An unusual weather pattern has stalled storms over southern and eastern states this week and will bring sudden downpours and potential flooding across Maine.

Known as an “omega block” for the shape that it gives the jet stream, the weather pattern forms when two low-pressure troughs trap a high-pressure system in between, locking fair or foul weather in place for days.

Low-pressure systems have stalled over Arizona and Kentucky, where they are cut off from the usual west-to-east flow of the jet stream, said Chris Legro, a meteorologist at the National Weather Service office in Gray.

“It can rain day after day after day in the same area,” Legro said Monday morning.

One corridor of concentrated downpours will hit from Maryland to Maine, where slow-moving bands of rain could cause isolated flooding from eastern New York into southern New England on Monday and Tuesday, according to national weather reports.

In Maine on Monday, crisp air was pushing in from the north and east, keeping rain at bay and giving most of the state a chance to dry out after recent storms, Legro said.

Downpours and thunderstorms were expected to move into Maine late Monday night, then spread across the state through Tuesday and into Wednesday, he said.

The greatest chance of flooding will be in southern New Hampshire and southwestern Maine, especially York County, where nearly 2 to 3 inches of rain fell over the weekend, Legro said.

“The ground there has absorbed all it can,” Legro said. “Pretty much anything that falls afterward is going to run off and could contribute to flooding.”

Flooding will be less likely across central, northern and eastern Maine, he said.

“If we start seeing six-hour rain totals exceeding 3 inches, we would be concerned about flooding,” he said.

The storms in Maine are expected to come in waves Tuesday, with downpours concentrated in the morning, afternoon and evening, he said.

Another corridor of concentrated downpours will run from eastern Colorado and New Mexico to Alabama, including Oklahoma and Texas, which experienced flooding last week, the Washington Post reported.

Some of the thunderstorms in southeastern New Mexico and western Texas will probably be severe Monday, when very large hail, damaging winds and a few tornadoes are possible.

The rain will be benefit some areas — swaths of the Mid-Atlantic and Northeast remain in a drought that developed in the fall.

FIRE RISK DAMPENED

In Maine, parts of York, Cumberland and Oxford counties are still experiencing abnormally dry to moderate drought conditions, according to the U.S. Drought Monitor.

The rain also has reduced the heightened wildfire risk that follows the spring thaw in Maine. The risk of brush fires is now rated “low” across the state, the Maine Forest Service reported.

“That’s another good thing about this rain,” Legro said. “It’s reducing the risk of dry underbrush catching fire, and everything else is greening up.”

But days of dangerous downpours are ahead for millions across the country, with signs that disturbances won’t take a break until the weekend.

Through Friday, rainfall over 4 inches is forecast across parts of 10 states: Alabama, Colorado, Connecticut, Louisiana, Massachusetts, New Jersey, New Mexico, New York, Rhode Island and Texas.

Because the omega block includes a cool system above, air that is warmed on the ground will have a stronger tendency to rise through the atmosphere and produce hail.