A kalbaisakhi (Bengali: কাল বৈশাখী /kaːl boiʃakʰi/, Odia: କାଳ ବୈଶାଖୀ /kaɭɔ̟ bɔ̟isakʰi/,), also known in Assam as a bordoisila (বৰদৈচিলা) and in some English publications as a nor'wester, is a localised rainfall and thunderstorm event which occurs in Bangladesh as well as the Indian states of Bihar, Jharkhand, Odisha, Tripura, Assam, and West Bengal during summer. These storms generally occur in the afternoon or just before sunset, when thick dark black clouds start appearing over the sky and then bring gale-speed wind with torrential rain, often with hail, but spanning only a short period of time.^[1][2]

In Bengali, "kalbaisakhi" refers to a "fateful thing" which occurs in the Bengali month of Baishakh.^[2] The term "nor'wester" is more commonly used in English newspapers since these storms travel from northwest to southeast.^[3] In Assam, the event is known as a bordoisila, which is derived from the Bodo word bardoisikhla, in which bar means wind, doi means water and sikhla means girl, represents the goddess of nature, wind and rain and also marks the beginning of the Assamese month Bohag or Baishagu.^[4]

The period of kalbaisakhi formation generally begins in April and lasts until the monsoon establishes itself over Northeast India, although March kalbaisakhi are not unknown.^[3] Based on event descriptions and the meteorological environments involved, these storms can be classified as progressive derechos.^[5] Kalbaisakhi originate over the Chhotanagpur Plateau between the states of Bihar and Jharkhand. They gradually gain strength as they move southeastwards and impact the states of Odisha, West Bengal, and Assam with gale-speed winds (often exceeding 100 km/h (62 mph)), torrential rains, and hailstorms. On some occasions, incursion of moist air in these elevated places, when combined with high surface temperatures, causes violent thunderstorms.^[1] Tornadoes also rarely form if there is enough moisture, convection and heat in the Ganga-Brahmaputra delta; one such tornado killed over a thousand people when it struck Bangladesh in 1989, making it the deadliest tornado in history.^[6][7]

Kalbaisakhi bring destruction by uprooting trees due to gale-winds and waterlogging roads due to heavy rainfall.^[7] It often damages crop by hailstorms.^[8] However it is extremely helpful for kharif crops like jute, paddy, etc. and give relief after the mid-day heat and give rainfall to the dry soils for the development of the crops.^[1]

Due to global warming, these storms are becoming infrequent and becoming stronger, causing more destruction.^[9]

Many Bengali poets and artists have been regarded kalbaisakhi as events of astounding beauty. Bengali poet Mohit Lal Majumder described the storm in his poem "Kalbaisakhi", and the storms also inspired Rabindranath Tagore's poem "Esho Hey Baisakh".^[1]