Why has the rain been moving backwards?

I've been asked several times over the past week why our showers and thunderstorms have been moving "the wrong way." Isn't it more typical in the summer for us to be watching skies to the south or southwest than to the east? While sometimes we just call them "Crazy Ivans" (you remember this scene from "The Hunt for Red October," right?), there is a real explanation for this phenomena.

A Typical Summertime Weather Pattern

To answer the second question first - yes. Our weather usually originates from the southwest or south in the summer. To understand the answer to the first question, we need to know that the wind at the surface does not determine the direction clouds or storms move. The wind at the mid-levels of the atmosphere direct most weather patterns. Specifically, the wind between about 5,000-10,000 feet up (or 850-700 mb if you are a meteorologist using pressure values rather than feet).

Typically, much of the summer in the southeast U.S. is controlled by the "Bermuda High," a high pressure system that takes hold in the western Atlantic centered near Bermuda. Remember that wind flows clockwise around high pressure. That means that the air around the western side of that high, which is where we are located relative to it, flows from the south tor southwest. Thus our weather  usually follows that same pattern.

Bermuda High pressure extending into the southeast U.S. Clockwise air flow means south to southwest wind for Memphis when the Bermuda High controls our weather. (Original graphic credit: NASA Goddard Space Flight Center)

But sometimes we get Crazy Ivans...

In the past week or so, the Bermuda High has not had control over our weather. More influential highs have been located to our north or northeast. You undoubtedly heard about the record heat from the Midwest to the Northeast last week? That was because of a massive, hot high pressure center over that region. Even this weekend, despite it weakening some, high pressure at that crucial 5,000-10,000 -foot level has been centered over the Ohio Valley.

Satellite imagery from July 4 show the clockwise motion of clouds around high pressure over Kentucky. That results in weather moving from east to west across the Memphis area.

Let's look a little closer at Sunday afternoon, when our scattered afternoon showers were moving from southeast to northwest...The national picture at 10,000 feet shows high pressure over southwest OH and a strong low off the Carolina coast (that is Tropical Storm Chris). The green arrows show the flow of the wind around those features, while the colors show the wind speed (less than about 25 mph is white). Note that over our area the wind is light from the southeast.

Zoomed in a bit, you can see how the high over southwest OH results in clockwise wind flow around it, or from the southeast in the Memphis area.

Finally, if we look just at the metro area, the wind barbs are angled from southeast to northwest. The little flags at the tail of the wind barbs tell us how fast the wind is blowing. The longer flags are 10 knots (mph) and the short flags are 5 knots (mph). So the wind over the metro yesterday afternoon was blowing from the southeast at roughly 15 knots (mph).

And guess which way yesterday's showers were moving? To the northwest at 15-20 mph! Now you know why sometimes a radar loop shows rain moving in a different direction than you would expect! It's all in the (movement of the) air up there! Or maybe, we need to watch out for those Crazy Ivans!

Erik Proseus
MWN Meteorologist

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