Omega block set to bring multiday flooding threat to central and northeastern U.S.

An omega block weather pattern will begin the week with an upper-level low-pressure system over the southwestern United States and an upper-level ridge over the Northern Plains and Midwest on Monday, May 5, 2025. Widespread unsettled weather is expected to persist across large portions of the country through the week.

But what’s an omega block? An omega block is a large-scale weather pattern named for its resemblance to the Greek letter Ω on meteorological maps. It forms when two low-pressure systems become cut off from the jet stream, flanking a stationary high-pressure ridge in between.

Areas beneath the high-pressure ridge typically experience warm and dry conditions, while regions under the low-pressure systems often see cooler and wetter weather.

In short, it’s a slow-moving weather pattern expected to produce rounds of unsettled weather across the contiguous United States throughout the week.

Showers and thunderstorms are expected across the Southwest, eastern Great Basin, and Rockies, with potentially heavy rainfall and isolated flash flooding in the Desert Southwest from Monday into Tuesday, May 6. Mountain areas in southwestern Montana, northwest Wyoming, Colorado, and north-central New Mexico will see heavy snowfall of over 20–30 cm (8–12 inches).

The precipitation will decrease on Tuesday as the low pressure system moves east.

A concentrated heavy rainfall event is expected to develop early in the week across parts of the central and southern High Plains, the southern Plains, and the Lower Mississippi Valley.

Moist air from the Gulf of Mexico will interact with a low-pressure system and upslope flow, generating thunderstorms and rainfall across the region. The southern High Plains are under a Slight Risk (level 2/4) of excessive rainfall and flash flooding through Monday, while southwest Texas faces an Enhanced Risk (level 3/5) of severe thunderstorms capable of producing large hail, damaging winds, and possible tornadoes.

On Tuesday, the threat will shift northeastward and expand. The ArkLaTex region and Lower Mississippi Valley are under a Moderate Risk (level 4/5) of excessive rainfall due to slow-moving storms, potentially causing widespread flash flooding. The southern and central High Plains are under a Slight Risk of excessive rainfall. Severe storms with large hail, damaging winds, and possible tornadoes are also possible from central Texas to western Louisiana.

Thunderstorms and showers are also expected in the Northeast on Monday due to the eastern upper-level low associated with the omega block. The strongest storms are forecast from the Lower Great Lakes through the Mid-Atlantic and into southern New England. A region including New York City and parts of southern New England is under a Slight Risk of excessive rainfall and localized flash flooding.

The storms are forecast to weaken and become more scattered and dissipate through Tuesday night into Wednesday, May 7.