  The formation of Middle Tropospheric Cyclones (MTCs) that are responsible for
a large portion of annual precipitation and extreme rainfall events over
western India is studied using an unsupervised machine learning algorithm and
cyclone tracking. Both approaches reveal four dominant weather patterns that
lead to the genesis of these systems; specifically, re-intensification of
westward moving synoptic systems from Bay of Bengal (Type 1, 51%), in-situ
formation with a coexisting cyclonic system over the Bay of Bengal that
precedes (Type 2a, 31%) or follows (Type 2b, 10%) genesis in the Arabian Sea,
and finally in-situ genesis within a northwestward propagating cyclonic anomaly
from the south Bay of Bengal (Type 2c, 8%). Thus, a large fraction of rainy
middle tropospheric synoptic systems in this region form in association with
cyclonic activity in the Bay of Bengal. The four variants identified also show
a marked dependence on large-scale environmental features with Type 1 and Type
2a formation primarily occurring in phases 4 and 5, and Type 2b and Type 2c in
phases 3 and 4 of the Boreal Summer Intraseasonal Oscillation. Further, while
in-situ formation with a Bay of Bengal cyclonic anomaly (Type 2a and 2b) mostly
occurs in June, downstream development is more likely in the core of the
monsoon season. Out of all categories, Type 2a is associated with the highest
rain rate (60 mm/day) and points towards the dynamical interaction between a
low pressure system over the Bay of Bengal and the development of MTCs over
western India and the northeast Arabian Sea. This classification,
identification of precursors, connection with cyclonic activity over the Bay of
Bengal and dependence on large-scale environment provides an avenue for better
understanding and prediction of rain-bearing MTCs over western India.
