Jim Andrews

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Southwest Monsoon Onset Four Weeks Early

June 16, 2013; 1:33 PM ET

Onset of the southwest monsoon has taken place deep into Pakistan as many as four weeks earlier than usual, according forecasters with the India Meteorological Department (IMD).

Monsoon onset was June 13, 2013, in Delhi, almost two weeks earlier than average, said the IMD. The June 15 onset at Karachi and Islamabad was more like three weeks ahead of schedule.

The abnormally early advance of the seasonal weather phenomenon has left unusually early soaking rain in many parts of Pakistan and northwestern India. The characteristic pre-monsoon heat has also been beaten back, thanks to widespread rain-cooled air.

Meanwhile, the widespread rains that accompanied the fast-charging monsoon were set to pull back significantly going forward in Pakistan and northwestern India.

Average and 2013 monsoon onset dates as plotted by the India Meteorological Department (IMD). The monsoon advance as of 16 June was effectively the same as the historical average for 15 July. (Credit: IMD website)

The exceptional northwestward thrust of the summer monsoon began after the first week in June, when a central Asian heat wave lifted the jet stream westerlies north of the subcontinent. At the same time, two monsoon lows at the critical precipitation level of about 3,000 meters whirled above the region, one off the west coast and the other tracking northwestward from the Bay of Bengal. These two lows were instrumental in spreading moist, rain-cooled air into all corners of the subcontinent, affording an environment primed for localized torrential rain.

The latter half of the week marked the furthest northwestward ahead of significant falls of rain, as drenching thunderstorms erupted north and west of the Indus River in Pakistan. It was at this time that low pressure over central Asia, a "Western Disturbance," dipped southward, forcing the jet stream westerlies to re-invade the north. At first, interaction between the Western Disturbance and the monsoon low from central India triggered further cloudbursts, especially along the foothills in northern India. These rains were ongoing as of Sunday night, local time.

Continued southeastward flow of dry air into the northern subcontinent will, by midweek, cut rain coverage to no more than isolated thunderstorms. At the same time, it will shut off the flooding rains along the foothills of northern India and western Nepal. As a result of these changes, the fraction of the subcontinent subject to significant falls of rain will return to about the historical average for the latter third of June.

By the end of the week, a new atmospheric player in the form of another monsoon low should enter the field of subcontinent weather. Its expected northward advance into India should instigate another boost in rain coverage and intensity.

-Some Rainfall Stats

Extreme falls of rain have hit the foothills region with tragic result. Eight people were reported dead as of Sunday in Uttarakhand, the Times of India website said. Rainfall since early Saturday, local time, was 568 mm (22.36 inches) within 45 hours, weather data accessed by AccuWeather.com showed.

New Delhi got its first heavy falls of the year, with at least 45 mm falling late Sunday, and a 60-mm late-day burst left Ahmadabad, Gujarat, with monthly rainfall above the normal June average.

In Pakistan, mid- to late-week rains left a number of cities and towns with many times the normal June rainfall. Jacobabad got a 50-mm downpour Saturday, giving nearly 20 times the average June rainfall.

Lahore rainfall rose well above historical June averages. In Karachi, rainfall was "hit or miss," with the "official" amount barely measurable.

The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com

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