April 4, 2010 in Idaho Voices

Finally, some weather to get excited about

Michelle Boss
 

This past week, the Inland Northwest received some much needed rainfall, saw the season’s first thunderstorms, and also experienced some strong wind gusts.

On Monday, southwest winds as strong as 54 mph were recorded at Fairchild Air Force Base. A peak wind gust of 44 mph was measured at the Spokane airport, 49 mph at Felts Field in Spokane Valley, and 49 mph in Coeur d’Alene. Sustained winds of over 20 mph were also prevalent during the late morning and early afternoon. What caused such windy conditions? One of the contributing factors was the location of a low-level jet stream, aligned southwest to northeast across northern Oregon and Eastern Washington. Winds in this jet stream had speeds up to 55 mph. Momentum from higher wind speeds aloft can be transferred down in several different ways.

One of the ways occurs through the mixing of air at different heights. This usually happens during the heat of the day as the sun warms pockets of air near the earth’s surface. Subsequent rising and sinking motions of the air serve to “mix” the energy of the higher speed air downward. This is why it is often windier in the afternoon than in the morning. Turbulent eddies caused by wind shear (change in speed and direction with height) or thunderstorms and even prolonged heavy rains can also mix higher momentum air downward.

Another contributing factor to the gusty winds was the passage of a cold front. Ahead of the front, winds were out of the south or southeast. Winds shifted to the southwest behind the front and picked up in speed due to a tighter pressure gradient and the push of the denser cold air.

Switching gears, I wanted to answer a question sent in by a reader recently. He wanted an explanation of the term “flood stage”. According to the National Weather Service, flood stage is “an established gauge height for a given location above which a rise in water surface level begins to create a hazard to lives, property, or commerce. Not necessarily the same as bankful stage.” In other words, flood stage may sometimes be set higher than bankfull stage (water flows out of its normal banks). In the case of rivers, gauges are located at various points on the river. A hydrologist with the National Weather Service would establish the flood stage for that particular point. Since the characteristics of the land surrounding the river, i.e. amount of development, slope, etc., are not homogenous, the determination of flood stage is only valid at the particular gauge location.

Feel free to continue to e-mail me with questions, comments, or ideas for future columns. I welcome the input!

Michelle can be reached at weatherboss@comcast.net


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