherwise, locally breezy southwesterly to westerly winds are forecast, and temperatures will be near to several degrees above normal.

The shortwave will likely (90% confidence) exit to the east by early Tuesday, with another wave and warm frontal passage slated for Tuesday. That said, precipitation chances on Tuesday will pivot to the Washington Cascades; locations east of the Cascades have low (20% or lower) chances of precipitation.

Ensemble guidance is in excellent agreement (at least 90% of members) that the 500-hPa pattern will amplify Tuesday through at least Thursday. Under this quasi-blocking pattern (troughing over the East, ridging over the Plains and West, and troughing upstream in the Pacific), well-above-normal temperatures and periods of breezy to windy southerly to southeasterly winds are forecast. Clusters of ensemble QPF show high likelihood (90% of members or greater) that notable precipitation will be confined to the Washington Cascades on Tuesday, with little to no QPF on Wednesday. Under this pattern, precipitation is largely restricted to the Cascades with warmer, drier conditions to the east under the upper-level ridge.

Significant uncertainty in the 500-hPa pattern develops in ensemble solutions Friday into the weekend. Ensemble guidance continues to trend towards a solution that splits off a piece of energy and develops a closed low offshore in the Pacific before eventually taking it south of our forecast area into California or the Southwest. That said, all members are showing another wave behind it, with a return to broad troughing in the Northeast Pacific, perhaps extending into the Pacific Northwest through the weekend. Should the additional troughing develop, precipitation chances will increase across the region heading into the weekend. Moreover, confidence is medium (40-60%) in breezy to windy conditions. Significantly cooler temperatures will be delayed if/when the trough/low moves onshore.

AVIATION /00Z TAFS THROUGH 00Z MONDAY/

VFR conditions currently across all sites, which will stay the course through the period. Breezy winds will subside this evening as a brief ridge of high pressure passes the area. An incoming system will push into the region from the southwest, bringing a return of light showers to most sites toward the end of the period. 75

Preliminary Point Temps/PoPs

PDT 39 59 43 64 / 0 10 60 0 ALW 43 57 46 62 / 0 10 90 10 PSC 38 57 38 61 / 0 10 50 10 YKM 36 55 36 56 / 0 40 40 20 HRI 39 58 39 61 / 0 10 50 0 ELN 29 50 31 50 / 0 30 70 30 RDM 31 59 37 64 / 0 60 30 0 LGD 37 58 39 59 / 10 10 70 0 GCD 36 60 38 63 / 10 10 60 0 DLS 42 56 43 59 / 0 60 90 20

PDT WATCHES/WARNINGS/ADVISORIES

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