The IMD expects heavy to very heavy rain over Sikkim and parts of the east on Monday, with heatwave conditions still likely over Uttar Pradesh and Delhi. A simple look at the science driving both. (Photo: PTI)

Very heavy rain to lash Northeast India Monday, heatwave to grip North

The IMD has forecast heavy to very heavy rain over the northeast and parts of the east on Monday, June 29, with heatwave conditions likely over Uttar Pradesh and Delhi. This piece explains the monsoon trough, orographic lifting and the systems driving the day's weather.

by · India Today

In Short

  • Northeast India faces heavy to very heavy rain alert Monday.
  • Heat wave still grips parts of Uttar Pradesh and Delhi.
  • Thunderstorms and gusty winds are likely across many Indian states.

The heaviest rain in the country on Monday will fall where the land begins to climb.

The India Meteorological Department (IMD) expects the wettest skies of the day to settle over Sub-Himalayan West Bengal, Sikkim and the hills of the northeast, with some slopes in line for more than 20 cm of rain in a single day.

WHICH STATES WILL SEE THE HEAVIEST RAIN ON JUNE 29?

A day when a region receives more than 20 cm of rain is something forecasters call an extremely heavy day, enough to flood low-lying streets within hours. Beyond the hills, other regions in Arunachal Pradesh, Assam and Meghalaya are braced for isolated very heavy rain, as are pockets of Kerala.

Heavy rain is likely over Bihar, Odisha, Telangana, coastal Karnataka, Konkan and Goa, Maharashtra, and Tamil Nadu.

Sub-Himalayan West Bengal and Sikkim are forecast to receive isolated extremely heavy rainfall of more than 20 cm on June 29. (Photo: PTI)

To set the scale, Mawkyrwat in Meghalaya gathered 39 cm in the 24 hours to the morning of June 28.

Thunderstorms, lightning and gusty winds of 40 to 50 km/h, touching 60 km/h, are forecast across a long list of states, from the northeast down to the southern peninsula.

WHY IS NORTH INDIA STILL SO HOT IF THE MONSOON HAS COME?

Because the monsoon has not reached everywhere. On June 28, the southwest monsoon's leading edge, which forecasters call the Northern Limit of Monsoon, ran through Surat, Indore, Mandla and Motihari.

Above that line, including much of Uttar Pradesh and the Delhi region, the air is still hot, dry and continental.

Heat wave conditions are likely in isolated pockets over Uttar Pradesh and the Delhi region, where the day temperature may touch 42°C. (Photo: PTI)

Heat wave conditions are likely in isolated pockets over Bihar, Uttar Pradesh and the Haryana, Chandigarh and Delhi belts on Monday. A heat wave, in IMD terms, is a day when the temperature reaches at least 40°C in the plains and stays well above normal.

Phalodi in Rajasthan touched 43.8°C on June 27. Delhi can expect 40°C to 42°C, with a short evening burst of light rain, thunder and winds gusting to 50 km/h.

WHAT MAKES THE HILLS FLOOD WHILE THE PLAINS BAKE?

Two ingredients. The first is the monsoon trough, a long belt of low air pressure now stretching from Punjab to Bihar. Air always flows in towards low pressure, then rises, cools and sheds its moisture. This trough is steering wet sea air straight into the east.

The second is orographic lifting, the plain fact that mountains push air upward. As moist Bay of Bengal air climbs the slopes of the eastern Himalaya and the Khasi Hills, it cools, the water vapour condenses, and the rain grows heavier. It is why Sikkim and Meghalaya so often top the country's rain charts.

The monsoon trough and the upward push of the hills, known as orographic lifting, explain why the eastern slopes flood while the northern plains stay dry. (Photo: PTI)

A few cyclonic circulations, swirls of inward-spinning wind over Madhya Pradesh, the Arabian Sea and Telangana, are wringing out extra showers across the middle of the map.

The next western disturbance, a rain-bearing system that drifts in from the Mediterranean, reaches the northwest from July 2, when the plains should finally begin to cool.

- Ends