Withdrawal of the South-West monsoon from West Rajasthan, already delayed by a month, may not begin until the second week of October, an India Met Department (IMD) outlook said.
Dynamical model guidance does not indicate the establishment of a seasonal anticyclone, which suppresses rainfall, over North-West India covering West Rajasthan until October 6.
Wind regime change
The anticyclone, the antithesis of a cyclonic circulations such as low-pressure areas and depression, represents air moving in a clock-wise direction (descending motion) and there rise in pressure and a dry environment.
This is unlike in a cyclonic circulation in which the air moves in an anti-clockwise direction (ascending motion) that lowers atmospheric pressure and temperature, cools down air and embedded moisture until it rains down.
A weak seasonal anticyclone is seen establishing over North-West India from October 7, according to the an extended forecast by the IMD valid for the next two weeks.
Lingering moisture from a low-pressure area over the Arabian Sea and arrival of a western disturbance is what could extend likely the monsoon-like situation over West Rajasthan into the first week of October.
Arrival of the western disturbance also indicates the gradual southward shifting of the sub-tropical westerly jet stream over to Indian latitudes from September 30 (Monday, tomorrow).
The jet stream returns after having cleared itself from the region to cede place for the monsoon easterlies from the Bay of Bengal during the four monsoon months.
Rains for now
The jet stream is also responsible for heralding the arrival of the winter season in another month once it starts carries the Arctic cold into the region, punctuated by thundershowers over North and Central India.
Frequency of western disturbances thus determines the extent of winter showers, which have a bearing of the Rabi crop, the amount of snowfall as well the intervening spells of respite from the biting cold.
Overall, model guidance indicates a changeover of atmospheric circulation pattern as well as reduction in the monsoon moisture content after October 6, the IMD said in its extended outlook.
As for the ongoing week (September 27 to October 3), a prevailing rain-driving trough in easterlies may help extend from to East to West along the plains of North India over the next three to four days.
The trough would act as a conduit for incursion residual monsoon moisture from the Bay of Bengal over major parts of India, leading to varying amount of rainfall.
The IMD sees large excess rain over Bihar, East Uttar Pradesh, Jharkhand and plains of Bengal and above normal rainfall over Punjab, Madhya Pradesh, Interior Maharashtra, North Odisha, Gujarat, Mizoram, Tripura, Kerala and Lakshadweep.
Second week outlook
Near normal rainfall likely over the rest of the country outside Andaman & Nicobar islands, Arunachal Pradesh and South Andhra Pradesh where the cumulative weekly rainfall is likely to be below normal.
The subsequent week (October 4 to 10), would witness the changeover in the atmospheric circulation regime over North-West India, allowing space for western disturbance to chug in with associated westerlies.
This would mean monsoon easterlies from the Bay of Bengal prevailing until then would give in, especially with the western disturbance deepening (entrenching or strengthening) over the region on October 5 and 6.
Rainfall amounts may vary over varied regions during this period from an interaction of the western disturbance with remnants of a low-pressure area/depression over Gujarat and neighbourhood.
So cumulative rainfall may be above normal over Jammu & Kashmir, Himachal Pradesh, Punjab, Haryana, Chandigarh & Delhi, North Rajasthan, Telangana and adjoining areas of Interior Maharashtra and Karnataka.
Near normal rainfall is likely over the rest of the country outside Kerala, Tamil Nadu, Lakshadweep, Andaman & Nicobar Islands, Bihar, hills of Bengal, Sikkim and Arunachal Pradesh where it is likely to remain below normal.