textproduct: Pendleton

KEY MESSAGES

- Elevated fire weather concerns due to breezy winds and low relative humidity across southern portions of the Blue Mountains region today

- Increasing monsoonal moisture early and mid-week will lead to increasing (15-40%) chances of high-based showers and predominantly dry thunderstorms

- Significant uncertainty in widespread Moderate (level 2 of 4) to locally Major (level 3 of 4) HeatRisk by mid/late week

DISCUSSION

Water vapor imagery reveals a broad area of high pressure centered over the Four Corners region. Meanwhile, a low-pressure system is churning just offshore of British Columbia. This is placing the Pacific Northwest under southwesterly flow which is transporting weak monsoonal moisture into the region from the Southwest, resulting in increasing high clouds. Hot temperatures are also in place at the surface, especially across the eastern half of our forecast area. A Heat Advisory remains in effect for Hells Canyon below 3000 feet through Sunday evening where afternoon high temperatures of 95-105 degrees are forecast.

Tonight, a shortwave visible offshore will round the base of the low and track across western Washington into Sunday, but little to no precipitation is forecast to accompany the wave with only a slight chance (15-20%) of precipitation for the Washington Cascade crest.

Looking ahead to Monday, ensemble NWP guidance is in reasonably good agreement in pattern details. Broadly, the high that is centered over the Four Corners is expected (95% confidence) to expand northeastward over the Great Plains region, and the development of additional troughing offshore in the Pacific will keep southwesterly flow over the Pacific Northwest. This will lead to increasing precipitable water (PWAT) values in the range of 100-200% of normal per ensemble means through mid-week.

While PWATs will be high enough to support wet thunderstorms, quick storm motions of roughly 30-40 kts coupled with dry boundary layer conditions will reduce the chance of wetting rains. Current probabilities of precipitation (PoP) range from 15-40% for central Oregon and the Blue Mountains Monday afternoon through Wednesday, with the best chances on Tuesday. Chances of thunderstorms are a bit lower in the 10-25% range. Chances of wetting rains are even lower.

Tuesday through the remainder of the week, calibrated probabilities from the National Blend of Models (NBM) indicate low (5-40%) chances of afternoon high temperatures reaching or exceeding 100 degrees across much of the lower elevations. Moreover, probabilities of Major (level 3 of 4) HeatRisk are also low (5-40%). This level of heat affects anyone without cooling/hydration.

Significant uncertainty in details of the 500-hPa pattern remains for Thursday as ensemble guidance is struggling to come to a concensus on the evolution of a closed low which may (80-85% chance per 00Z ensemble clusters) track inland sometime between Thursday and Friday. The alternative scenario is that the area of low pressure will remain offshore and a strong ridge of high pressure will build over the Pacific Northwest and Rockies. The former scenario would provide shower and thunderstorm chances along with somewhat cooler temperatures while the latter scenario would lead to dry conditions and hot temperatures.

AVIATION /00Z TAFS THROUGH 00Z MONDAY/

VFR conditions continue with mid to high level clouds moving into the area with no low ceiling concerns expected. Breezy winds initiate at DLS/PDT/RDM/BDN/YKM with gusts up to 20-30 knots expected to persist through the rest of the afternoon through the early evening in some spots (with the DLS expecting to linger later into the evening and early morning hours). Otherwise no CIG or VIS issues expected.

FIRE WEATHER

Elevated fire weather conditions will continue today due to breezy winds and low relative humidity (RH). The highest chances of overlapping wind/low RH thresholds will be centered across the Blue Mountains region, especially the Southern Blue Mountains (OR698), southeast portions of the Central Mountains of Oregon (OR700), and the John Day Valley (OR696) where there are widespread 40-70% chances (locally higher) of meeting low- end Red Flag criteria. Have held off on issuing any headlines due to marginal confidence. Chances of reaching low-end Red Flag criteria are lower (20-40%) farther north in the Columbia Basin region, particularly in and around the Yakima Valley (portions of WA691, WA690, and the lower elevations of WA694).

Lighter winds will yield limited concerns Sunday, though there is still a low (10-30%) chance of meeting wind/RH thresholds across portions of OR698/OR700 during the afternoon hours.

Increasing monsoonal moisture in the early to mid-week time frame will facilitate increasing chances of high-based showers and predominantly dry thunderstorms. Chances of measurable precipitation peak Tuesday afternoon at 15-40% across central Oregon and the Blue Mountains. Confidence is still low in forecast details, especially by Thursday.

Preliminary Point Temps/PoPs

PDT 56 83 56 90 / 0 0 0 0 ALW 61 84 60 92 / 0 0 0 0 PSC 60 87 57 93 / 0 0 0 0 YKM 58 87 56 94 / 0 0 0 0 HRI 59 86 58 93 / 0 0 0 0 ELN 53 81 52 90 / 0 0 0 0 RDM 51 86 52 91 / 0 0 0 0 LGD 57 87 57 92 / 0 0 0 0 GCD 56 91 58 92 / 0 0 0 10 DLS 59 87 59 94 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...None. OR...Heat Advisory until 7 PM PDT Sunday for ORZ050.