Breaking Down Monday’s Isolated Severe Weather Threat
A marginal risk of severe weather is in place for southern and eastern Cassia County Monday
TWIN FALLS, Idaho (KMVT/KSVT) — The Storm Prediction Center has placed portions of southern Idaho under a marginal severe thunderstorm risk on Monday.
In the above image, the regions in dark green are considered to be within the marginal risk. What this means is any thunderstorm that pops up within this dark green area has a chance at becoming severe, with damaging winds in excess of 50 mph.
However, the ingredients aren’t terribly strong, and not every storm within the risk will become severe. What are the ingredients coming together today? A combination of thunderstorm energy and somewhat dry air in the lower levels of the atmosphere
Above is a screenshot from one of the forecast models looking at thunderstorm energy. In essence, any value above 500 indicates that there is the potential for some isolated strong updrafts, which in turn will generate a thunderstorm. It’s a good idea to also look for regions of upward motion, like a mountain or converging winds to prognose where a thunderstorm may go up.
As you can see by the arrows, there doesn’t appear to be a terribly strong region of converging winds. Therefore, thunderstorms will likely go up in the mountains and South Hills. With stronger values into Cassia county, it looks like stronger updrafts will set up there.
Another ingredient coming together with this overall environment is dryer air near the earth’s surface. The above image is a hypothetical vertical snapshot of the atmosphere over southern Cassia county around 4 PM. I’ve circled two areas that are very important.
Where the red line and green line are far apart, it means the air is very dry. Notice how the red and green lines are very far apart lower in the atmosphere. This in turn has created very high values of ‘DCAPE’, or the energy available to the winds coming out of a thunderstorm.
When This value exceeds 1000, there’s a good shot that winds will be strong in thunderstorms that initiate in this environment. Why does dry air enhance this number? Dry air causes evaporation of falling precipitation, which in turn creates cooler air. This cooler air is more dense, and hits the ground a lot faster than the less dense air, causing a heightened wind threat.
Later in the afternoon and into the evening hours is when we can expect this threat to occur Monday. As always, have multiple ways to receive warnings, and stay up to date with KMVT, as we’ll keep you updated throughout the day.
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