From AMS Glossary
It is attributed by Bn. Banerji (1938) to a cool dry upper current from the north or northwest meeting a shallow surface flow of warm air from the Bay of Bengal along a quasi-stationary front. It is similar to the shamal. The front advances northwestward as the season progresses, and similar phenomena termed loo or uala-andhi extend into the United Provinces and Punjab.
Banerji, Bn. 1938. Nature of ‘Nor'westers’ of Bengal and their similarity with others. Beitr. Physik fr. Atmos. 24. 231–233.