An unusual weather pattern is causing storms to stall over eastern and southern states this week. The phenomenon, called an omega block, forms when two low-pressure systems trap a high-pressure system in between, locking adverse weather in place for days.

As a result, flooding rain and thunderstorms are possible beginning Monday in a corridor from eastern Colorado and New Mexico to Alabama, including Oklahoma and Texas, which experienced flooding last week.

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

Another corridor of concentrated downpours will hit areas 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.

For some, the rain will be beneficial — swaths of the Mid-Atlantic and Northeast remain in a drought that developed in the fall. But days of dangerous downpours are ahead for millions across the country, with signs the disturbances may not depart until the weekend.

Drenching downpours

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

The National Oceanic and Atmospheric Administration’s Weather Prediction Center has issued a moderate risk (Level 3 out of 4) for excessive rainfall in an area that includes parts of southeastern Oklahoma, eastern Texas, southern Arkansas, Louisiana and Mississippi — home to about 6.7 million people — from Tuesday into Wednesday.

What to know about the East

Monday: Heavy showers and thunderstorms will cover a wide area, from North Carolina to New Hampshire and westward to Indiana. The highest rainfall amounts are forecast in Pennsylvania, New York, New Jersey, Connecticut and Massachusetts, where isolated flooding can’t be ruled out.

Tuesday: Showers and storms will focus from Maryland to Maine, probably heaviest in New England. Similar to Monday, pockets of flooding will be possible.

Wednesday-Friday: Additional scattered showers and storms are likely in the Northeast on Wednesday before a drier Thursday. A new storm may bring more rain Friday.

Because the omega block is associated with a cool pool of air aloft, air warmed on the ground will have a stronger tendency to rise through the atmosphere, increasing the risk for hail in the East this week.

What to know about the South

Monday: Showers and storms will affect areas from Southern California to Texas, northward to Montana.

Parts of western Texas and eastern New Mexico have a slight risk (Level 2 out of 4) for excessive rainfall and/or an enhanced risk (Level 3 out of 5) for severe thunderstorms capable of producing very large hail, damaging winds and a few tornadoes — including Midland and Odessa in Texas.

Tuesday: A cluster of strong to severe storms will spread eastward, causing a risk for flash flooding in several states, including eastern Colorado, northeastern New Mexico, Oklahoma, northern and eastern Texas, Arkansas, Louisiana and Mississippi. Affected by these downpours will be Pueblo, Colorado; Dallas, Houston and Amarillo in Texas; Oklahoma City; Little Rock; and Shreveport, Louisiana.

Wednesday-Friday: Scattered showers and storms, some heavy, will stretch from the Plains to the Southeast and Florida, but the risk for flooding won’t be as high or widespread as early in the week.

Does your region need rain?

As summer approaches, drought risk rises. A higher sun angle, long days and warmer temperatures can quickly dry out the ground and vegetation and deplete water storage.

According to the latest U.S. Drought Monitor readings, more than 56 percent of contiguous states were abnormally dry, while just under 20 percent were experiencing severe drought or worse.