Weekend Weather Blog: The uniqueness of this next system

Weekend weather lab: Inversions
Published: Feb. 19, 2022 at 7:58 PM MST
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TWIN FALLS, Idaho (KMVT/KSVT) — Hopefully you got out and enjoyed the nice weather on Saturday this weekend, because Sunday isn’t going to be quite as pleasant. An area of low pressure off to our west will bring a cold front into the forecast area Sunday evening into Sunday night, creating some interesting weather over the next 24-48 hours.

However, this system won’t be a typical system coming into southern Idaho. While over the past 5 months or so we’ve seen systems bring in periods of widespread precipitation, this one will be different. While we will likely see periods of precipitation, especially in the mountains, this will fall in a very narrow and concentrated band - one that will bring a quick burst of moderate to heavy precip, then fade out. This is known as convective precipitation - but how does it form?

Convective precipitation is nothing new to southern Idaho. In fact, anytime the region sees a summertime thunderstorm, that’s convective precipitation. It’s caused by bubbles of warmer air near the surface rising above cooler air above it. If air cools at a rate fast enough, the atmosphere is known as an ‘unstable atmosphere’ For these conditions, air must cool at a rate faster than 3.3°F per 1000 ft, or 6°C per 1 km. If this is the case, mountains or any type of front is able to initiate precipitation.

Where does Sunday’s event stack up? Let’s look at one of the best weather tools out there, the skew-t, to find out.

HRR Sounding
HRR Sounding(Pivotal Weather)

This may look chaotic, but there is an easy way to interpret this. Look at the box on the left: notice a couple of things: The red line, white dashed line, and green line. The red and green lines represent the temperature and moisture the atmosphere as going vertically in height above the earth’s surface. The White dashed line represents the threshold for instability.

When the red line is to the left of the white dashed line, the air is unstable. This allows for development of this type of precipitation to occur. Precipitation that develops by these methods can fall in fast rates, and in quick bursts - especially if the atmosphere is exceptionally unstable. While the atmosphere in this case isn’t extremely unstable, some moderate bursts of snow will be possible here.

Combining this with the air that will be forced to slope up the south hills or northern Blaine county mountains, and you’ll get some good snowfall rates over a short period of time. This will add up to quite a few inches of snow over the mountains - likely at least up to 5″, if not more by sunset on Monday.

What’s bringing this all about though? It all starts higher up in the atmosphere.

RAP 700 mb
RAP 700 mb(Pivotal Weather)

Look at how quickly the winds are pushing in cooler air in the middle levels of the atmosphere. This colder air aloft is moving in cooler air more rapidly than at the surface, allowing for air to cool much more rapidly with height, making the atmosphere fairly unstable.

Now not all of us will see snow. The mountains up north will act as a wall and prevent northern Magic Valley areas from seeing widespread accumulations. However, it should be noted that many areas will see at least a snowflake by Tuesday morning. As far as specific amounts? You’ll have to wait to see in my full forecast on KMVT news at 10.

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