August 29, 2010 in Idaho Voices

Weather combinations lead to red flag warnings

Michelle Boss

Growing up in the Midwest, I experienced severe thunderstorm warnings, flash flood warnings, and even tornado warnings.

Not once, however, do I ever recall hearing the term red flag warning. The National Weather Service issued a red flag warning Thursday from the east side of the Cascades through North Idaho and into Western Montana.

In Oklahoma we had lightning, plenty of hot days, and storms which brought a lot of wind. The difference between summer weather in the Midwest and that across the Inland Northwest, has to do with moisture. Across the Inland Northwest, our driest time of the year happens to coincide with the hottest. When we finally do get thunderstorms, they are often starved for moisture and end up producing a lot of lightning, with very little rainfall. In the Midwest, summer storms often blanket the area with drenching rains.

Here, the combination of low humidity, dry lightning and gusty winds often results in the National Weather Service issuing a fire weather watch or red flag warning. The warning indicates that conditions are favorable for explosive fire growth. Each weather service office has different criteria for issuing these types of warnings, but in Spokane, here is what the forecasters are looking for:

• Dry thunderstorms with sufficiently dry fuels. Lightning would provide the spark, while dried out vegetation would provide fuel for the fire.

• Sustained surface winds exceeding 10 minute average of 15 mph, in combination with low relative humidity of less than 15 percent to 25 percent depending on location. Strong sustained surface winds help fires spread by not only fanning the flames, but by transporting embers. Low humidity encourages continued drying out of vegetation. These conditions are typically associated with the passage of a dry cold front.

• A Haines index of 6 (in a range of 2 to 6 with 6 indicating the highest potential for wildfire growth). The Haines Index was developed by Forest Service meteorologist Don Haines. The index takes into account the stability of the atmosphere, as well as the dryness of it. An unstable atmosphere is associated with rising motion, which encourages the growth of the smoke column in an existing fire. This can increase the chances of spotting or crowning. Spotting concerns the transporting of embers, and crowning has to do with the movement of the fire at the top of area vegetation, which can be independent of the movement of the surface fire.

Though we may think of fire weather season as coinciding with summer, the season can extend well into October depending on weather conditions. It has been a relatively quiet year for large wildfires across our area, but this month’s drier-than-normal conditions could turn things around rather quickly if Mother Nature brings abundant lightning to the area, or people are careless with fire.

Michelle Boss can be reached at

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