Weekend Weather Blog: The southern Idaho radar hole

Weekend weather lab: Inversions
Published: Feb. 5, 2022 at 6:30 PM MST
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TWIN FALLS, Idaho (KMVT/KSVT) — I’m sure you’ve all heard either Eric, Max, or I talk about how it can be tough to see precipitation over southern Idaho on the radar - especially in the winter time when there’s snow. The two closest radars to Twin Falls and other surrounding communities are located in Boise and Pocatello, obviously not ideal.

The radars being far away is only part of the problem. How the radar works is the other part of the problem. Let’s talk about how radars work.

Radar beam propagation
Radar beam propagation(Jana Houser)

First and foremost, radars generate microwave energy that is sent out in a narrow beam at a certain elevation angle. This angle, combined with the earth’s curvature makes the radar beam get further and further away from the ground as you move away from the radar.

This leads us to another problem - how high up in the sky wintery precipitation extends. During the summer, thunderstorms can tower up to 50,000 feet above the ground. This allows radar beams at fairly high heights miles away from the actual radar sight to be able to see these thunderstorms. Below is an example from this past summer

Radar Example from Summer
Radar Example from Summer(KMVT)

Notice how the high values of rain are showing up well, even into western Twin Falls county. However, winter is a different story. Cold season precipitation is flatter in nature, only reaching heights of around 15,000 feet. This leads to the radar missing the precipitation altogether - known as overshooting. Below is an example from the winter season

Winter Example
Winter Example(KMVT)

Notice how when the precipitation is near the radar sites in Boise and Pocatello it shows up well. However, over Blaine county, there seems to be a precipitation hole - a result of the radar overshooting the precipitation.

Another problem that can be presented to us here in Southern Idaho is the mountains. Radars see precipitation by sending out pulses of microwave energy, and waiting on a reflection back from objects to see where exactly they are. When this energy is blocked by mountains or buildings, the radar can’t see what’s behind the barriers.

Let’s go back to our winter weather example. Notice over Camas and Boise counties how the precipitation is mysteriously disappearing. This is a result of the foothills to the north and west of Boise. The energy from the Boise radar is being blocked by the foothills to the north and west of Boise, keeping the radar from seeing precipitation over the Wood River Valley.

Bottom line, we need a radar somewhere in the Magic Valley. It would be a big help to any meteorologist who needs to forecast for the general area. For now, though, we’ll just have to wait.

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