If you are one of the few but hopefully growing readers of this blog, you’ve heard me throw out the phrases ‘elevated storms’ and ‘surface based storms.’ These are terms many in the weather world understand but are not often discussed in public. Hopefully this blog will help explain to you what it means and why it’s so important in forecasting severe weather.
It’s a commonly accepted theory that surface based storms generally occur in warm sectors, the area of warm atmosphere on the back side of the warm front. Elevated storms tend to reside in the cold sector of a storm system.
Surface based storms are usually the type of storms that can produce tornadoes. Elevated storms are more associated with large hail and strong, damaging winds. Of course this doesn’t mean either type of thunderstorm has a corner on any mode of severe weather.
Last week’s storms across Iowa provided us a classic example of surface and elevated based storms all at the same time. A warm front draped from east to west across Iowa was the focal point for thunderstorm development. To the north of the front, temperatures were in the 50’s. Just south of the front, temperatures reached the 80 degree mark. I drew a red line to show you the sharp temperature contrast in the Des Moines area on the afternoon of April 6th. All of this indicated a sign of a strong frontal boundary.
Now before the storms developed, we were concerned over the threat of a tornado outbreak. The threat was there, but it all hinged on where the storms developed and what type would take shape.
That morning, the warm front pushed north out of Missouri and into Iowa. But by the afternoon, the front reversed course and began slowly moving to the southeast. As storms developed along the boundary, they moved in an opposite direction, to the northeast.
This direction pushed the storms away from the front and into much colder air. What did that lead to? Elevated based storms. These storms lacked the surface heating to generate super cellular signatures that would lead to organized rotation and possibly tornadoes.
What the cold air instead did was greatly enhance the threat of large hail inside those storms. One particular storm struck Black Hawk County, dropping large damaging hail over the Waterloo-Cedar Falls area.
The hail caused widespread damage across many areas, damaging homes and cars. Fortunately no serious injuries were reported.
Had those same storms taken shape south of the warm front, or lingered along the boundary, they could have taken root in the heating at the surface. That difference could easily have produced the conditions favorable for tornadic development.
What saved Iowa this day from such an event in my mind was the direction of the storms in contrast to the direction of the front itself. Storms were unable to stay along the front. Although they tapped into the potential energy in the atmosphere, the fine details in supercell development simply weren’t there.
I hope this better explains the difference in surface and elevated based storms and what they mean during a severe weather event. They are key words when it comes to the development of thunderstorms and better help us understand why certain storms behave the way they do based on where and when they develop during a stormy day or night.