Origin, track of twin end-November cyclones over Bay surprise weathermen

Ditwah was the first cyclone after Burevi (November 30-December 2, 2020) that crossed east Sri Lanka coast on December 2
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BALACHANDAR L

End-November cyclones Senyar and Ditwah in Bay of Bengal surprised meteorologists and weather watchers not just in regard to the respective areas of their origin but also in terms of their timing and tracks. 

Two cyclonic disturbances sprung up over the Bay during last week of November, a special note brought out by India Meteorological Department (IMD) said. Earliest such occurrence was in 2013 when back-to-back cyclones Helen (November 19 to 23) and Lehar November 23 to 28) crossed the Andhra Pradesh coast. 

Unique cyclone Senyar

Senyar proved to be a unique system as birth of cyclones in such lower latitudes as the Malacca strait is quite rare, the IMD note recalled. It was the first cyclone in that region as per IMD’s recorded history since 1891. 

Senyar had a recurving track. Initially, it moved west-southwestwards till early morning on November 26, but gradually recurved and moved southeastwards till early hours of next day; and then eastwards till midnight. Senyar returned six-hourly average translational speed of 10.5 km/hr against normal of 15.5 km/hr for post-monsoon cyclonic storms over the Bay from 1990-2013. 

Slow-moving systems 

Thus, it was a slow moving cyclone, leading to extremely heavy rainfall activity over Indonesia. Senyar’s life lasted two days and 15 hours against long period average (1990-2013) of about three days and 14 hours. 

It did not undergo rapid intensification/weakening during its lifecycle and was noted for low wind impact, if any. It caused extremely heavy rains, flash floods and landslides in Indonesia and Malaysia, but no adverse weather over next-door Andaman and Nicobar Islands, though adjoining seas were ‘rough’ to ‘very rough.’ 

Ditwah sinks Lanka 

In comparison, Ditwah was even more slow-moving than Senyar at a six-hourly average speed of 9.8 km/hr, bringing in its wake extremely heavy rainfall activity over Sri Lanka during November 25 to 29. Interaction with orography (land features) over central Sri Lanka led to enhancement of rainfall activity. 

Unpredictable track 

Operational track and intensity forecast errors were comparatively higher than long-period average since Senyar was involved in multiple recurvatures and interactions with circulationsover southern Bay and the north-west Pacific. But intensity forecast skills were markedly above long-period average. Track forecast errors for Ditwah were markedly less than long-period average though intensity forecast skills were above long-period average.

First after Burevi

Ditwah was the first cycloneafter Burevi (November 30-December 2, 2020) that crossed east Sri Lanka coast on December 2. Ditwah’s track was unique since no such track is on IMD record during satellite era since 1961. It had struck an almost northward track from south of Sri Lanka to south-west Bay close to North Tamil Nadu coast.