The Hindu Business Line : Rains pound East as depression crosses in

Thiruvananthapuram, June 17 The monsoon depression in the Bay of Bengal has crossed over into Gangetic West Bengal and adjoining Bangladesh, and is expected to keep tracking to the northwest to preside over a productive wet session in east and central India.

‘Monsoon depressions’ are a class apart from normal tropical depressions in that they are much larger in size, have extended life over land, and produce copious rainfall over larger swathes of geography. They are basically ‘low’s forming within the monsoon trough.

Upper winds in the Bay are too strong to allow a tropical storm or stronger cyclonic circulation to develop. So, the system is left with maximum strength above the surface, but beneath strong upper level wind shear that prohibits it from developing into a tropical storm.

COPIOUS RAIN

The large size and typical slow track toward the west or northwest lends the system the firepower to produce copious amounts of rain and cause widespread flooding. Since winds buffeting the system rarely exceed 64 km/hour and their circulation is strongest above the ground, they can persist for many days over land with little weakening. Hence, they can be prolific rain producers.

In view of this, the India Meteorological Department (IMD) has forecast widespread rainfall with heavy to very heavy falls at a few places, while isolated extremely heavy falls of up to 25 cm have been warned of over an already soaked Gangetic West Bengal during the next 24 hours.

The heavy rain belt is shown to hang over adjoining north Orissa and Jharkhand for another 24 hours, the IMD update said on Tuesday. Fairly widespread rainfall with isolated heavy to very falls is also likely over south Orissa, Chhattisgarh and the North-East during this period.

Also falling under footprint of the enhanced rain belt are Uttar Pradesh, north Madhya Pradesh and Uttarakhand to the northwest, which are predicted to receive fairly widespread rainfall with isolated heavy falls, the IMD said.

Extended forecast for three days from Friday suggests that the heavy rainfall over east and northeast India would relent, with the monsoon depression (or ‘low’) moving further northwest and dragging the rain belt along.

MAIN FEATURES

The main monsoon features that drive the rainfall activity is the west-to-east seasonal trough passing through Anoopgarh, Churu, Kanpur, Varanasi, Dumka, the centre of depression and southeastward into the east-central Bay. Additionally, an upper air cyclonic circulation hung over Uttar Pradesh and neighbourhood.

The eastern end of the trough is shown to retreat into interior India in line with the movement of the depression that holds the trough pinned to that end. This movement will be maintained until Saturday, when the likely formation of another weather system would cause the trough to revert and dip back into the Bay.

The European Centre for Medium-Range Weather Forecasts (ECMWF) and the US Centre for Ocean-Land-Atmosphere Studies (COLA) are in agreement with the above outlook. But they differ in the articulation of the exact co-ordinates where this is expected to happen.

Ensemble forecasts from the US National Centres for Environmental Prediction (NCEP) indicate that the monsoon would continue to be active until June 30 with blow-ups of rainfall over the west coast, the Heady Bay and adjoining northeast India, east India and parts of central India. But Rajasthan in the northwest and Tamil Nadu and coastal Andhra Pradesh in the south would remain largely dry.

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