Dangerous "Atmospheric River" To Cause Mid-Atlantic, Northeast Flooding

Three to seven inches of rain, and locally higher amounts, are
likely, especially in the red areas of this map during the next seven days.

An atmospheric river is set to dump tremendous amounts of rain on a broad strip running north and  northwestward off the Atlantic Ocean through areas around North Carolina and Virginia, northward through Pennsylvania and central New York. 

That means these areas need to be really, really on guard for serious flooding over the next few days.

First, we need to define for us all what an atmospheric river is. I'll let NOAA do it:

"Atmospheric rivers are relatively long, narrow regions in the atmosphere - like rivers in the sky - that transport most of the water vapor outside the tropics. These columns of vapor move with the weather, carrying an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River.

When the atmospheric rivers make landfall, they often release this water vapor in the form of rain or snow."

The problem is, atmospheric rivers often release extreme amounts of rain, causing extreme floods.

Atmospheric rivers are to blame for extreme floods around Nashville, Tennessee in 2010; and another extreme, deadly flood around South Carolina in October, 2015 that dumped up to two feet of rain in that state.

Not all atmospheric rivers are bad. They're common in California in the winter, and help replenish reservoirs and add to the snowpack in the Sierra Nevada mountains. Melting snow in the summer then keeps reservoirs and drinking water supplies at reasonable levels.

Like anything in weather, atmospheric rivers vary in intensity. Some aren't too tough to take, dropping a few inches of rain on a drought-stricken area, for example.

But often, they are terrible. And they can unload their tremendous rainfall well inland from wherer they come ashore -- like in the 2010 Tennessee flood I just mentioned.

The atmospheric river slated to hit parts of the East Coast this week is being created by a really wonky weather pattern now in place.

Usually this time of year, the jet stream, which controls the paths of storms and fair weather systems, sputters in a general west to east course near the United States/Canada border. In July, there's rarely much in the way of big southward dips or huge northward bulges in the jet stream.

There's an exception going on now and the weather pattern is really off the rails.  There's a big dip in the jet stream centered over the Great Lakes. This dip is causing unusually chilly weather for this time of year near the Great Lakes, and strangely severe storms in the South. Those storms created hail as big as tennis balls in Georgia and some other southern states.

Meanwhile, the Bermuda high is pushing a big heat ridge northward in the western Atlantic Ocean. Part of the Bermuda High has pushed as far north as Nova Scotia.

This is setting up the atmospheric river flowing northward from the tropics between the big Bermuda High to the east and the dip in the jet stream to the west.

This pattern is kind of stuck until later Thursday and Friday, when things will start moving. So the heavy rains will keep going over the same places from North Carolina  to New York through then. This could extend into southwestern Quebec as well.  Remember, this is tropical moisture, so any showers and thunderstorms can create one to two inch per hour rainfall rates.

This can cause life-threatening flash flooding. There was already destructive flooding and water rescues in Pennsylvania this morning.  That flooding will likely worsen this afternoon in parts of Maryland, Pennsylvania and the southern tier of New York.

We here in Vermont are on the eastern edge of this atmospheric river pattern. That means we are subject to the torrential downpours, but not the constant fire hose of rain. I noticed during a batch of rain that moved northward through Vermont this morning, an inch of rain fell on Rutland in just an hour.

We desperately need the rain here in Vermont, so precipitation this week is a good thing. We still have to watch out, especially Wednesday night and Thursday, when the axis of heaviest rain will probably shift to near or over the Green Mountain State.

Despite the dry conditions, an excessive downpour, or a series of downpours going repeatedly over the same spot, can cause dangerous flash flooding. It's impossible at this point to determine if, or even whether Vermont will see any flash flooding from this system.

The National Weather Service is eyeing the possibility of that happening, so stay tuned.