The Southwest Monsoon is well under way over the southern half of the Subcontinent as of Monday, June 10, 2013. The India Meteorology Department (IMD) have drawn the line of northern Monsoon advance between the Gulf of Kutch in Gujarat and the West Bengal-Jharkhand border in the east, with the line also drawn northward to the Himalayas in Sikkim.
It is a bit early to have the SW Monsoon under way in Kathiawar, as the northward onset has got a boost in the west from a well-defined Monsoon low near Gujarat. At the same time, advance has been a little slow in the east.
In a nutshell, what is happening as of Monday is that the high-level westerlies of the jet stream have lifted well northward coincident with a central Asia heatwave. This northward shift has afforded room for the building of a well-marked, twin-centered 700-mb trough across the middle of India.
Going forward, the Monsoon trough is forecast to continue to have tandem centers going forward, although forecast scenarios have been varying with each new computer model run. What has been consistent, however, is that moist, rain-cooled air would be driven unusually far to the north and west (for mid-June), even crossing the Indus River Valley in Pakistan. Most likely timing of widespread, locally very heavy, falls of rain in northwestern India and Pakistan would be between Wednesday and next Monday, based open the most consistent aspects of recent numerical model scenarios.
What a dramatic event if this scenario of rainfall play out! GFS forecast for cumulative rainfall through 1200 UTC Tuesday, 18 June, 2013, based up 1200 UTC Monday, 10 June. Dark magenta to black areas imply 20- to 30-cm cumulative falls. For Pakistan and northwestern India, it would be truly extraordinary to have so much rain so early in the season.
This commutative rainfall forecast, based upon GFS run of 1200 UTC Monday, 10 June, 2013, speaks volumes. A student of the SW Monsoon could be forgiven for thinking it for late July or August rather than mid-June. Highest rainfall, shown in dark magenta or black, is between about 20 and 30 cm (8 to 12 inches). Hot spots worthy of mention are near Karachi, Pakistan (!), along the western coast of Indian and along the southern coast of Myanmar.
It may all be fantasy in the end, but if it did work out this way, spots in the area of Karachi would get an average year's rainfall within a few days!
To conclude, it is still possible that this scenario is over wrought as to the intensity and spread of rain in Pakistan and northwestern India. However, it is the hunch of the present forecaster that some very unusual weather is going to unfold in the Subcontinent during the next week!
The plum rains are known as "meiyu" in China, the "baiyu" or "tsuyu" in Japan, and the "jangma" in the Koreas. The heart of the plum rain season stretches from early June to mid-July, with a tendency to shift south to north across the affected area.
In the wake of the mid-June cloudbursts, most of Pakistan to northwestern India last week saw a return to dry, hot weather typical of the weeks leading up to the Monsoon onset. It was as if the Monsoon withdrew to its "normal" position for the latter half of June.
The 38.5 degree C (101 degrees F) reading Tuesday in Ajaccio, Corsica, may have been tops in Europe.
Monsoon Onset was June 13th, 2013, in Delhi, almost two weeks earlier than average. The June 15th onset at Karachi and Islamabad was more like three week ahead of schedule.
In Pakistan, hit-or-miss downpours missed the Sindh capital, Karachi. One did hit Pad Idan, where it left 60 mm of rain Wednesday. This was more than 20 times greater than the normal June rainfall.
It is still possible that this scenario is over wrought as to the intensity and spread of rain in Pakistan and northwestern India. However, it is the hunch of the present forecaster that some very unusual weather is going to unfold in the Subcontinent during the next week!