Weather Bee | How unusual is the heavy rain flooding northeastern states

The trend in rain is usually checked against the average of rain over a long time period, called the Long Period Average (LPA). The IMD currently uses the 1971-2020 period for calculating the LPA. An analysis of 2024 monsoon rain (the season officially begins on June 1) shows that four of the eight northeastern states — Meghalaya, Sikkim, Assam, and Nagaland — have more than 20% surplus over the LPA, which is IMD’s threshold for “excess” or “large excess” rain. The rain for the season so far is also ranked very high for Meghalaya, Sikkim, and Assam: fifth, ninth, and 12th highest for this period since 1901.

To be sure, the other four states not breaking records for the season overall does not mean that they are dry. The past week has been exceptionally rainy in all northeastern states except Sikkim. The rain in 2024 for the week ending July 3 is among the top ten highest recorded since 1901 in all these states except Sikkim, where it is the 40th highest. Last week’s rain is between 1.5 and 2.3 times the LPA in the seven states, excluding Sikkim.

How much of these states have been affected by excess or large excess rain? In all states except Manipur, where overall rain in the season is not ranked high, at least 25% of the state’s area is affected by excess or large excess rain this monsoon. In Sikkim, which has been affected by floods and landslides since the beginning of the season, 92% of the area is affected by such a large surplus of rain. Similarly, in Assam, Meghalaya, and Nagaland, at least half of the state’s area is affected by excess or large excess rain.

IMD’s data also shows that parts of all northeastern states have been affected by more than usual days of heavy or extreme rain (rain of more than 35.5 mm in 24 hours). Over 50% of all northeastern states, except Manipur, have experienced more days of heavy or extreme rain than in the LPA. In Sikkim, Meghalaya, Assam, and Nagaland, such area covers 80%-100% of the state’s area.

The data described above makes it clear that all northeastern states are experiencing an exceptionally wet monsoon season so far, although in Manipur this trend might have started slightly later than in other states. However, the loss of lives and property is almost always a function of the resilience we build to extreme weather events. Such resilience can be built either through the management of the river’s flow or its banks, early warning systems and effective communication of adverse weather events, protecting the natural environment from exploitation, and other measures that are well-documented by the scientific community. The lack of action on these measures causes loss of lives and property in extreme weather events.

Abhishek Jha, HT’s assistant editor-data, analyses one big weather trend in the context of the ongoing climate crisis every week, using weather data from ground and satellite observations spanning decades.