textproduct: Pendleton

KEY MESSAGES

- Elevated to locally critical fire weather conditions through the week due to periods of localized breezy conditions, low RH, and hot, dry, and unstable conditions

- Widespread smoke and haze through at least mid-week, likely (90% confidence) persisting through Thursday

- Widespread Moderate to locally Major HeatRisk middle to late week.

DISCUSSION

Satellite imagery shows widespread smoke and haze across the forecast area this afternoon. Water vapor imagery reveals a weak shortwave trough passing through northwest flow aloft over the Northern Rockies. Meanwhile, an upper-level ridge is amplifying offshore in the Pacific. This pattern has set up a classic West Coast thermal trough (WCTT) to the west of the Cascades, leading to an offshore-directed pressure gradient and continued southward and westward smoke transport from the wildfires to our north in northern Washington and Canada.

Wildfire smoke is expected (99 percent confidence) to persist across the region through at least mid-week as the upper-level ridge amplifies and slowly tracks eastward. Winds will remain diurnally and terrain-driven and supportive of continued advection of smoke into the region. Air quality headlines have been expanded by air quality agencies to cover the entirety of our forecast area, and are in effect until further notice.

The ridge axis will move inland Wednesday through Friday, with increasingly hot temperatures forecast during the period. Currently, widespread Moderate to locally Major HeatRisk is forecast due to afternoon temperatures in the 95-100 degree range and lows in the 50s and 60s. That said, wildfire smoke inhibits efficient solar heating so afternoon temperatures may end up being several degrees cooler than currently forecast if smoke is sufficiently dense; forecast highs were decreased by roughly 2-5 degrees for this afternoon to account for how the smoke affected temperatures yesterday. Conversely, the dry air mass (PWATs of 0.2-0.6") should facilitate efficient nocturnal cooling, and smoke does not significantly affect nocturnal cooling.

While 12Z ensemble NWP guidance has reduced the strength and/or backed off on timing relative to yesterday's forecast, the upper-level pattern is still likely to change (70 percent chance) by late week into the weekend to a quasi-zonal pattern as ensemble clusters indicate some flavor of upper-level low or shortwave trough will likely approach or move into British Columbia; roughly half of ensemble uncertainty in the 500-hPa height field during this period is driven by differences in the magnitude of heights over the Pacific Northwest. A troughing pattern, or partial breakdown of the upper ridge, would lead to a reversal of surface pressure gradients and result in breezy to windy westerly winds through the Cascade gaps and across the Blue Mountain foothills and Columbia Basin. Wind coupled with low relative humidity in this scenario would lead to increasing fire weather concerns (see FIRE WEATHER section below). Moreover, breezy to windy westerly winds represent the best chance of scouring the widespread smoke.

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/

Aviation concerns revolve around smoke and smoke layers aloft from area wildfires. Latest satellite shows smoke aloft is largely translucent. Smoke is commonly reducing visibility down to MVFR 3SM-5SM at many sites with smoke layers central in central and north central OR around 3-4 kft/low-end VFR. Easterly-northeasterly flow regime aloft will continue to keep smoke at sites, however, some improvement owing to a lower transport of smoke is expected to work into northeast OR terminals based on hi-res guidance. Highest confidence in KPDT and KALW though a moderate signal is seen in central OR.

Otherwise, winds will remain largely light and predominantly N-E with increased winds in the afternoon/evening before lessening overnight.

FIRE WEATHER

Elevated to locally critical fire weather conditions are anticipated today across north-central Oregon as locally breezy east to northeast winds overlap with dry air.

Wednesday through Friday, the combination of dry air and hot temperatures will drive afternoon RHs into the single digits to teens. Predominantly poor to fair overnight RH recovery is forecast through the week, except for locally better recovery in drainages. Of note, deep mixing across the region, especially central Oregon and vicinity, may support plume-dominated fire. Have held off on issuing any Red Flag Warnings for hot, dry, and unstable conditions, but will continue to monitor the need. Moreover, high-resolution weather models (12Z HREF and 12Z REFS) indicate some potential (broadly 30-70%) in overlap of critical wind/RH thresholds in central and east-central Oregon tomorrow and Thursday.

Late week into the weekend, confidence is high (70 percent) in a return to breezy westerly winds through the Cascade gaps and across the Columbia Basin as a low-pressure system will likely (70 percent) approach or move into British Columbia. Confidence in the combination of critical winds and relative humidity is medium-high confidence (40-80 percent) for Friday and Saturday.

Preliminary Point Temps/PoPs

PDT 48 93 58 96 / 0 0 0 0 ALW 53 95 64 98 / 0 0 0 0 PSC 55 95 58 99 / 0 0 0 0 YKM 55 94 60 98 / 0 0 0 0 HRI 52 95 60 99 / 0 0 0 0 ELN 53 92 61 96 / 0 0 0 0 RDM 47 95 52 98 / 0 0 0 0 LGD 51 95 56 100 / 0 0 0 0 GCD 51 98 57 102 / 0 0 0 0 DLS 56 97 63 99 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...Air Quality Alert until further notice for WAZ024-026>030- 521>523. OR...Air Quality Alert until further notice for ORZ041-044-049- 050-502-503-505>511.