Critical Fire Weather Returns to Pacific Northwest: High Pressure, Low Humidity, and Wind

Forecasters at the National Weather Service are tracking a rebuilding high-pressure ridge aloft across the Western U.S. this week that is bringing a pronounced warming and drying trend to the Pacific Northwest. The pattern is generating critical fire weather conditions across large portions of eastern Oregon, eastern Washington, and southern Idaho.

Current Conditions

Relative humidity values throughout much of eastern Oregon and southern Idaho are forecast to fall into single digits today and tomorrow, with areas of 5-8% RH possible across the most exposed terrain away from the moderating influence of the Pacific. In eastern Washington, RH values in the 10-15% range are expected across the Columbia Basin and Yakima Valley. Combined with westerly wind gusts up to 30 mph on the Snake River Plain and afternoon temperatures well into the 90s, these conditions create a setup for rapid fire spread.

Red Flag Warnings Issued

The National Weather Service offices in Pendleton and Seattle issued Red Flag Warnings for central Washington and Oregon on June 20, citing low relative humidity and windy conditions across the Yakima Valley, Columbia Basin, Eastern Columbia River Gorge, and the mountains of central Oregon.

A Red Flag Warning means that critical fire weather conditions are either occurring now or will shortly. Outdoor burning is strongly discouraged and may be prohibited under local ordinances. All campfires should be fully extinguished before leaving any site.

Lightning Holdovers a Concern

Thunderstorms moved through portions of the northern Intermountain West late last week, producing lightning strikes across rangeland and forest. Fire managers are monitoring for holdover fires from those strikes, as lightning-caused smolders can remain dormant for several days before emerging as active fires when temperatures rise and humidity drops.

Fuels Condition

Drought conditions across much of the Pacific Northwest have left fine fuels -- the grasses and shrubs that carry the initial spread of most wildfires -- significantly drier than normal for this time of year. Below-normal snowpack in the Oregon Cascades, the Blue Mountains, and the Idaho ranges means that moisture that would normally still be feeding into the soil through June has already been depleted.

The combination of dry fuels, low relative humidity, high temperatures, and gusty winds represents the full suite of conditions that drive catastrophic wildfire behavior. Residents in fire-prone areas should review their evacuation plans and ensure their properties have been defensibly spaced ahead of the coming days.

Outlook