textproduct: Pendleton

KEY MESSAGES

- Upper level ridging over the region will bring a warming and drying trend across the forecast area, with high end moderate to major HeatRisk developing in and around the Columbia Basin

- Monsoonal moisture push with shortwave impulses will result in a return of isolated to scattered thunderstorm activity across central OR and the northeast OR mountains

- Upper level troughing offshore will produce breezy winds in the Cascade Gaps and Columbia Basin late Thursday into the weekend

DISCUSSION

Partly cloudy conditions across the forecast area are evident on satellite imagery tonight, with little activity to note on radar. Surface observations across portions of the forecast area do show lowered visibility from smoke settling near the surface.

Sensible weather concerns through Thursday will revolve around a strengthening upper level ridge producing hot temperatures in and around the Columbia Basin; daily shortwave impulses riding up the ridge will tap into monsoonal moisture and increasing instability, producing isolated to scattered thunderstorms in the afternoons and evenings.

Through early Thursday, the PacNW will see an upper level ridge of high pressure gradually strengthen each day while deep southerly flow persists aloft. This will introduce increasingly hot temperatures across the lower elevations, with the brunt of the heating expected in the Columbia Basin/Gorge and adjacent foothills/valleys through Thursday. Portions of the OR/WA Columbia Basin and the eastern Gorge are forecast to reach the 100 degree mark as early as this afternoon, with the Blue Mountain foothills, Yakima/Kittitas valleys, portions of north central Oregon, and portions of the John Day Basin reaching 100 degrees Wednesday(confidence 60-85%). While model guidance depicts the warming temperatures, there is plenty of uncertainty surrounding the impacts regional smoke may have on daytime heating. Model guidance struggled with the impacts of the smoke yesterday, with many areas seeing afternoon temperatures well below forecast high temperatures. With smoke expected to continue impacting portions of the region today, there is moderate confidence (45-55%) that those areas(portions of the Blue Mountain Foothills, Columbia Basin, central OR, and eastern mountains) under the heaviest smoke will see afternoon temperatures lower than forecast. Hot temperatures will continue in most areas Thursday, however increasing winds through the Cascade Gaps late afternoon may limit heating through the eastern Gorge and the Kittitas valley (confidence 50-65%). Between elevated afternoon and overnight temperatures, HeatRisk values across much of the lower elevations will be in the moderate to major categories starting Tuesday, resulting in the issuance of Heat Advisories across the Columbia Basin/Gorge, Yakima Valley, and the northern Blue Mountain foothills.

Through Thursday, ensemble and deterministic guidance depicts a monsoonal moisture push riding the southerly flow into the intermountain PacNW, with daily PWATs around 0.75 to 1.25 inches. Instability will be increasing each day, with the best chances of thunderstorm activity expected Wednesday and Thursday. This afternoon, there is a slight chance (15-20%) of isolated thunderstorms developing across the central OR Cascade east slopes and across the mountainous terrain of southern and eastern Grant county. MUCAPE this afternoon will peak around 250-500 J/kg, with low level lapse rates around 8-9 C/km and 0-6km shear between 20-30kts. On Wednesday, the main thunderstorm activity will be focused across central OR and the mountainous terrain from the Cascade east slopes to the Elkhorn crest. In these areas, MUCAPE will be between 500-1000J/kg, low level lapse rates 8-10 C/km, and 0-6km shear between 20-40kts. These conditions will be more than enough for thunderstorms to develop in the afternoon and become organized as they propagate north. By Thursday, similar conditions will be present along the WA Cascades, Blue Mountains, and Wallowa county. Expect at least small hail, gusty outflow winds, frequent lightning, and a few storms to produce heavy downpours.

Late Thursday through the weekend, there is good agreement amongst ensemble cluster guidance that upper troughing will develop off the coast of the PacNW while upper level ridging pushes east and south. Southwest flow aloft will prevent monsoonal moisture from advecting into the region, resulting in drying conditions across the forecast area. The proximity of the upper trough Friday and Saturday will also result in breezy west to northwest winds(15-25mph with gusts 25-35mph) through the Cascade gaps and across the Columbia Basin. Confidence remains moderate (45-65%) that peak winds will overlap with critically low RHs in portions of the Columbia Basin and Cascade Gaps Friday and Saturday, resulting in elevated fire weather conditions those days. Sunday through Monday, there remains good agreement that the trough will retrograde slightly into the northeast Pacific, resulting in weaker Cascade gap winds and continued dry conditions(confidence 50-60%). Lawhorn/82

AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/

VFR conditions generally expected through Tuesday, with winds 10 kts or less.

However, smoke from area wildfires could cause MVFR or even lower conditions at almost any site through the period.

FIRE WEATHER

Conditions will remain hot and dry into the middle of the week, with hottest conditions in the Columbia Basin and adjacent foothills/valleys. However, areas under heavy smoke will see less heating than anticipated. Monsoonal moisture and increasing instability will bring isolated to scattered thunderstorm chances through Thursday. Today, a slight chance of isolated storms will be focused over the central OR Cascade east slopes and elevated terrain in southern/eastern Grant county. Wednesday, chances (15-30%) will be focused across central OR and the eastern mountains. Chances (15-30%) Thursday will be focused along the WA Cascade crest, Blue Mountains, and Wallowa county. Thunderstorms will be capable of frequent lightning, gusty outflow winds, small hail, and brief heavy downpours. Winds will increase through the Cascade Gaps and Columbia Basin Friday and Saturday, and are expected to overlap with near to critically dry conditions. Expect elevated to critical fire weather conditions those days. Lawhorn/82

Preliminary Point Temps/PoPs

PDT 96 66 99 68 / 0 0 0 0 ALW 98 71 101 72 / 0 0 0 0 PSC 100 68 103 70 / 0 0 0 0 YKM 99 70 103 72 / 0 0 0 10 HRI 99 70 103 72 / 0 0 0 0 ELN 97 68 101 70 / 0 0 0 10 RDM 94 62 96 61 / 0 10 20 40 LGD 97 67 99 67 / 0 0 0 10 GCD 96 64 96 63 / 0 10 10 10 DLS 101 73 104 73 / 0 0 0 20

PDT WATCHES/WARNINGS/ADVISORIES

WA...Heat Advisory from 11 AM this morning to 9 PM PDT Thursday for WAZ024-027>029. Air Quality Alert until 9 AM PDT Wednesday for WAZ028>030. OR...Heat Advisory from 11 AM this morning to 9 PM PDT Thursday for ORZ041-044-507. Air Quality Alert until 4 PM PDT Wednesday for ORZ044-049-050- 502-503-505>509-511. Red Flag Warning from 11 AM to 11 PM PDT Wednesday for ORZ698- 700-705.