textproduct: Pendleton

KEY MESSAGES

- Drying and warming trend will continue through early Tuesday as upper level high pressure moves over the region

- A series of upper level low pressure systems will bring periods of rain, breezy westerly winds, and isolated thunderstorm chances Tuesday into Friday

DISCUSSION

Little to talk about this afternoon as satellite imagery shows some wisps of cirrus streaming east of the Cascades and pockets of denser smoke around active wildfires in the Cascades and Elkhorn crest. At the surface, winds have mainly be light, though a few weak gusts have developed across portions of the Columbia Basin.

Today through early Tuesday: While temperatures will remain cooler than normal this afternoon behind yesterday's upper low passage, an upper level ridge sliding into the PacNW will result in another warm-up that will persist into Tuesday. Conditions will remain dry under the high pressure aloft, with mostly light winds into early Tuesday.

Behind the upper level ridge, an upper shortwave trough will amplify southward offshore out of the Gulf of Alaska, with southerly flow developing over the region by Tuesday morning.

Tuesday and into the weekend: Confidence remains high (75-85%) that an active period of weather will be in store for the forecast area as two low pressure systems impact the area.

Tuesday afternoon, the aforementioned shortwave trough offshore the PacNW will lift northeast with scattered rain showers developing along the Cascades, central OR, and the Blues mountains through Tuesday evening. The deep southerly flow ahead of the trough will result in weak instability developing over the area, though isolated thunderstorm chances (10-20%) will be limited to far northeast OR Tuesday afternoon. The approaching upper low will also result in pressure gradients tightening across the region, with breezy westerly winds developing in the lower elevations by Tuesday afternoon. Breezy winds will overlap with low relative humidity in the Harney Basin, promoting elevated fire weather conditions along the southern periphery of the Southern Blue mountains and central OR fire weather zones.

Wednesday, there is good agreement amongst ensemble guidance that the upper trough will lift northeast across the PacNW, providing the best chance of widespread rain showers across the mountains and lower elevations through the remainder of the period. During transit, the upper trough will take on a negative tilt that will promote isolated to widely scattered thunderstorms developing across the Blue mountains in the afternoon and early evening. Breezy west to southwest winds will develop across the lower elevations throughout Wednesday, but the fire weather threat will remain low owing to cooler temperatures and higher relative humidity across the area. The probability of wetting rains (>0.1 inches) will be 30-65% in the lower elevations, and 70-90% in the mountains for Wednesday. Meanwhile, ensemble cluster solutions remain in good agreement that a potent upper low will dive south along the BC coastline and arrive offshore Vancouver Island by Wednesday evening.

Thursday into Saturday, ensemble cluster solutions are in general agreement that the upper low offshore the PacNW will continue to impact the region with shower activity (especially over the Cascades), but diverge on the evolution of the upper low in regards to location and timing. The two main solutions are for the upper low to move inland and across the PacNW or stay offshore through the period. While there is agreement on the former occurring, ensemble cluster solutions are split on the timing of upper low moving inland. As what this means for the forecast, the solutions favoring the upper low moving inland will bring light to moderate rain showers to portions of the eastern mountains and the Blues; solutions favoring the upper low offshore keep shower chances tied to the Cascade crest and east slopes. While confidence remains moderate (60-75%) in rain showers continuing into the weekend, confidence is low (20-25%) in the details and timing. Lawhorn/82

AVIATION /18Z TAFS THROUGH 18Z MONDAY/

VFR conditions with mostly light winds at less than 12 knots. Not expecting any major impacts from smoke or haze, but the small chance (5 to 15 percent) still persists for all TAF sites seeing smoke makes it way down to the surface from the overnight to early morning hours.

Preliminary Point Temps/PoPs

PDT 44 86 51 83 / 0 0 0 0 ALW 50 89 57 85 / 0 0 0 0 PSC 49 89 54 88 / 0 0 0 0 YKM 54 86 59 87 / 0 0 0 0 HRI 49 88 55 86 / 0 0 0 0 ELN 47 81 54 82 / 0 0 0 10 RDM 41 87 46 80 / 0 0 0 40 LGD 44 89 52 87 / 0 0 0 10 GCD 47 92 54 87 / 0 0 0 0 DLS 55 88 62 81 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...None. OR...Air Quality Alert until noon PDT Monday for ORZ041-505-506- 509>511.