The Seasons Battle It Out Over The May Outlook

The May outlook has some major temperature changes in the U.S., including what could be one of the nation's chilliest Mays in 15 years, according to the latest update from The Weather Company and Atmospheric G2.

Key Takeaways

• May is forecast to be persistently cooler than average in the Midwest and Northeast.
• It's expected to be hotter and drier than average in the Pacific Northwest.
• Any May warmth is expected to be muted and not persistent in the South.

A Big Change

The animation below shows the May outlook issued in mid-April compared to the latest forecast.

Notice the sharply cooler change over basically the eastern two-thirds of the nation, as well as the hotter change to the West Coast and Great Basin, particularly from Nevada to Oregon and Washington state.

You'll also notice the South is much less hot, now trending toward a slightly warmer than average May.

This could also lead to some later than usual frosts and freezes in parts of the Great Lakes and Midwest that could damage early blooming flowers and crops.

Overall, it could be the second chilliest May in the U.S. in the past 15 years, according to Todd Crawford, chief meteorologist at Atmospheric G2.

In all, we estimate about 141 million people in the Midwest and Northeast are forecast to be chillier than usual this May, just over 40% of the U.S. population.

(Further beef up your forecast with our detailed, hour-by-hour breakdown for the next 8 days – only available on our Premium Pro experience.)

What's Average In May?

The map below shows high temperatures in May averaged over 30 years from 1991 through 2020. You can think of these as typical or sometimes called "normal" highs during the month.

(MORE: When Is The Last Spring Freeze, Typically?)

Why The Big Change?

As it turns out, one impetus for this started in late February.

It involves the polar vortex, a spinning, whirling dervish of low pressure high over the North Pole in the colder months. And by high, we mean 6 to 30 miles above the ground, in the stratosphere, above which most of our weather happens.

Sometimes this polar vortex is strong, which works to keep cold air bottled up in the far north.

But in late February, the stratosphere suddenly warmed, weakening the polar vortex.

Sometimes, that warmer stratospheric air can leak down into the levels of the atmosphere where our jet stream is several weeks later. And that can set up more persistent blocking patterns that can tap colder air from Canada into the U.S.

So, the dominant jet stream pattern in May could feature a southward plunge in the East with generally higher pressure in the West. That's a cool pattern for the East and a warm one for the West.

(MORE: What Is The Polar Vortex?)

It May Snow

Seeing how much cooler May could be in parts of the Great Lakes might leave you wondering if we'll see at least a little snow during the last full month of spring.

It turns out May snow is typical, not weird, in Upper Michigan, as well as the Rockies and Sierra, as the map below shows.

Marquette, Michigan, averages 1.4 inches of snow each May. Three years ago, the city was buried by 26 inches of May snow, snowier than April or November that season.

And we've seen some weird May snow recently, including an Appalachians snowstorm in 2023, some flakes in New York City on Mother's Day weekend 2020, and snow as far south as Arkansas in 2013.

This persistent cool May pattern could have a few tricks up its sleeve.

Northwest Concern

Perhaps the most concerning part about this outlook to us was in the Northwest.

Both a much warmer and drier than average May is expected in the Northwest, particularly in parts of Idaho, Oregon and Washington.

As of late April, drought was already widespread in the West, due to one of the least snowy winter seasons in decades.

Increasing heat, less rain and mountain snow during one of the last months before the summer dry season is a troubling sign.

In its seasonal outlook released on April 16, NOAA's Climate Prediction Center forecasts drought to spread through the rest of the Northwest by the end of July. Those conditions could be ideal for more widespread wildfires this summer in the Northwest.

Jonathan Erdman is a senior meteorologist at weather.com and has been covering national and international weather since 1996. Extreme and bizarre weather are his favorite topics. Reach out to him on Bluesky, X (formerly Twitter) and Facebook.