textproduct: Pendleton

KEY MESSAGES

- Region-wide precipitation chances today as a frontal system moves inland.

- Weak warm frontal passage Tuesday, with precipitation chances pivoting to Washington and northern Oregon.

- Dry, warm, and locally breezy conditions east of the Cascades Wednesday into Friday.

- Active weather will likely (90% confidence) return later Friday into the weekend.

DISCUSSION

GOES-18 imagery shows a shortwave trough and decaying frontal band moving inland early this afternoon. Radar reveals showers across roughly the western half of our forecast area, which will spread eastward to encompass the remainder of the region through the afternoon and evening hours. Otherwise, there are some locally breezy southeasterly winds through the Grande Ronde Valley, as well as some southwesterlies down into the lower elevations of central Oregon.

The shortwave will likely (99% confidence) exit to the east by early Tuesday, with another wave and weak warm frontal passage slated for Tuesday. That said, the best precipitation chances on Tuesday will pivot to the Washington Cascades; locations east of the Cascades across northern Oregon and southeast Washington have low-medium (15-50%) chances of measurable precipitation, and the southern half of our forecast area will likely (90% chance) remain dry. Anticipating another round of locally breezy southerly to southeasterly winds across central to northeast Oregon on Tuesday, but not expecting them to reach advisory thresholds (90% confidence).

Ensemble guidance indicates 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 locally windy southerly to southeasterly winds are forecast. Additionally, moisture transport will be pushed northwest of our forecast area into northwest Washington and British Columbia; correspondingly, clusters of ensemble QPF show high likelihood (~95-100% of members) that notable precipitation will be across northwest Washington and British Columbia with little to no precipitation for our forecast area.

Significant uncertainty in the 500-hPa pattern develops in ensemble solutions Friday into the weekend, but ensemble clusters do suggest some flavor of active weather is likely (90% confidence). Ensemble guidance continues to trend towards a solution that splits off a piece of energy in the Pacific on Thursday and develops it into a closed low offshore and well south of our forecast area by Friday. That said, all members are showing another wave or closed low behind it, with a return to broad troughing or a large closed low in the Northeast Pacific and into the Pacific Northwest Friday into the weekend. Precipitation chances increase across the region later Friday and persist through the weekend. Moreover, confidence is medium-high (40-70%) in breezy to windy conditions accompanying the trough/low. Significantly cooler temperatures will be delayed if/when the trough/low moves onshore, and confidence in advisory-level snowfall is low (<40%) for any mountain zone over the weekend.

AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/

An arc of precipitation spreading rapidly eastward across the Columbia Basin into the Blue Mountain foothills this evening. By about 8 UTC, the more organized rains should have ended with only lingering isolated to widely scattered showers around PDT and ALW. All terminals are in VFR category and expected to remain in that category as no widespread fog development is showing in the models at this time for the overnight. Another round of rain is expected to be focused farther north of the Columbia River for the late morning to early afternoon hours and could impact terminals like YKM/DLS/ALW/PDT from late morning to late afternoon with mainly VFR/Brief MVFR vsby/cigs (30 percent confidence).

Preliminary Point Temps/PoPs

PDT 43 67 43 66 / 80 10 10 0 ALW 47 64 48 66 / 90 30 10 0 PSC 38 61 37 59 / 70 30 20 0 YKM 35 57 38 59 / 90 50 30 0 HRI 39 62 39 60 / 70 10 10 0 ELN 30 50 35 56 / 90 80 30 0 RDM 37 65 38 69 / 50 0 0 0 LGD 39 59 43 63 / 80 10 10 0 GCD 39 64 38 69 / 40 0 0 0 DLS 43 61 44 61 / 100 30 10 0

PDT WATCHES/WARNINGS/ADVISORIES

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