Scattered strong storms have fired up across north Alabama this week, producing strong winds that have knocked down trees and power lines.
Summer storms are more simple than the severe storms of the other seasons: updrafts and downdrafts. Hot, humid air rises (quickly) creating the storm, but eventually that air cools, condenses out water vapor, and gets too heavy for the updraft to hold. That’s when it crashes to the ground in what we call a ‘downburst.’
A downburst that exhibits damaging winds for 2.5 miles or less in diameter for 2-5 minutes is known as a microburst. A downburst with damaging winds greater than 2.5 miles in diameter for 2-5 minutes is known as a macroburst.
Those downbursts can fall to the ground at up to 100 miles per hour, and when they hit the ground all of that energy spreads out on the ground as damaging winds.
Here’s an example of what a downburst looks like in person. Often, the heavy rain-cooled air surges to the ground in a matter of seconds to a few minutes, almost a “blink and you miss it” situation.
Here’s an example of what it looks like on radar looking at the storm’s vertical characteristics. Why can’t see see this in ‘real time?’ It’s simple. Radar gives us a series of snapshots that we have to piece together. Once this ‘collapse’ occurs, the wind has already happened and it’s too late to warn you about it.
We’re over halfway to the end of the summer storm season
Let’s put some boundaries on summer storm season in North Alabama. We move from the spring tornado season straight into the scattered daily downpours sometime around mid-May, and we usually stay in that pattern until at least early September. The true peak of the season is the month of June; after that, we usually see a slow recession in the number of storms week-to-week through August and into September.
You can see that trend in the climatological (historic) odds of severe weather because a lot of summer storms get nasty with high winds. The normal trend is for fewer of those through the end of the summer to the point that August, September and October end up being the driest months of the year.
Worst all year you say?
Yep! Some of the nastiest storms of the year happen in the summer season, but they are not always ‘severe’ like the powerful, destructive storms of Spring and Fall (the ones that bring the extreme winds and tornadoes). In fact, the video below shows heavy rain and relatively intense winds — however, the wind gusts remain below severe limits (58+ mph or greater).
Summertime storms work differently than those driven by dynamic jet-stream-driven storm systems, and it can be very, very difficult to ‘see‘ what they’re doing on radar because they aren’t spinning or causing long-term strong winds that are more easily tracked in the Spring/Fall storms.
A storm does not have to be ‘severe’ to bring strong winds that can break branches on trees, heavy rain that can cause flash flooding, and dangerous (deadly) cloud-to-ground lightning.
Sometimes these storms congeal into a large MCS: a mesoscale convective system. Those big masses of thunderstorms can do a lot of damage while producing only downbursts. One particular MCS from last summer stands out as a prime example of how nasty these things can be: the derecho of June 28, 2018.
Here’s the deal. It does not matter if the wind is swirling in a funnel with a Tornado Warning or if it’s blowing along the ground in a straight line; wind gusts that high are dangerous.
Anytime there’s a storm in the area, take it seriously: especially if there is a Severe Thunderstorm Warning!
You can get free lightning alerts, warnings, and notifications about storms in the general area with Live Alert 19!