May 1
edit2012: 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
edit1982: The Weather Channel went on the air in the United States.
May 3: Start of National Hurricane Preparedness Week 2020
edit2000: The GOES 11 weather satellite was launched. Part of the Geostationary Operational Environmental Satellite program, GOES 11 was used as a backup satellite until it replaced the GOES 10 in 2006, and would provide weather forecasting data mainly for the Western United States and Pacific Ocean from 2006 until its retirement in 2011.
May 4
edit1971: Saint-Jean-Vianney, Quebec was partially destroyed in a devastating landslide after weeks of heavy rain. The unscathed parts of the town were subsequently abandoned due to the threat of future landslides.
May 5
edit1995: 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
edit1965: A series of violent tornadoes affected the Twin Cities area of Minnesota.
May 7
edit1981: Tropical Storm Arlene moved across Cuba. Minimal damage was reported.
May 8
edit2003: Cyclone Manou reached peak intensity just 19 kilometres (12 mi) off the coast of Madagascar. Though it did not make landfall, the storm killed 89 people across the country.
May 9
edit1990: A severe tropical cyclone made landfall in the Andhra Pradesh state of India, killing nearly 1,000 people.
May 10
edit1933: A tornado killed almost every resident of the small town of Beatty Swamps, Tennessee. Because of the destruction, the town was abandoned permanently.
May 11
edit1996: A severe blizzard killed eight people near the summit of Mount Everest.
May 12
edit1997: An F1 tornado affected downtown Miami, Florida, causing minor damage but providing for spectacular photos and news footage.
May 13
edit1998: 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
edit2007: 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
edit1998: A rapidly moving squall line raced across Minnesota and caused over $1 billion dollars of wind–driven hail damage.
May 16
edit1986: Severe flooding from an ice jam completely destroyed the town of Winisk, Ontario, which was subsequently abandoned. Two people were killed.
May 17
edit2001: A Yakovlev Yak-40 aircraft crashed while flying through a thunderstorm near Sari, Iran, killing 30 people, including several members of the Islamic Consultative Assembly and other government officials.
May 18
edit1902: A tornado killed 114 in Goliad, Texas.
May 19
edit1780: An unusual darkness, lasting from noon until after sunset, fell over parts of New England.
May 20
edit1970: Hurricane Alma reached peak intensity in the Caribbean Sea.
May 21
edit1976: Typhoon Pamela passed over the island of Guam, damaging or destroying 80% of the island's buildings but only killing one person.
May 22
edit1951: Hurricane Able, the strongest off-season Atlantic hurricane ever recorded, reached its peak strength of 115 miles per hour (185 km/h) off the coast of Cape Hatteras.
May 23
edit1995: The GOES 9 weather satellite was launched. GOES 9 would provide weather forecasting data to the National Oceanic and Atmospheric Administration until 1998, and was leased to the Japan Meteorological Agency from 2001–2003.
May 24
edit1988: TACA Flight 110 lost power to both its engines due to water ingestion after flying through a heavy thunderstorm. Unable to reach an airport, its pilots made a successful deadstick landing on a grass levee near NASA's Michoud Assembly Facility.
May 25
edit1989: Tropical Storm Cecil made landfall near Hội An, Vietnam, causing severe flooding that killed more than 700 people.
May 26
edit1917: 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
edit2018: For the second time in two years record flooding struck Ellicott City, Maryland.
May 28
edit1879: Milutin Milanković, famous for relating the cycle of ice ages to characteristics of Earth's orbit, was born in Erdut, Croatia (then Austria-Hungary).
May 29
edit2010: Tropical Storm Agatha made landfall near the Guatemala–Mexico border, killing more than 200 people due to flooding, landslides, and a sinkhole.
May 30
edit1998: 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
edit1953: Tropical Storm Alice moved over the western tip of Cuba.