The India Meteorological Department (IMD) has decided to put the low-pressure area over the Andaman Sea and the South Bay of Bengal under watch for next five days, even dropping hints that it may have overstayed its welcome. But the European Centre for Medium-Range Weather Forecasts (ECMWF) has persisted with a watch for a cyclone developing in the area.

This, in a nutshell, brings into sharp focus the unpredictability of predicting cyclogenesis (birth of a cyclone) during the pre-monsoon season in the tropical waters of the Bay of Bengal. At least two timelines set by the IMD had passed before the preparatory low-pressure area showed up four days ago. Ever since, it has kept weathermen on tenterhooks as to its real intentions. The delayed genesis had, in a way, signalled the waning forecaster confidence vis-à-vis its shelf life.

Contender in the offing?

Interestingly, the IMD’s analysis of the genesis potential parameter, a product based on a few critical dynamical meteorological variables, is now pointing to the development of a contender circulation to the South-West (closer to Sri Lanka and South Tamil Nadu coasts) on May 9 (Saturday).

This would automatically mean the demise of the extant low-pressure area over the Andaman Sea in due course. In any case, the IMD has ruled out its its intensification for the next five days. The contender circulation developing over the South-West Bay on Saturday is seen moving in a North-North-West track (towards the Tamil Nadu coast) till May 11.

Satellite-based assessments suggested that winds had picked up speeds of up to 56 km/hr over the South-West Bay on May 4 and, into the night, 37-46 km/hr to the South-West of the current area of low pressure. The total precipitable water imagery indicated a significant decrease in warm moist air incursion from the Equatorial region, needed for strengthening of the existing system.

ECMWF sticks its neck out

The IMD aid that most numerical models — including IMD-GFS (Global Forecast System), NCEP-GFS (US National Centers for Environmental Prediction), GEFS (Global Ensemble Forecast System), NEPS (Ensemble Prediction System of India’s National Centre for Medium-Range Weather Forecasts, NCMRWF) and NCUM (A global model from NCMRWF) — are still not able to capture the current low pressure, and do not predict any cyclogenesis during the next five days.

This is where ECMWF seeks to stick its neck out by suggesting cyclogenesis over South Andaman Sea on May 8, with intensification into a cyclonic storm by May 11 and gradual weakening by May 14 over West-Central Bay of Bengal. As regards onward movement, the European model is suggesting a track to North-West till May 12, followed by a shift to North-East and a recurvature away from the Indian coast.

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