For much of the United States, September played out as follows: Rain tonight, followed by more rain in the morning. Watch for occasional drizzle and downpours, which will be interspersed with rain.
“It really worked out to be an odd September. It’s been a very wet month for two-thirds of the country,” says Jon Gottschalck, the chief of operations at the Climate Prediction Center in the National Weather Service.
So, what gives? Gottschalck told me that all this abnormal amount of moisture can’t be blamed on any one weather factor. What makes September “odd,” he said, is that the cause of the rain tended to vary by region.
In the north, much of the woe can be blamed on the jet stream, the high-atmospheric river of air that whisks storms across the country. The stream has spent most of the month unusually far north, lingering in Southern Canada, and moving very little air across the continental United States. So storms have tended to crawl across the country, dropping more rain on fewer places before they make it to sea. This northern jet stream also blocked drier, cooler pockets of air from making it to the United States.
In the upper Midwest, that helped lead to stalled-out rainstorms and dangerous flooding earlier this month. A slow-moving storm system also led to record rainfall and flash flooding in Oklahoma this weekend, killing one. (Meanwhile, the northerly jet stream has brought a rare bout of tornadoes to Ontario and Southern Canada.)