Weekend Feature: Do Rivers Weaken Thunderstorm Complexes?

It's a warm and muggy summer evening, lightning flashes in the distance as a complex of thunderstorms races eastward through Iowa heading right towards the Sauk Valley. You pull up the latest radar loop on your phone thinking, "Wow, that's one impressive storm complex! Looks like we are in for quite the soaking!" An hour passes by and as the storms near the Mississippi river, a sudden weakening trend appears in the newer radar scans. "NOT AGAIN!"

It seems like a common occurrence especially either early in the morning or late at night. So what gives? Is the Mississippi River really affecting thunderstorm complexes that try to move into northern Illinois?

The simple answer is, NO. Here's why:

Thunderstorms can extend several miles into the atmosphere and when we compare the scale of these two features it's easy to see why the river really doesn't have much of an impact. The river typically spans a width under a mile, usually around a half a mile compared to the nearly 7 to 11-mile span a thunderstorm occupies vertically in our atmosphere. That's a big difference in real estate!

We also have to take into account that the river basin itself is situated even further away from the storm because of its location in a valley compared to higher elevations that surround the river. Years of data have proven that river basins don't have a truly meaningful impact on thunderstorm complexes.

So if the river isn't affecting storm complexes, what is?

Well, in our case here in the Sauk Valley, a number of things.

  1. The influence of Lake Michigan providing a more stable environment due to a strong easterly wind ahead of some storm complexes, especially early on in the spring and summer months when the lake temperatures remain quite cool. An unfavorable environment will eventually weaken any storm complex significantly as it enters the local area and this is often the case early in the season.
  2. Location & Timing (the usual suspects): A number of times these thunderstorm complexes will develop out in parts of Iowa. Take this complex for example that formed in central Iowa and then began racing eastward towards the area. Take a look at the timestamp on the radar image. Without the aid of a low-level jet, the storms will more than likely be entering an unfavorable environment to the east as we lose daytime heating. These same storms will also likely produce an outflow boundary/gust front ahead of them that often times out-runs the line, cutting off its source of instability.
  3. Storms that form closer to the area have a better chance overall of producing more widespread rainfall as opposed to storms that form even 100 miles to the west. Take for example June 22, 2015, when we saw supercell thunderstorms form and cause extensive damage to Northland mall. These storms formed right over top of the area allowing their peak intensities to be realized in the Sauk Valley region.
  4. MORE DATA IS NEEDED: In the weather world the more data we have the better. As of right now, the number of observation stations remains limited but the hope is that with finer model resolution and more data in the years to come we'll be better able to depict the behavior that a thunderstorm complex will exhibit within its given environment.