The Hindu Business Line : `Low' materialises, to decide monsoon onset in Delhi

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`Low' materialises, to decide monsoon onset in Delhi

Vinson Kurian

Thiruvananthapuram June 26 True to predictions, a remnant of erstwhile tropical storm `03B' has set up a low-pressure area in the northwest Bay of Bengal on Tuesday, which is expected to whip itself up as a depression by Wednesday.

This will activate a round of unremitting rains over Orissa, south Chhattisgarh and north Andhra Pradesh to begin with, propagating further in a likely west-northwest direction subsequent to the system making a landfall over the Orissa coast.

International weather models indicate that this would go on to become another numbered tropical storm (`04B') or even a named tropical cyclone (`Yemyin') in the Bay. But this bears some watching, as does the exact direction the system is likely to take, said Dr Akhilesh Gupta, Adviser to the Department of Science and Technology.

The wind shear and vorticity tendencies are positive, while the `ocean residency' factor militates against the system deepening beyond a threshold level. Another favourable indicator is the likely shifting of the Tibetan anti-cyclone to the east from its anomalous position currently.

`Locking up'

On the flip side, this will cause the `locking up' and weakening of a western disturbance coming from the opposite direction. In fact, the onset of monsoon in Delhi and west Uttar Pradesh hinges on the interaction that a properly evolved western disturbance sets up with the monsoon easterlies. This works on the premise that the supportive `low' from the Bay moves in a west-northwest direction, which, again, is something that needs to be watched.

Dr Gupta said Delhi would get some rain over the next two days or so but not some areas in Uttar Pradesh. This pattern is attributable more to the western disturbance than monsoon proper. But orderly behaviour of the monsoon `low' from the Bay can still work things in Delhi's favour. June 29 is the normal date of onset for the national Capital.

Quoting data spanning over the last 106 years, Dr Gupta said the monsoon onset in Delhi was late during 63 years, early in 37 and `on the dot' in six. The data was skewed in favour of a `late than normal' onset.

Onset pattern

The onset happened a week beyond the normal in 36 years while there are only 10 cases (out of total 106) of it taking place a week earlier than normal. The most delayed onset was July 26 in 1987 while the earliest onset took place on June 17 in 1998.

The European Centre for Medium-Range Weather Forecasts (ECMWF) has predicted that the Bay system will move in a west-northwest direction initially before settling into a more westerly track. This will take the system into Gujarat, and cause overlapping of areas traversed and flooded by `03B' earlier.

But an exclusively west-northwest track would take the system to areas as yet uncovered by seasonal rains and would help the cause of evenly distributed precipitation.

On Tuesday, the monsoon advanced into some more parts of east Uttar Pradesh, remaining parts of Uttarakhand, entire Himachal Pradesh, Jammu and Kashmir and the northern parts of West Uttar Pradesh, Haryana and Punjab. The northern limit passed through Deesa, Idar, Hoshangabad, Pendra, Ambikapur, Varanasi, Sultanpur, Lucknow, Bareilly, Ambala and Amritsar.

IMD update

An IMD update said fairly widespread rainfall with isolated heavy to very heavy falls is likely over Orissa, south Chhattisgarh and north Andhra Pradesh during the next 24 hours.

Subsequently, the rainfall activity is likely to enhance further with scattered heavy to very heavy falls and isolated extremely heavy falls (25 cms and above) over these areas. The rains will later extend into Vidarbha and Madhya Pradesh. Fairly widespread rainfall with isolated heavy to very heavy falls are likely over Konkan and Goa (including Mumbai), Madhya Maharashtra and Gujarat region during from Thursday onwards.

Scattered heavy to very heavy falls with isolated extremely heavy falls are also likely over Konkan, Goa, Madhya Maharashtra, Gujarat and southeast Rajasthan from Saturday onwards.

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