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 Weather outlook for October first week: Strong Western Disturbance for Western and Central Himalayas, twin low pressures and Monsoon persisting

Chances of moderate-heavy rain in low to mid elevations and moderate-heavy snow in high elevations of Jammu, Kashmir, Ladakh, Himachal Pradesh, Uttarakhand and Nepal during 4th to 7th October. Rains/thunderstorms for the northern plains.

RED alert for Himalayas!

Possible cause: Jet Stream Dynamics...reasoning given below:

• A very strong dip in the Polar Jet Stream over Europe will cause it to split, with southern part merging with the subtropical westerly jet stream near Mediterranean/west Asia region with a powerful midlatitude storm at the surface over the Mediterranean Sea and a very strong trough aloft.

    Trough in the subtropical westerly Jet Stream over the eastern Mediterranean on 3rd October (note: Map does not depict the actual boundaries of all the countries)

• The upper level trough over the Mediterranean is likely to get further amplified by the merging of Hurricanes Humberto and Imelda into the Jet Stream across the Atlantic (energy propagating downstream from the region of peak winds, downstream amplification of trough).
• The energy from this trough over eastern Mediterranean will propagate east and cause another strong trough to develop downstream (further east of the parent trough) over the Western Himalayas from 4th/5th October onwards.

IMD GFS model forecast of 200 hPa winds for 6th October.

ECMWF model forecast for the same (note: Map does not depict the actual boundaries of all the countries)

• This WD trough as it deepens, can interact with the newly formed monsoon depression BB-15 (which is moving in a northwest direction and expected to become a deep depression as per IMD) and cause heavy rains for central and eastern Nepal, Sikkim, sub-Himalayan West Bengal, Bhutan and parts of Uttar Pradesh and Bihar during 4th/5th October.

850 hPa winds showing BB-14 off Saurashtra coast and BB-15 off the AP coast, both are depressions.

• The WD trough will become strong enough to dip south up to the northern Arabian Sea by 5th/6th, pulling in strong moist southwesterly winds till the western Himalayas, bringing unseasonably heavy rain/snow.

• There is already good moisture over western states from SW winds because of the depression BB-14 off the Saurashtra coast. BB-14 will continue to push SW winds into Gujarat and Rajasthan.

Due to this, rains/thunderstorms also expected across Jammu, Punjab, Haryana, Delhi NCR, parts of Rajasthan, Gujarat, western Maharashtra and western Madhya Pradesh from 5th to 7th October.

SW Monsoon withdrawal will be delayed due to the additional moisture push from the Arabian Sea and Bay of Bengal due to the WD and monsoon depressions BB-14 and BB-15.

However, after the WD moves away eastwards, winds are expected to quickly reverse with a ridge establishing over Rajasthan and dry northerlies expected across north, west and central India.

A quick exit of monsoon could happen from entire Gujarat and many parts of Maharashtra around 13th/14th October...more details on monsoon withdrawal in subsequent days.