The month's tornadic reputation is one of the first weather concerns that comes to mind, but the tropics also have to be watched, especially since we've seen multiple recent examples of hurricane season starting early. For many, warmer temperatures are one of the month's highlights as the first signs of summer creep into the forecast.

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 of coming together is from Texas to Iowa.

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

Based on data from 2001 to 2020, the average number of May tornadoes in the U.S. is 276. That's notably higher than April and June, which on average are the second (200) and third (194) busiest months for twisters, respectively.

Of course, these are averages and the actual tornadic outcome can vary in any given year. May over the past three years illustrates this, since it had 510 twisters in 2019 and then just 126 tornadoes in 2020. The final count for May 2021 hasn't been determined, but there were 289 reports of tornadoes, which is close to the month's average.

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.

Tropical cyclones also occasionally form in the Atlantic prior to the official June 1 start of the hurricane season. 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 2021, 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.

Since 2015, at least one named storm has developed before June 1 in each hurricane season, some of which had impacts in the U.S. and elsewhere in the Atlantic Basin.

Last May, Tropical Storm Ana formed late in the month to the northeast of Bermuda, but it did not impact any land areas.

In May 2020, the Carolinas were impacted by a pair of tropical storms late in the month.

Tropical Storm Arthur brought soaking rain to far eastern North Carolina as it tracked just off the Southeast coast May 16-19.

That was followed by Tropical Storm Bertha, which made landfall near Isle of Palms, South Carolina, just hours after it formed on May 27. Bertha's impacts were minor, but it did produce gusty winds and locally heavy rainfall.

Wet Season Starts in Florida

Miami receives about 52 inches of rainfall, or almost 77% of its average annual rain, from May to October.

Cold fronts no longer push as far south into Florida, which allows for warmer temperatures and increasing humidity across the state.

Thunderstorm activity increases, bringing an increase in rainfall. Thunderstorms develop along sea-breeze fronts as cooler air slides inland from the Gulf of Mexico and the 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 in the region.

Starts to Feel Like Summer

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

The Northeast and Midwest regions both typically have signs of summer by Memorial Day.

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 temperature from May 1 to May 31, rising from 65 degrees to 76, and the average low climbs from 46 degrees to 56.

Farther west, Denver has an average high in the mid-60s in early May but reaches 78 degrees by May 31. Average low temperatures also increase by about 10 degrees, reaching the upper 40s by late May.

Heat continues to build across the South. Dallas has an average high of 79 on May 1, but it climbs to 88 by May 31, when the average low is 69 degrees. The average high in Atlanta reaches the mid-80s by May 31, and the average low is in the mid-60s.

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.

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 2020 and 2021.

Wet snowflakes flew through the air in parts of Indiana, Ohio and western Pennsylvania a year ago on May 9.

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.

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