- May is usually the most tornado month of the year.
- The Atlantic hurricane season sometimes starts early in the month.
- However, many are looking forward to May as most areas can begin to enjoy longer daylight and warmer temperatures.
May begins this weekend, and the start of the new month ushers in several notable changes to the country’s weather from the start of spring.
The tornado of the month reputation is one of the first weather concerns that comes to mind, but the tropics should also be watched, especially since we’ve seen several recent examples of hurricane season starting early. For many, warmer temperatures are one of the highlights of the month as the first signs of summer creep into the forecast.
Tornado Peak Month
In most years, May is the peak of tornado activity in the United States. This is the time of year when the ingredients for severe thunderstorms most often come together.
The severe thunderstorm pattern typically includes disturbances in the jet stream out of the Rocky Mountains and across the plains, interacting with warm air and abundant moisture.
In May, the area where these ingredients are most likely to come together is from Texas to Iowa.
Severe tornadoes are more likely this time of year, and many outbreaks of tornadoes and severe weather events have occurred in May.
Based on data from 2001 to 2020, the average number of May tornadoes in the United States is 276. This is significantly higher than April and June, which average the second (200) and third (194 ) busiest months for tornadoes, respectively.
Of course, these are averages and the actual tornado outcome may vary in any given year. May in the past three years illustrates this, as there were 510 tornadoes in 2019, then only 126 tornadoes in 2020. The final tally for May 2021 has not been determined, but there were 289 reports of tornadoes, which is close to the month. medium.
The tropics wake up
The Eastern Pacific hurricane season officially begins on May 15. This is a bit earlier than the Atlantic season because sea surface temperatures are generally warmer and wind shear is generally weaker in the eastern Pacific than in 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 United States.
Tropical cyclones also occasionally form in the Atlantic before the official start of hurricane season on June 1. In May, areas from the western Caribbean to the southeast coast of the United States and into the Gulf of Mexico are favored for development.
From 1851 to 2021, 41 named storms have formed in the Atlantic Basin by 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 every hurricane season, some of which have had impacts in the United States and elsewhere in the Atlantic Basin.
Last May, Tropical Storm Ana formed late in the month northeast of Bermuda, but did not affect any land areas.
In May 2020, the Carolinas were hit by a pair of tropical storms at the end of the month.
Tropical Storm Arthur brought torrential rains to far eastern North Carolina as it tracked just off the southeast coast May 16-19.
This was followed by Tropical Storm Bertha, which made landfall near Palms Island, South Carolina, just hours after forming on May 27. Bertha’s impacts were minor, but produced gusty winds and locally heavy rain.
The rainy season begins in Florida
Florida’s rainy season – the time of year when most annual precipitation occurs – begins in May and lasts until October.
Miami receives about 52 inches of precipitation, or nearly 77 percent of its average annual precipitation, from May through October.
Cold fronts no longer push as far south of Florida, allowing warmer temperatures and increased humidity across the state.
Thunderstorm activity increases, leading to increased precipitation. 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 of rainfall. It doesn’t take a powerful hurricane to bring heavy rain to the Sunshine State. Slow-moving tropical storms or tropical depressions can bring excessive rainfall to the region.
Starts to feel like summer
Average high and low temperatures are fairly mild for the most part in the United States in early May. However, by the end of the month, average temperatures begin to erase thoughts of winter in most places.
The Northeast and Midwest regions generally have signs of summer before Memorial Day.
As of May 31, the average high in New York reaches 75 degrees and the average low is 59, a big improvement from the average low of 40 in early April. Chicago is seeing a temperature increase of about 10 degrees from its average high from May 1 to May 31, 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 hits 78 degrees on May 31. Average low temperatures also increase by around 10 degrees, reaching the upper 40 degrees in late May.
The heat continues to rise in the South. Dallas has an average high of 79 on May 1, but it climbs to 88 on May 31, when the average low is 69 degrees. The average high in Atlanta hits the mid-80s on 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 in the northern part of the United States
Daylight hours in Boston and New York increase by almost an hour from the start of the month to the end. Minneapolis sees a 65-minute increase during that time, while Seattle has a 72-minute increase. Areas from Atlanta to Los Angeles have daylight increases of more than 40 minutes.
Snow becomes less likely
Of course, the warmer conditions come with a very reduced chance of snow.
Although snowfall becomes rarer as May progresses, snow can still fall. Areas that typically see snowfall in May are at the highest elevations in the western United States and Alaska, but there can be outliers each year, as we’ve seen 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 last measurable snowfall 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 last of the season that snowflakes were observed. Pittsburgh reported a trace of snow on May 8 and 9, 2020, while Detroit has had five straight days of at least a trace of snow.
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