Saturation point: Tornado warnings are ‘interesting,’ but the bigger threat remains flash flooding

⇒The National Weather Service has issued a flash-flood warning until 6 p.m. in our area.

MANCHESTER, NH – Sunday morning’s tornado warnings and watches got everyone’s attention, and to those in the northeast unaccustomed to them, they can be “interesting,” but the real continued threat is flash flooding. According to the National Weather Service, the potential for flooding in and around Manchester and regionally is due to, in layman’s terms reaching a saturation point on the ground and in the air.

“We haven’t heard of any tornados on the ground – and if that happened we probably wouldn’t have confirmation of it right away,” says National Weather Service meteorologist Andy Pohl, reporting from Gray, Maine. “And while a tornado might be interesting the major threat right now is flooding.”

Simply stated, flooding is due to oversaturation below and above.

“The precipitation we’ve had over the past several weeks has caused the ground to become hydrophobic,” says Pohl. “What that means is the ground – especially in the mountains – will repel water; it won’t soak in at all.”

Another way to describe it is that the soil is completely saturated and, like a sponge, it can only hold so much water.

And the humidity in the atmosphere is also at critical mass. Right now we’re in the midst of a record-breaking phenomenon caused by the extreme humidity, says Pohl. Based on Sunday morning’s weather balloon sounding, the amount of precipitation in the upper atmosphere registered 2.34 inches, the highest ever.

“A sounding measures the precipitable water and from our sounding, 2.34 inches is the highest we’ve ever seen. That means the air above us is holding a record amount of water vapor so whenever it rains it will be torrential. Some locations could pick up 3-4 inches in a short period of time and since the ground won’t soak up any more water, it has to go somewhere,” Pohl says.

The previous record for precipitable water was measured on Aug. 20, 2021 at 2.26 inches, Pohl says.

He notes that flooding issues can develop especially quickly in the mountains where there’s more terrain as precipitation comes into a valley or basin much faster than it does on a flat surface.

“That’s why you get flash flooding more in mountainous terrain, although during this weather pattern everyone will be under the threat of it,” Pohl says.

And while the rain seems endless right now, it’s not.

“We should see the back end of it later tonight and by Monday morning it should be gone. In Manchester, you should see some clearing by sunrise and it will actually be sunny and nice,” Pohl says.

Nice, but still stinkin’ hot and muggy.

“The bad news is that it will hit about 90 degrees with 60 percent relative humidity,” Pohl says.

According to the National Weather Service a tornado watch remains in effect until 3 p.m. in our area.