Why is the Weather Going Backwards?

One question I've seen a lot on my Facebook this week is: Why is the weather going backwards? What people mean by this is: "Why are the rain and storms in the Northeast quadrant of the country moving from southeast to northwest?" This is shown in this afternoon's Pennsylvania radar animation:

The answer is surprisingly simple. Yes, in general, weather systems move west to east in the United States (and east to west in the Tropics - remember hurricanes?) But on the smaller scale, storms can come from any direction because low pressure systems rotate like a top (this is also the reason why some crazy youtube guy thinks the world is ending).

What's happening this week is that we have an upper-level low pressure system over the Carolinas (I've explained before why upper-level lows generate showers due to instability, so I won't rehash that here).

In today's case, the upper-level low, located in Virginia, is being undercut by dry air (orange on the map above, which shows atmospheric moisture) which is cutting off showers on the southern side. That's adding to the confusion because all the showers and storms are on the northern side of the low pressure, meaning they are moving westward. The map also shows the previous positions of the low, and it's barely moved in the last three days, meaning people are seeing this kind of behavior again and again, perplexing the public further.