textproduct: Pendleton

KEY MESSAGES

- Warm and dry conditions will persist through the week

- Breezy to locally breezy conditions through most of the week in the Cascade gaps and OR Columbia Basin

- Elevated fire weather concerns for wind and relative humidity in the Cascade Gaps through Saturday, then elevated fire weather concerns for unstable conditions and thunderstorms Monday and Tuesday

DISCUSSION

Today through Sunday: Satellite imagery this afternoon shows a deck of cumulus pushing into far northeast OR, while the remainder of the forecast area is under mostly clear skies. At the surface, breezy winds have developed through the Cascade gaps, but are having a tough time pushing further east into the Columbia Basin and central OR.

Through Saturday, an upper level low in the Gulf of Alaska is expected to drop down to Vancouver Island then slide into southern BC. Ahead of the upper low, flow aloft will turn more southwest, allowing for slight warming into the lower to mid 90s in the lower elevations Friday afternoon. Saturday and Sunday, lower elevation temperatures are expected to lower back into the mid 80s to lower 90s behind the upper low exit. The exception is going to be the Snake River area along the ID/OR border. Temperatures in the river valley are expected to increase up to 105 degrees Friday and Saturday, bringing HeatRisk into the upper moderate to lower major category.

Otherwise, the upper low approach will result in increasing winds through the Cascade gaps Friday and Saturday, with west to northwest winds 20-30 mph and gusts up to 40 mph developing in the Kittitas valley and the eastern Gorge. This will bring elevated fire weather conditions into these areas, but a weak push of moisture with the incoming trough may keep relative humidity values just above critical thresholds. Of the two days, Friday will see the biggest threat, though probabilities of critical fire weather conditions being met is widely 25-45% across the Basin and central OR, however there is some localized areas of 50-75% chances in the OR Columbia Basin. At this time, have opted not to issue Red Flag warnings.

By Sunday, weak troughing will be left over from the main low exiting the region. While temperatures will moderate between Saturday and Sunday, winds will become locally breezy in the afternoon (confidence 65-80%).

Monday through Wednesday: Monday into Tuesday, there is moderate confidence (55-65%) that the PacNW will remain under a southwest flow aloft while a secondary upper low dives down and sets up off Vancouver island. The persistent southwest flow aloft will lead to increasing unstable conditions for Monday, which will bring elevated fire weather concerns to our existing fires over the southern Blue Mountains. A monsoonal moisture push rounding the upper high over the southwest CONUS will also bring a low chance (10-15%) of an isolated thunderstorm threat from the central OR Cascades to the Elkhorn crest Monday afternoon. By Tuesday, chances (15-25%) for isolated to scattered thunderstorms will spread across the high terrain between the Central OR Cascades and Wallowa county, bringing a elevated fire weather threat due to lightning. As of this discussion, confidence is moderate (55-65%) that red flag warnings may be needed for Monday and Tuesday.

Heat will be the secondary concern through this period, as temperatures are expected to warm back into the 90s across the lower elevations (confidence 65-85%). However, the lack of upper level ridging over the region will limit how hot it will get across the region, with chances of hitting the 100 degree mark only 15-35% across portions of the Columbia Basin.

Tuesday night into Wednesday, uncertainty grows amongst ensemble cluster solutions in regards to what happens with the upper level troughing. Solutions range from the trough weakening as it comes onshore, slowly coming onshore, or remaining offshore the PacNW. Looking for the most impactful solution, the slow moving onshore trough solution during this period (about 28% of members) would result in another round of thunderstorm activity across the mountains of the forecast area. Lawhorn/82

AVIATION /06Z TAFS THROUGH 06Z SATURDAY/

Westerly winds under 12 knots will prevail overnight and much of Friday until boundary layer mixing kicks off stronger gusts in the afternoon. Gusts of 20 to 30 knots will become more common after 21z with strongest gusts at DLS. VFR with clear sky and very dry near surface layer.

FIRE WEATHER

Breezy Cascade gap winds will continue into the weekend, producing an elevated threat of fire weather conditions. Afternoon RHs will generally be in the teens to mid 20s across the region through the period. Early next week, temperatures will be on the rise, as well as unstable conditions over the eastern Oregon mountains. In this area, a low chance (10-15%) of isolated thunderstorms will be possible Monday, with slight chance (15-20%) for isolated to scattered thunderstorms Tuesday.

Preliminary Point Temps/PoPs

PDT 56 90 56 87 / 0 0 0 0 ALW 61 93 62 90 / 0 0 0 0 PSC 59 95 60 92 / 0 0 0 0 YKM 59 94 57 90 / 0 0 0 0 HRI 60 93 60 89 / 0 0 0 0 ELN 56 87 54 83 / 0 0 0 0 RDM 50 89 47 88 / 0 0 0 0 LGD 54 94 55 94 / 0 0 0 0 GCD 53 96 54 96 / 0 0 0 0 DLS 59 88 60 85 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...None. OR...Heat Advisory from noon Friday to 7 PM PDT Sunday for ORZ050.