textproduct: Pendleton

KEY MESSAGES

- Hot, dry, and windy conditions enhance fire weather concerns. *Red Flag Warnings Active*

- Heat Peaks today, near normal temperatures by Thursday.

- Slight chance (10-20%) for isolated thunderstorms today.

DISCUSSION

Current radar and infrared satellite imagery shows dry conditions under mostly clear skies. This is in response to a transient upper level ridge building briefly in the wake of the departing shortwave that brought some isolated thunderstorms across Central Oregon on Monday. The present ridge will linger through the beginning of the day, ahead of an approaching upper level trough that will move onshore this afternoon. As this trough nears, a pressure gradient will develop between the ridge and trough, increasing winds across the Lower Columbia Basin and the Kittitas Valley. The earlier ridge of high pressure will keep skies clear through the early afternoon to allow for conditions to dry rapidly as temperatures soar. Afternoon humidities as low as 13% and west winds of 15-25 mph with gusts of up to 35 mph have warranted the issuance of a RED FLAG WARNING across the Kittitas Valley (WA690) and the Lower Columbia Basin of Washington (WA691) and Oregon (OR691) between the hours of 1 PM and 10 PM today.

The building of the transient ridge and approaching trough, as mentioned earlier, will also enhance southwest flow aloft and advect drier air from the south. The lingering high level clouds overnight have lead to morning lows of around 60 degrees, setting up potentially the hottest day so far this summer as highs reach to around 100 degrees across the Lower Columbia Basin and along the Blue Mountain foothills. The HREF advertises a 40-60% chance of 100 degree or greater high temperatures across the Tri-Cities area today, with a 20-40% chance for Yakima, Hermiston, and Walla Walla. This would be the first time since August 24, 2025 that Tri-Cities reached 100 degrees, with Yakima as August 25, 2025, Hermiston as August 30, 2025, and Walla Walla as August 23, 2025. Due to the lack of overnight relief and high temperatures about 10 degrees above normal, widespread moderate HeatRisk (2 or 4) extends across the Lower Columbia Basin, Blue Mountain foothills, John Day Basin, and the Yakima/Kittitas Valleys. This moderate category primarily relates to concern for members of heat-sensitive groups, but it also extends to the general public who are exposed to the sun for longer periods of time. Make sure to stay hydrated and reduce time or move events during peak afternoon hours.

The earlier mentioned synoptic features will also lead to a slight chance (10-20%) of isolated thunderstorms developing across portions of the area. The best chances will be across the Blue Mountains, Central Oregon, John Day Basin, and Union/Wallowa counties. However, cannot rule out a rouge cell developing across the Lower Columbia Basin, albeit low probability (5-10%). Several ingredients for thunderstorms will be present, including CAPE of 200-300 J/kg in the Basin and 700-900 J/kg across higher chanced areas, low level shear of 20-35 kts, and lapse rates of 7-8C/km. The limiting factor is a direct result of the present transient ridge through the early afternoon and the late arrival of the trough that would break up the warm, stable air of the ridge. As a result, there is too much Convective Inhibition (CIN) across the Basin and not enough over Central and Eastern Oregon. The lower CIN does provide enough atmospheric forcings to initiate isolated thunderstorms early in the afternoon, but will stay rather weak in nature as they increase cloud cover and cool lower levels of the atmosphere. Main concerns associated with any developing storm cells will be breezy winds and lightning. ECMWF AI ensembles and GFS ensembles both highlight precipitable water amounts of 0.7-1.0", which is 130-160% of normal - so any develop storm cells should provide a wetting rain (=>0.10").

Temperatures begin to cool and near normal values by Thursday behind the passing upper level trough and accompanying west- northwest flow aloft. Another upper level low slides down the British Columbia coast Friday as the high pressure center strengthens over the 4 corners region to bring southwest flow back to the area. ECMWF AI ensembles and the GFS AI ensembles both suggest these features will stall and slowly dissipate through the weekend and into next week. This will keep temperatures slightly above normal as dry conditions persist. 75

AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/

VFR conditions across all sites, which will stay the course through the period. Widespread breezy winds of 10-20kts this afternoon, with KDLS/KPDT/KRDM/KBDN/KYKM gusting to between 20-30kts this afternoon and evening. 75

FIRE WEATHER

A RED FLAG WARNING is in effect between 1 PM and 10 PM today for fire weather zones: WA690, WA691, and OR691 due to strong winds and low relative humidities. West-northwest winds of 15-25 mph and gusts up to 30 mph and humidities as low as 13% is expected across these areas. Critical fire weather conditions should materialize first along the western boundaries of WA690 and WA691, before including western OR691 by 1400 with concerns extending east through the remainder of the warning. There is a low, 5-10% chance of isolated thunderstorms for the Basin, but timing is expected toward the end of the period - likely after 2000 with minimal to no lightning expected.

Isolated fire weather concerns return for OR690 and WA/OR691 on Wednesday afternoon, but are currently confined to the eastern and northern borders of the zones. Afternoon humidities will be improved to around 20% with winds of 10-20 mph out of the west- northwest. These are marginal conditions and would result in elevated fire weather concerns - but may not reach Red Flag criteria. Further analysis is necessary.

Due to the overall pattern late in the week that persist through the weekend and into next week, elevated fire weather concerns linger across the Columbia Basin and Kittitas Valley. This is related to a stalled offshore trough and inland high pressure ridge that slowly dissipates through the period. 75

Preliminary Point Temps/PoPs

PDT 96 59 87 54 / 0 0 0 0 ALW 98 65 89 59 / 0 0 0 0 PSC 100 63 93 56 / 0 0 0 0 YKM 99 60 91 56 / 0 0 0 0 HRI 98 63 90 58 / 0 10 0 0 ELN 93 58 81 53 / 0 0 0 0 RDM 92 51 86 47 / 0 10 0 0 LGD 95 59 87 52 / 20 10 0 0 GCD 95 55 89 50 / 10 10 0 0 DLS 94 62 85 58 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...Red Flag Warning from 1 PM this afternoon to 10 PM PDT this evening for WAZ690-691. OR...Red Flag Warning from 1 PM this afternoon to 10 PM PDT this evening for ORZ691.