Forecasters say an “omega block” is keeping parts of the U.S. stuck in opposite extremes as May turns to June—warm and dry in the north-central region, cooler and wetter along coasts and the South, and unsettled conditions that can raise flood and fire risks.
As May gives way to June, the United States is heading into a familiar kind of weather standoff: the same high-pressure pattern that can freeze conditions in place, leaving some regions stuck under heat and others under repeated rain.
Forecasters say an “omega block” is already guiding the country’s weather and is forecast to linger in some form for a while—locking in cool air for parts of the coasts and hot air for stretches of the interior. It can also steer storms in unusual ways, depending on where the block stalls.
The pattern is producing a stagnant, split setup. Very warm and dry conditions are expected across the north-central U.S., while cooler, wetter conditions are forecast along the coasts and into the South. With slow-moving systems in the wetter areas, forecasters are warning of elevated flood risks.
Blocking patterns like this are known for “wacky weather” as high pressure jams the jet stream and displaces storm tracks to the north and south, according to a late-May note by ABC 17 meteorologist Nate Splater. The pattern could persist for weeks.
A more open, weaker version of the omega block is expected to set up June 4–12, according to AccuWeather meteorologist Paul Pastelok.
An omega block is essentially a stalled jet stream pattern over the United States that keeps weather locked in place. It can maintain a middle-country mix of warmth and dryness while stormier, flood-prone conditions linger along the coasts for days. Pastelok described it as a large, nearly stationary, upper-level high-pressure area caught between two upper-level low-pressure areas.
The name comes from the shape. Weather.com meteorologist Jonathan Erdman said that if you trace the path of the jet stream across the U.S. from west to east, it outlines the Greek letter omega (Ω).
Where the country lands under the pattern will determine what people feel day to day.
NOAA’s Weather Prediction Center said the general setup will feature troughing over both the West and East. with ridging in between—meaning cooler conditions for the two coasts and very warm weather for the central U.S. The forecast includes multiple regional tracks with very different day-to-day impacts.
For the Northeast, the Weather Prediction Center said unseasonably cool conditions over the East are expected to be reinforced through early to midweek. The relatively cool air mass across the Northeast and east-central U.S. is expected to keep heat and humidity suppressed to the south.
In the northern Plains. Pastelok said temperatures are expected to run well above average from central and west-central Canada to the northern Plains. As the pattern settled. summer heat reportedly arrived across the northern tier. shattering nearly 50 records. with temperatures up to 20 degrees above average. according to Fox Weather.
Down in the South. the blocking pattern is expected to support a very warm. humid. and unstable air mass south of a wavy frontal boundary. That setup is forecast to keep much of the South and Southeast active with daily rain and thunderstorm chances into early June. the Weather Prediction Center said.
Over the West, the trough is expected to close off into a broad upper low and spin over the northern Rockies. The Weather Prediction Center added that low-elevation heavy rain and mountain snow are possible from May 31 into June 1.
In the Southwest, temperatures are expected to moderate after a period of cooler-than-normal conditions linked to increased cloud cover and precipitation, the Weather Prediction Center said. The National Weather Service forecast that Phoenix should reach 104 degrees by June 1.
The pattern’s extremes can also carry risks beyond comfort.
Pastelok warned that during this time of year, drier-than-average conditions and higher temperatures can lead to fires—especially across the northern High Plains and central Canada.
The overall shift is clear in the way the block divides the map: heat and dryness tend to settle into the middle. while storminess. moisture. and repeated impacts are more likely along coasts and the South. As the block persists—and even as a weaker version is expected to return from June 4–12—how long any given region stays under its assigned weather lane may matter as much as the forecast itself.
Right now, the focus for many communities will be on whether persistent systems keep rain rates slow enough to raise flood concerns, or whether heat and dryness linger long enough to raise fire danger—both outcomes that become more likely when the atmosphere refuses to move on.
omega block United States weather National Weather Service NOAA Weather Prediction Center jet stream flooding risk heat wave records AccuWeather Phoenix 104 degrees May 31 June 1 rain and snow wildfire risk