Is there really a seven-day weather cycle?

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I once read something about the famous sinking of the Edmund Fitzgerald in Lake Superior that said at the time of the event, people believed that storm systems only blew through every seven days. The ship’s captain thought his route and timing were safe bets because a storm had recently passed. Then a storm with gale-force winds took them by surprise while crossing the lake, and the ship was sunk. I don’t know how true that little piece of background information was. I have tried to find that source again for more information about it, but with no luck. What I do know is believing weather has set cycles is easy for people to do, especially when the cycles seem obvious to casual observers.

Rainy Fridays get us down.

For the past year, most of our weekends have had rain. For at least the last month, every Friday has featured a chance for storms. It’s easy to take those facts and extrapolate that we’re in a seven-day cycle for rain that’s centered on the weekends. However, there’s not much to it. We’ve had rain during the workweek, too. In fact, RDU recorded 26 days in April that had at least 0.01 inch of rain. Weekday precipitation just isn’t as bothersome as weekend storms that cause canceled outdoor events such as April’s Friday Night on White in Wake Forest. We just notice weekend rain more.

Sometimes, we do slip into short-lived, repetitive patterns like having a chance for rain every Friday, but eventually, those patterns break. For example, we have a chance for storms this Friday night again, but as of this morning, it looks like next Friday will be dry. In fact, if the forecast holds, we’ll be drying out after a rainy Thursday. The pattern is about to change.

If these rainy weekends have you thinking that we’ve been experiencing wetter-than-normal weather, you are correct. Since January 1, the rainfall at RDU International Airport is 3.9 inches above our 30-year average. Last year ended with much of the state experiencing its wettest year on record. So, for the last 16 months, precipitation has been leaning toward excessive. Instead of wondering when the seven-day pattern will change, I’m wondering when the long-term moisture pattern will change.

Or will it ever?

It is possible that warmer global temperatures are affecting the moisture levels in our atmosphere. For years, climate scientists have predicted that while we may not necessarily have more storms as the earth warms, we’ll see more rainfall with the storms we do have. The reason is higher temperatures mean the atmosphere has the capacity to hold more moisture.

The Climate Prediction Center’s projections for May through July are above-average chances for higher-than-normal precipitation over most of the country, so it doesn’t look like there will be much change in the near future. While we may be tired of rainy weekends, at least we can say we’re not in a drought. Too much rain can be pretty bad, but no rain at all is usually worse.

CPC MJJ precip outlook

The Climate Prediction Center‘s Three-month Outlook Precipitation Probability map shows most of the nation has above-average chances for more rain than normal from May through July.