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

Much like yesterday, little to talk about this afternoon as visible satellite imagery shows mostly clear skies across the region. While not evident on satellite, however, layers of near surface smoke have filtered into portions of central OR and along the Blue mountain foothills last night resulting in a light haze in these areas.

Today through early Tuesday: An upper level ridge sliding across the PacNW today has resulted in modest warm-up across the forecast area, with most locations looking at afternoon temperatures 7 to 10 degree warmer than yesterday. Otherwise, winds will remain light through early tomorrow.

To the west of the upper level ridge, an upper shortwave trough has already made it's way south out of the Gulf of Alaska, and per water vapor imagery is around 550 miles west of the PacNW at this time. The upper trough will continue to amplify southward, with southerly flow developing over the region by tonight.

Tuesday through Friday: Confidence remains high (80-90%) 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 Blue 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 (10-25% chance) 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 50-80% in the lower elevations, and 85-100% 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 through Friday, ensemble cluster solutions are in agreement that the upper low will park along the PacNW coast through this period, resulting in light to moderate rain showers across the mountains and lower elevations. The extent of the cold core of the upper low over the region and weak surface instability developing over the mountains will also result in isolated to scattered thunderstorm development Thursday afternoon. By Friday, overall ensemble members favor the low weakening, limiting activity to just rain showers over the mountains, however ensemble cluster solutions show a disagreement on whether the closed low remains or if it becomes an open wave, with the open wave solution resulting in lower precipitation amounts.

Saturday and Sunday: Disagreement grows amongst ensemble members in regards to the evolution of the upper low/trough over the PacNW. About 60% of members favor the low remaining centered along the PacNW coastline, resulting in continued light showers over the mountains. Whereas the remaining 40% of the members favor the upper trough pushing inland, with precipitation diminishing west of the trough axis. Confidence in forecast details over the weekend is low (15-20%). Lawhorn/82

AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/

VFR conditions prevail through the period. Low chances (5 to 15%) that surface smoke will push conditions to MVFR temporally. Winds will start to strengthen in the Central Oregon sites, also including the DLS with a weather system approaching. Gusts could reach 20 to 30 knots at times, especially during the later part of the TAF period.

Preliminary Point Temps/PoPs

PDT 52 84 49 76 / 0 0 20 30 ALW 58 85 55 77 / 0 0 30 40 PSC 54 88 53 78 / 0 0 20 30 YKM 59 88 56 74 / 0 0 20 30 HRI 56 88 53 79 / 0 0 20 30 ELN 54 84 50 71 / 0 0 20 30 RDM 46 80 41 67 / 0 20 30 70 LGD 54 87 50 75 / 0 0 60 50 GCD 55 88 51 73 / 0 0 20 30 DLS 61 81 57 73 / 0 0 10 80

PDT WATCHES/WARNINGS/ADVISORIES

WA...Air Quality Alert until 10 AM PDT Wednesday for WAZ026-522-523. OR...Air Quality Alert until noon PDT Wednesday for ORZ041-505-506- 509>511.