Vinson Kurian

Thiruvananthapuram , June 2

With model predictions tending to defer the formation of the Bay `low' by a day, meteorologists have been forced to get back to their drawing boards and plot the monsoon itinerary afresh for Central and Northwest India.

Cross equatorial flows that feed the monsoon flows both over the Arabian Sea and Bay of Bengal during the onset phase are weakening, which means that a major enabler of an `in situ' Bay system may be making itself increasingly unavailable.

Bay systems taking shape during monsoon onset phase have to fend for themselves, not having the luxury of back-up support from migrating systems from neighbouring oceans. It's mostly in July/August that such systems migrate in from the South China Sea/Western Pacific, said Dr Akhilesh Gupta of the National Centre for Medium Range Weather Forecasting (NCMRWF).

In its forecast on Friday, the NCMRWF said predictions continue to indicate weakening of the monsoon current over Arabian Sea from Sunday, leading to a decrease in rainfall over the west coast. The Bay `low' is now being forecast to form on `Sunday or Monday,' based on the assumption that the monsoonal flow over Bay of Bengal region will keep strengthening.

But this is considered untenable, since it's the same cross equatorial flows that go into feeding the two arms of the monsoon system. The periodic monsoon-friendly Madden-Julian Oscillation (MJO) wave travelling to the east is not expected to traverse the Indian Ocean next until June 19. A passing westerly wave, the MJO becomes amplified during the monsoon season and helps stimulate the Inter-Tropical Convergence Zone (ITCZ) into activity.

Another major irritant to the monsoon system is a western disturbance crossing in from the Northwest border. On Friday, it was seen interfering with the monsoonal current over Gujarat and disturbing the flows.

Their interaction resulted in a string of untimely thunderstorms lining up in an arc reaching from West India through Central India to the Northeast. At least 75 people are estimated to have died from lightning strikes as powerful storms tore through the geography over the past two days.

There are now indications that the western disturbance is dominating over the feebler southwest monsoon current. The northwesterlies packed in the westerly system are considered inimical to the orderly progress of monsoon. The `heat low' over West Rajasthan has also been compromised with.

The NCMRWF said in its forecast that Northwest and Central India may continue to experience scattered rainfall activity and below normal day temperatures during the next 24-36 hours. This north-south oriented rainfall zone is expected to weaken and shift eastward after 24 hours, before petering out.

Meanwhile, the southwest monsoon advanced into parts of Gujarat on Friday. The northern limit of monsoon passed through Porbandar, Rajkot, Ahmedabad, Baroda, Pune, Sholapur, Kurnool, Ongole and Gangtok.

`Not yet time for easterly jet'

The Tropical Easterly Jet (TEJ) along Chennai latitudes is not expected to take its normal position until the monsoon makes a westward progression from the Bay of Bengal, says Dr Akhilesh Gupta.

This will happen only with a low-pressure area taking shape in the Bay and starts moving in a west-northwestward direction, he said in response to a related report appearing in Friday's Business Line. "A close monitoring of the TEJ may reveal that this might move northward and take a position close to Chennai latitude once the monsoon progresses westward and establishes itself fully over the country," Dr Gupta added.

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