A monsoon low will likely provide the trigger to lift rain coverage and intensity in the Indian Subcontinent during the next one to two weeks.
The low will promote northward monsoon onset, potentially reaching the central and eastern Himalayas before the end of next week.
As of Thursday, June 6, 2013, the leading edge of the South West Monsoon was plotted from about Vishakhapatnam, on the Bay of Bengal, west to the Maharashtra coast south of Mumbai, the India Meteorological Department (IMD) website showed.
On schedule Monsoon onset was imminent in Mumbai, based on the assessment of the present forecaster.
Analysis showed that northward shift of westerly jet stream winds was already well underway as of Thursday. Such a shift is characteristic of the early stages of monsoon onset on the subcontinent. Further, a more marked northward shift than usual was being projected by numerical forecast models, coincident with the formation of monsoon low pressure over central India during the first half of next week, or by about June 12.
Computer forecast (GFS) of winds, relative humidity and height (above sea level) of the 700 millibar surface (roughly 3,000 meters a.s.l.) valid 1200 UTC Wednesday, June 12, 2013. In this scenario, a monsoon low is depicted over central India, marked by a vigorous anti-clockwise wind field. Highest humidity in dark green most favors heavy rain.
During the latter half of next week, indications were that the Monsoon low would either shift bodily westward or split, one center breaking away to the west. One way or another, the likely result would be heavy to very heavy falls of rain westward from central India to the Gujarat and Maharashtra coasts.
North of the expected monsoon low, moist, rain-cooled air should flow northward to the Himalayas, even westward into the Indus Valley of Pakistan, the result being scattered downpours along with a break in the pre-monsoon heat next week.
As of Thursday, AccuWeather.com is forecasting significant cooling along with thunderstorm potential for next Wednesday, June 12, from New Delhi to Islamabad.
All this said, it is too early for this forecaster to definitely attribute the forecast downpours to an early monsoon onset over north-central and northwestern India, not to speak of Pakistan, which is normally last to feel the effects of the South West Monsoon.
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!