May 1
2012: The National Centers for Environmental Prediction in the United States replaced the Rapid Update Cycle (RUC) weather prediction model with the Rapid Refresh (RAP) model.
May 2
1929: Part of a larger tornado outbreak, a tornado destroyed a school in Rye Cove, Virginia, killing 12 students and a teacher.
May 3: Start of National Hurricane Preparedness Week 2020
2016: A major wildfire, sustained by high winds and record-high temperatures, swept through the city of Fort McMurray, Alberta, Canada, forcing an evacuation of all 88,000 of the city's residents and destroying 2,400 buildings.
May 4
2007: A three-day tornado outbreak featuring more than 100 tornadoes began across a large area of the Central United States. One tornado destroyed 95% of the town of Greensburg, Kansas, and was the first tornado ever rated EF5.
May 5
1995: A major hailstorm caused around $1 billion in damage in the Dallas–Fort Worth metroplex. This was the costliest hailstorm in United States history.
May 6
1965: A series of violent tornadoes affected the Twin Cities area of Minnesota.
May 7
1981: Tropical Storm Arlene moved across Cuba. Minimal damage was reported.
May 8
1995: More than 20 inches (510 mm) of rain in just 12 hours caused major flooding in southern Louisiana, causing more than $1 billion in damage.
May 9
2002: Cyclone Kesiny made landfall near Antsiranana, Madagascar, killing 33 people. Kesiny was the first tropical cyclone on record in the South-West Indian Ocean to make landfall in the month of May.
May 10
2002: A tropical cyclone struck the coast of Oman, causing severe damage and several deaths due to drowning in flash floods.
May 11
1953: An F5 tornado destroyed areas of downtown Waco, Texas, killing 114 people.
May 12
1948: A Douglas DC-4 crashed while flying into a thunderstorm near Libenge, Congo, killing 31 of the 32 aboard.
May 13
1998: The NOAA-15 weather satellite was launched into a polar orbit from Vandenberg Air Force Base, California. Originally planned for just a 5 year mission, as of early 2020 the satellite was still returning useful atmospheric and space data from several instruments.
May 14
2007: Cyclone Akash made landfall in Bangladesh, causing almost $1 billion in damage.
May 15: Start of the East Pacific hurricane season and end of the Mauritius and Seychelles cyclone season
1966: The Nimbus 2 weather satellite was launched. It collected information about atmospheric temperature and cloud cover until early 1969.
May 16
1986: Severe flooding from an ice jam completely destroyed the town of Winisk, Ontario, which was subsequently abandoned. Two people were killed.
May 17
2021: Cyclone Tauktae made landfall near Mumbai, killing at least 174 people.
May 18
1902: A tornado killed 114 in Goliad, Texas.
May 19
2018: Cyclone Sagar struck northwestern Somalia, killing 56 people in three countries. It was the strongest tropical cyclone on record to strike Somalia.
May 20
1916: A tornado struck the town of Codell, Kansas. Incredibly, the same town would be hit by two more tornadoes on May 20, 1917 and May 20, 1918.
May 21
1976: Typhoon Pamela passed over the island of Guam, damaging or destroying 80% of the island's buildings but only killing one person.
May 22
2004: An F4 tornado, one of the widest on record peaking at 2.5 miles (4.0 km) wide, destroyed the village of Hallam, Nebraska.
May 23
1917: Edward Lorenz, meteorologist and original founder of the field of chaos theory, was born in West Hartford, Connecticut.
May 24
1686: Daniel Gabriel Fahrenheit, inventor of the Fahrenheit temperature scale used primarily in the United States, was born in present-day Poland.
May 25
1955: Several significant tornadoes, including two F5's, killed more than 100 people in the Central United States
May 26
1917: At one point thought to be the longest single tornado track on record, a family of several tornadoes killed over 100 people in Illinois and Indiana.
May 27
1896: A violent tornado tore through downtown Saint Louis, Missouri, killing at least 255 people.
May 28
1941: After searching for several hours, the German weather ship Sachsenwald recovered two survivors from the sinking of the battleship Bismarck
May 29
2013: Hurricane Barbara made landfall in Tonalá, Chiapas, Mexico, killing 4 people. This was the easternmost landfall for a Pacific hurricane on record, as well as the second-earliest in the year.
May 30
1998: After a supercell spawned an F4 tornado that nearly wiped out the town of Spencer, South Dakota, the storms organized into a destructive derecho that tracked over 1,000 miles (1,609 km) across the southern Great Lakes region of the United States and Canada.
May 31
1985: More than 40 tornadoes were spawned around the eastern Great Lakes in the 1985 United States-Canadian tornado outbreak, including the only F5 ever recorded in that area.