textproduct: Pendleton

KEY MESSAGES

- Precipitation across Washington and northern Oregon will taper off overnight.

- Dry, warm, and locally breezy to windy conditions east of the Cascades Wednesday into Friday, albeit with areas of fog and cooler temperatures in the lower Columbia Basin and adjoining valleys.

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

DISCUSSION

GOES-18 satellite imagery shows extensive cloud cover draped over the forecast area, thickest across Washington and northern Oregon where radar shows light to moderate returns. RAP-based mesoanalysis indicates ample northeastward moisture advection from the Pacific into northwest Oregon and western Washington this afternoon, with warm advection aloft supporting light to locally moderate precipitation across south-central to southeast Washington and far northern Oregon despite more modest inland moisture transport. Moisture advection will pivot northwestward overnight as an upper-level ridge develops across the West, and the region will dry out as a result.

Under the upper-level ridge, areas of fog will be possible overnight, especially where precipitation falls today; chances will be especially high (50-70%) in the lower Columbia Basin and adjoining valleys. Fog development will modulate temperatures over the next couple of days in these regions.

Ensemble guidance indicates the amplified 500-hPa pattern will then persist 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), above- normal temperatures, dry conditions, and periods of breezy to locally windy southerly to southeasterly winds are forecast. Highest confidence in above-normal temperatures is for central and eastern Oregon, with much higher uncertainty in the Columbia Basin where fog is forecast to have an impact. Confidence was a bit too low (50-70%) in localized advisory-level winds overnight and Wednesday night along the base and slopes of the northern Blue Mountains and in the southern Grande Ronde Valley to issue any headlines. 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 Wednesday through Thursday.

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 (95% 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 (40-80%) 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 THURSDAY/

Potential exists for development/expansion of fog and stratus following the recent rains. Light winds are expected at all terminals while mid and high clouds continue spilling over the cascade crest. Stratus in the the Gorge has led to visibility at DLS to bounce between one quarter mile up to one mile though the evening. DLS/YKM and PSC appear to have the greatest probabilities for LIFR/VLIFR conditions heading though the overnight and and early Wednesday morning based on HRRR model fields expanding the low visibility across the Columbia Basin combined with GFS LAMP guidance trends. RDM and BDN are the only terminal likely to remain VFR through 18z (80%).

Preliminary Point Temps/PoPs

PDT 45 66 39 63 / 0 0 0 0 ALW 49 65 45 63 / 10 0 0 0 PSC 38 58 36 56 / 10 0 0 0 YKM 38 60 37 58 / 20 0 0 0 HRI 39 60 36 57 / 10 0 0 0 ELN 36 56 36 55 / 60 0 0 0 RDM 39 72 37 70 / 0 0 0 0 LGD 44 64 44 62 / 0 0 0 0 GCD 39 70 36 68 / 0 0 0 0 DLS 45 61 43 59 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

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