Tornado Safety and Preparedness

Five Interesting Changes May Weather Typically Brings

At a Glance

May is typically the most tornadic month of the year.
Hurricane season begins in the Eastern Pacific and sometimes starts in the Atlantic.
Longer daylight and warmer temperatures become more evident in May.

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May ushers in several notable changes in the nation's weather compared to earlier in the spring.

The month's tornadic reputation is one of the first weather concerns that comes to mind, but the tropics also have to be watched. For many, warmer temperatures are one of the month's highlights as the first signs of summer emerge.

Below, we take a look at five changes that typically occur in the weather in May.

1. Peak Tornado Month

In most years, May is when the peak of tornado activity occurs in the United States. This is the time of year when ingredients for severe thunderstorms come together most often.

The setup for severe thunderstorms typically includes disturbances in the jet stream pushing out of the Rockies and into the Plains, interacting with warm air and ample moisture.

In May, the area where these ingredients have the highest chance to come together is from Texas to Iowa.

Violent tornadoes are more likely this time of year. Numerous tornado and severe weather outbreaks have occurred in May.

Based on data from 2001 to 2021, the average number of May tornadoes in the U.S. is 277. That's considerably higher than April and June, which, on average, are the second (197) and third (187) busiest months for twisters, respectively.

Of course, these are averages and the actual tornadic outcome can vary in any given year. May over the last four years illustrates this, since it had 510 twisters in 2019 and then just 126 in 2020.

(MORE: What's Typically The Busiest Three Months For Tornadoes)

2. The Tropics Wake Up

The Eastern Pacific hurricane season officially begins May 15. That's a little earlier than the Atlantic season because sea-surface temperatures are typically warmer and wind shear is usually weaker in the Eastern Pacific compared to the Atlantic earlier in the season.

Tropical cyclones that form in the Eastern Pacific can impact Mexico and Central America and moisture from these systems can reach the southwestern U.S.

The Atlantic hurricane season officially begins June 1, but tropical cyclones occasionally form prior to that date. In May, areas from the western Caribbean to the southeastern U.S. coast and into the Gulf of Mexico are favored for development.

From 1851 to 2022, 41 named storms formed in the Atlantic Basin before June 1, a long-term average of one such early storm every four to five years.

At least one named storm developed before June 1 in each hurricane season from 2015 to 2021 and there were impacts in the U.S. and elsewhere in the Atlantic Basin. One recent example was in late May 2020, when the Carolinas were impacted by a pair of tropical storms.

(MORE: Changes You'll See In Tropical Forecasts This Hurricane Season)

Each point on the map represents a tropical cyclone that formed in the Atlantic Basin in May between 1950 and 2022.

3. As One Wet Season Begins, Another One Ends

A wet season is the time of year when most of the annual precipitation occurs.

Florida's wet season begins in May and lasts through October. Miami, for example, receives about 52 inches of rainfall – almost 78% of its average annual rain – during this time period.

Cold fronts usually do not push as far south into Florida from late spring to early fall, which allows for warmer temperatures and increasing humidity across the state.

Thunderstorm activity increases, bringing an increase in rainfall. Storms develop long sea-breeze fronts as cooler air slides inland from the Gulf of Mexico and Atlantic Ocean.

Hurricane season also brings an increased chance for rainfall. It doesn't take a strong hurricane to bring heavy rainfall to the Sunshine State; Slow-moving tropical storms or tropical depressions can result in excessive rainfall.

Cooler air slides inland from the Gulf of Mexico and the Atlantic Ocean to replace warm air that is rising into the atmosphere.

Meanwhile, California's wet season comes to an end. The wet season, or water year, runs from October through April.

The jet stream pattern in May typically moves northward and brings an end to low-pressure systems impacting California.

This is one reason why snowmelt is so important for the Golden State – it provides much-needed water for the reservoirs in the dry season.

4. Starts To Feel Like Summer

Average high and low temperatures are fairly mild across most of the U.S. in early May. However, by the end of the month, average temperatures begin to erase thoughts of winter in most locations.

By May 31, the average high in New York City reaches 75 degrees and the average low is 59, a big improvement from the average low of 40 in early April. Chicago experiences about a 10-degree temperature increase in its average high from May 1 to 31, rising from 65 degrees to 76, and the average low climbs from 46 to 56.

Heat continues to build across the South. Dallas has an average high of 79 degrees on May 1, but it climbs to 88 by May 31 and the average low is in the upper 60s by the end of May. The average high in Atlanta reaches the mid-80s by May 31 and the average low is in the mid-60s.

Farther west, average high temperatures in Las Vegas reach the mid-90s by late May and average lows are in the lower 70s.

(FORECAST: May Temperature Outlook)

One of the most noticeable signs of summer is the increase in daylight hours. This is especially evident across the northern tier of the U.S.

Daylight hours in Boston and New York City increase by almost an hour from the start of the month to the end. Minneapolis experiences an increase of 65 minutes during this time, while Seattle has an increase of 72 minutes. Areas from Atlanta to Los Angeles have an increase in daylight of more than 40 minutes.

5. Snow Becomes Less Likely

Of course, along with the warmer conditions comes a much-reduced chance of snow.

Although snowfall becomes rarer as May progresses, snow can still fall. Areas that typically experience snowfall in May are in the higher elevations in the western U.S. and Alaska, but there can be some outliers each year, as we saw in recent years.

Wet snowflakes flew through the air in parts of Indiana, Ohio and western Pennsylvania in 2021.

In May 2020, Cleveland set a record for its latest measurable snow with 0.2 inches falling on May 11.

New York City had a trace of snow on May 9, 2020, which tied the city's record for the latest in the season that snowflakes have been observed. Pittsburgh reported a trace of snow on May 8 and 9, 2020, while Detroit saw five straight days of at least a trace of snow.

(MORE: Here's When Your Final Accumulating Snow Typically Happens)

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