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The rain shadow effect: Why many storms leave the Treasure Valley dry

Meteorologist Rick Lantz explains how the topography of mountains and valleys affect southern Idaho's weather.

BOISE, Idaho — Winter storms create some inconvenient, even hazardous, situations at times, but they're also important to southern Idaho.

One benefit: those winter storms fill our reservoirs with water for next summer. After a spring and summer in which every part of Idaho experienced some level of drought, that's going to be crucial for all who depend on a plentiful water supply.

Meteorologists especially look for those wet storms that come from the south. Those southerly storms provide snow, especially heavy snow, in the mountains.

Mountains surround the Treasure Valley, and that topography can affect the region's weather to such an extent that you may wonder, “What happened to the storm I heard about in the forecast?”

The phenomenon is known as the “Rain Shadow Effect”.

When storms come in over the mountains, the storm lifts and intensifies, providing plenty of snow in the mountains.

Yet, the same storm can be tricky. When the storm arrives over the Treasure Valley from the south, it can change because of the Owyhee Mountains. The same southerly storm rises on the south side of the Owyhees, cools and condenses, resulting in a lot of snowfall on the south side of the mountains.

When the same wet air falls on the north side into the Treasure Valley, the weather begins to change.

As the wet southerly air advances and falls on the north slopes of the mountains, the air begins to dry in the mountains' shadow.

Credit: KTVB

This Rain Shadow Effect is responsible for large storms weakening as they head into the Treasure Valley from the south, leaving the valley dry. The mountains will still get the snow, even if the valley remains dry.

It is our hope that these storms will drop heavy snow in our mountains during the winter, as the runoff from melting mountain snow is good for our reservoirs.

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