Chance of rain? Seems very likely

NASHUA – A big blue “L” floating over the Northeast on radar screens is to blame for the seemingly endless rain and thunder storms that have soaked southern New Hampshire in the last month.

The bad news is there’s no end in sight.

“It seems there’s this persistent upper air low that’s been hanging over the Great Lakes and the Northeast, and it’s created enough instability. When you have instability, you have more chance of showers,” said Kathryn Vreeland, a climatologist at the New England Regional Climate Center in Ithaca, N.Y.

The cool, low-pressure air that has squatted in the upper atmosphere above a large swath of upstate New York, Vermont, New Hampshire, Maine and parts of Massachusetts is relatively light, Vreeland said, which allows denser, warmer air to rise. That upward flow is a key process in the mechanics of storm formation, she said.

The ideal storm-brewing conditions help produce days with alternating periods of sun and rain, she said.

“You might have a day when it’s half sunny and half rainy,” she said. “That seems to be the predominate weather pattern this summer. There’s a big ‘L’ over us, and we want it out of here.”

It probably won’t be moving on anytime soon, according to Bill Simpson, a meteorologist with the National Weather Service in Taunton, Mass.”Indications are it’s kind of locked in. There’s no real expected change,” he said. “It seems the active pattern will continue at least until next week.”

More than 7 inches of rain fell at the Manchester-Boston Regional Airport last month, Vreeland said.

Concord had 6.48 inches of rain in July, almost twice the long-term average, and has had 3.3 inches – five times the average – already this month, according to Simpson.

The downpours haven’t swelled rivers on a wide scale, Simpson said, because most of the storms have struck relatively small areas.

“It’s kind of scattered enough flooding isn’t really an issue. Sometimes flash flooding is,” he said. “The rivers aren’t really an issue just yet.”

Flash flooding, Simpson said, is a different phenomenon than a major river cresting its banks. Downbursts over a small area can quickly flood small streams or city streets.

On Wednesday, a 150-yard mudslide took down about a dozen trees in Bath, N.H., closing Route 112. Heavy rain also washed out River Road there. Areas of central Vermont were hit by flooding this week, when several inches of rain washed out roads and bridges in southern Addison County.