The onset of the monsoons over the Kerala coast is expected in the first week of June. And by next week the weatherman will be in a position to give his official forecast about how the monsoons will do this year.

Says Dr M Lal, Chief Scientific Officer at the Centre for Atmospheric Sciences: "Data for the period 1991-1998 indicates that the percentage of rainfall excess of the normal has been on the rise. This is especially true in the case of North India, including Punjab, Haryana, Chandigarh and Delhi. Data on Kerala also indicates a slight departure from normal."

Taking thecountry as a whole, IMD statistics indicate a trend of above-average rainfall right through the '90s. Between 1991 and 1998, a majority of the 35 meteorological sub-divisions had either normal or excess rains. Last year, 20 subdivisions recorded excess rainfall.

Data available with the Centre for Atmospheric Sciences at the Indian Institute of Technology, Delhi, indicates that monsoons tend to follow alternating patterns. Between 1871 and 1900 there was excess rainfall during the monsoons; between 1901 and 1930 it was deficient. The next three-decade period saw excess rains, while between 1961 and 1990 it was deficient. Keeping to the pattern, the rainfall recorded since 1991 has been in excess.

So will the pattern continue? No, say IMD officials. According to them, there are too many parameters and factors involved and at present the only tried and tested model for the forecast is a statistical model called the `Parametric and Power Regression Model'.

"People working in the field use differentparameters in their models," says Dr U S Dey, officiating director general of the IMD. "Research work is on to try and improve on the present model, but as of today that is the most dependable model we have."

"May is a crucial month for us," explains Dey. "Data collected in this month is significant when we make our calculations. With the normal onset of monsoons on the Kerala coast between the last week of May and the beginning of June, we work under a lot of pressure to come out with the forecast before the rains. Also, rains at this time of the year don't always mean monsoons. We declare the onset of monsoon only when other physical elements indicate it."

However, with the weather elements known to break the rules over the last few years, all forecasts and predictions are accompanied by a five per cent variable. "Forecasting today predicts the average weather for a given period of time; extreme conditions have become the problem," says a Met Department official.