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Low-pressure off Sri Lanka to spark wet weather over South India

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True to forecasts, the Equatorial Indian Ocean to the South of Sri Lanka and the adjoining South-West Bay of Bengal are readying themselves for what could likely be the last major stirring during the ongoing North-East monsoon season with a low-pressure area being thrown into the scheme of things.

India Meteorological Department (IMD) expects the ‘low’ to develop by Friday and kick up squally weather (wind speeds of 40-50 km/hr gusting to 60 km/hr to almost depression strength) until Sunday. Fishermen (from Sri Lanka or Tamil Nadu) are advised not to venture into these areas.

Minimum purchase for South

The South Peninsula of India may not get much purchase from the ‘low’ except in light to moderate rainfall at isolated places over Tamil Nadu, Puducherry, Karaikal, Kerala and Mahe for next five days.

Light to moderate rainfall is also likely at a few/many places over Andaman & Nicobar Islands until Monday while being heavy rainfall over the Nicobar Islands on Monday, the IMD said.

This aligns with the outlook of the Climate Prediction Centre of the US National Weather Service which points to the development of the ‘low’ thanks to a Madden-Julian Oscillation (MJO) pulse lurking over the Maritime Continent (Indonesia et al) across the international waters farther to the East.

Different takes by models

The US agency says that major models, Global Ensemble Forecast System (GEFS) and the European Centre for Medium-Range Forecasts (ECMWF), have different takes on the evolving scenario.

Several GEFS ensemble members indicate development of the ‘low’ near Malaysia or southern Thailand while the ECMWF ensemble favours a system developing closer to Sri Lanka and a track to the East (farther into the South-West Bay and away from Sri Lanka and Tamil Nadu).

While these models depict formation on opposite sides of the Bay of Bengal, the projected tracks converge near the Andaman and Nicobar Islands with a broad moderate risk area is designated to account for the scenarios depicted in the GEFS and ECMWF models.

Cold to severe cold wave

Elsewhere in the country, the IMD has forecast a dip in night (minimum) temperatures) by 2-4 degrees Celsius over most parts of North-West and adjoining Central India and Gujarat during next 4-5 days and by 2-3 degrees Celsius over most parts of East India and Maharashtra during next four days.

Cold wave/severe cold wave conditions may develop over Punjab, Haryana, Chandigarh, Saurashtra and Kutch until Tuesday; over North Rajasthan during Saturday-Tuesday; over West Uttar Pradesh during Sunday-Tuesday and over Gujarat on Sunday/Monday as western disturbances keep away.

Moisture-laden western disturbances, depending on their status (moderate to active), have a major role in modulating winter weather by kicking up of snow in the hills and thundershowers in the plains of North-West India. These are welcome rain for the standing Rabi crop as well.

Modulating winter climes

They warm up the atmosphere since associated clouds prevent solar radiation from escaping into the atmosphere, thus keeping at bay cold wave or severe cold wave conditions.

Mid-December also marks the beginning of the fog season over North, North-West and East India that lasts into mid-January with implications for vehicular, train and air traffic.

The IMD has forecast dense/Very dense fog in the morning hours over Punjab and Haryana on Friday and Saturday; over North-West Rajasthan, Assam, Meghalaya, Nagaland, Manipur, Mizoram and Tripura on Friday, as a weak western disturbance travels from West to East.

Fog, ground frost predicted

On the other hand, ground frost conditions are likely to develop in the morning hours in isolated pockets over Saurashtra, Kutch, Punjab and Haryana during next four days as a corollary to the cold wave. Ground frost is not a good augury for the standing crop.

Dense fog combines with lingering moisture left behind by a western disturbance, which can either drift as thick fog or settle on ground as frost. This is because colder westerly to north-westerly winds fill the space vacated by warm western disturbances crossing in from the Pakistan border.

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