Will it or won’t it? India’s monsoon forecast gamble
The India Meteorological Department (IMD) has released the long-awaited update of its long range forecast for the 2012 monsoon.
Stripped of its scientific jargon, this is what the update has said. There is a July model and an August model. For both months, there are three forecast categories: below normal in which rainfall in less than 94% of the long period average (LPA), normal in which the rainfall is between 94% and 106% of the LPA, and above normal in which rainfall is more than 106% of the LPA. Under the three categories, the forecast probabilities for July are (in the same order) 36%, 41% and 23% and for August they are 42%, 36% and 22%. Under any combination of probability therefore, this means that both July and August are going to be drier than usual, and coming on top of an unusually dry June, the scenarios for water availability and for agriculture come early September are all looking tough.
The volatility of the 2012 monsoon over north-eastern India can be seen in the images of the district weekly rainfall deviations for those states. Please bear in mind that with the late beginning of the 2012 monsoon, the week of June from 07 to 13 was for all practical purposes the first monsoon week. The colours signify major deviations – red for 50% of the average and below, green for 150% of the average and above. In Arunachal Pradesh, for the first week the average rainfall in districts was around 45%, the second week it was 41% and the third week it shot up to 124% – red is evenly scattered through the districts in the first two weeks and green districts appear in the third week.
In Assam, the first week’s average for all the state’s districts was 65% of the long period average, with red dominating. In the second week the average was 103%, with ‘red’ districts declining and a few greens appearing. In the third week the average zoomed to 184% with most districts being ‘green’. In neighbouring Meghalaya, the average for the districts in the three weeks was 63%, then 51% and then a steep rise to 225% in the third week. In stark contrast Nagaland and Manipur have for the duration of these three weeks seen a combined district rainfall average of 33% and if we remove the ‘green’ districts from both states of the third week, we get a dismal 15% average – it is of course quite likely that there are data anomalies in the numbers that IMD has collected from the north-east region, as automated weather stations that actually work are likely to be fewer in number than in ‘mainland’ India. (There is a spreadsheet for this data. If you want the data till date please write to me here: makanaka at pobox dot com.)
In the update, there is also a separate set of forecasts and probabilities for four major regions of India – North-West India, Central India, South Peninsula and North-East India. There are small variations for each of these in the definitions of below normal, normal and above normal. Here are the forecast probabilities for the regions:
The list of states in each of these four geographical regions is:
Northwest India: Jammu and Kashmir, Himachal Pradesh, Punjab, Rajasthan, Haryana, Chandigarh, Delhi, Uttaranchal and Uttar Pradesh.
Northeast India: Arunachal Pradesh, Meghalaya, Assam, Nagaland, Manipur, Mizoram, Tripura, Sikkim, West Bengal, Bihar and Jharkhand.
Central India: Gujarat, Madhya Pradesh, Chattisgarh, Maharashtra, Goa and Orissa.
South Peninsula: Andhra Pradesh, Karnataka, Tamil Nadu, Kerala, Lakshadweep and Andaman and Nicobar Islands.
The first stage forecast for the nation-wide season rainfall was issued on 2012 April 26 and this update was issued on 2012 June 22. The summary of the first stage forecast is:
“Southwest monsoon seasonal rainfall for the country as a whole is most likely to be Normal (96-104% of Long Period Average (LPA)) with the probability of 47%. The probability (24%) of season rainfall to be below normal (90-96% of LPA) is also higher than its climatological value. However, the probability of season rainfall to be deficient (below 90% of LPA) or excess (above 110% of LPA) is relatively low (less than 10%). Quantitatively, monsoon season rainfall is likely to be 99% of the LPA with a model error of ± 5%. The LPA of the season rainfall over the country as a whole for the period 1951-2000 is 89 cm.”
The IMD has said that it has taken into account the experimental forecasts prepared by the national institutes like Space Applications Centre, Ahmedabad, Centre for Mathematical Modeling and Computer Simulation, Bangalore, Center for Development of Advanced Computing, Pune and Indian Institute of Tropical Meteorology, Pune. Operational/experimental forecasts prepared by international institutes like the National Centers for Environmental Prediction, USA, International Research Institute for Climate and Society, USA, Meteorological Office, UK, Meteo France, the European Center for Medium Range Weather Forecasts, UK, Japan Meteorological Agency, Japan Agency for Marine-Earth Science and Technology, Asian-Pacific Economic Cooperation (APEC) Climate Centre, Korea and World Meteorological Organization’s Lead Centre for Long Range Forecasting – Multi-Model Ensemble have also been taken into account.