The uncommon phenomenon bringing downpours to the East and South into next week

An uncommon atmospheric phenomenon known as an omega block will bring days of rain and thunderstorms to the Eastern Seaboard, starting on Saturday and lasting until Wednesday or Thursday.

With more than three inches forecast in about eight northeastern states, the rain could be heavy enough to lead to localized flooding into early next week. The area that includes central and eastern Pennsylvania, southern New York, New Jersey and western parts of New England may be at particular risk, with scattered severe storms on Saturday afternoon and drenching downpours on Monday and Tuesday.

Still, even as the rain could drive hazardous impacts, the Mid-Atlantic and Northeast could use the precipitation. Abnormal dryness to severe drought continues to cover parts of the region.

Another area of intense, potentially flooding rainfall and thunderstorms will cross the Rockies on Monday, then the southern Plains and parts of the Gulf Coast from Tuesday into Wednesday, with eastern Texas and Louisiana at highest risk.

What is an omega block?

An omega block is a slow-moving weather pattern in which two low pressure systems become stalled on either side of a high pressure area, creating a configuration that resembles the Greek letter omega (Ω) on weather maps.

The pattern forms when jet stream winds, around 40,000 feet above the ground, slow down, causing the systems to become nearly stationary. As a result, an omega blocks often lead to prolonged periods of adverse weather, including heavy rain and thunderstorms.

Over the next few days in the U.S., the disturbances will stall over the Southwest and Midwest, where rising air ahead of the systems cools and condenses to produce rain and storms.

How much rain may fall

Through Wednesday, more than an inch of rain is forecast across parts of 42 contiguous states, meaning that the omega block will have wide-reaching effects.

The heaviest rain is forecast across the Ohio Valley, Mid-Atlantic, Northeast, Rockies, southern Plains and Gulf Coast, where localized areas of flooding may develop in the hardest-hit places.

Ohio Valley, Mid-Atlantic and Northeast

Areas of rain and thunderstorms, some of which could produce damaging winds, will become more widespread and intensify across the Ohio Valley, Mid-Atlantic and Northeast on Saturday, as southerly wind flows add moisture and humidity to the air.

A band of heavy rain will cross Tennessee, Kentucky, Indiana and Ohio — states that were drenched in April — with the potential for isolated flooding.

Showers and storms are expected to develop along or just west of the Interstate 95 corridor on Saturday afternoon, which may put a damper on any evening plans.

The unsettled weather will continue across the region on Sunday, though it will probably be less intense than Saturday.

A pinwheeling disturbance in the upper atmosphere, forming part of the omega block, will slowly move eastward on Monday and Tuesday, leading to downpours from Virginia to Vermont — heaviest in eastern Pennsylvania, southern New York, New Jersey and western New England on Tuesday, when some flooding is possible.

Rockies, southern Plains and Gulf Coast

The western side of the omega block will bring areas of heavy rain and thunderstorms to the Rocky Mountains on Monday.

This moisture may coalesce into a cluster of heavy rain and thunderstorms in Colorado, Kansas, Oklahoma, Texas, Arkansas, Louisiana and Mississippi on Tuesday, where flooding can’t be ruled out.

NOAA’s Weather Prediction Center has a slight (Level 2 out of 4) to moderate (Level 3 out of 4) risk for excessive rainfall in several of these states.

Heavy showers and storms will likely continue across the region on Wednesday, heaviest from Texas eastward to Alabama.

When it will end

While the rain will probably ease across the above regions by late next week, the Southeast may become a focal point for more storms.

There’s a remote chance that a second omega block develops during the second week of May, locking in yet more unsettled weather.

In the meantime, the initial omega block will interrupt summerlike warmth and clear skies, reinforcing its reputation as a disruptive weather phenomenon.