First of June is one of the most looked for dates in the Indian Weather calendar. The traditional date for Monsoon onset over Kerala is 1st of June when it first touches the mainland of India though it checks into India much earlier over the Andaman & Nicobar Islands through the Bay branch. After making onset over Kerala it gradually starts to climb latitude through surges covering the country in roughly 6 weeks time. Places in Rajasthan bordering Pakistan is the last to come under Southwest Monsoon when it reaches these places over NW India around 15th of July.
This year Southwest Monsoon marked its first entry over the southern parts of Andaman & Nicobar Islands and adjoining South bay areas on 19th May on the back of westerlies strengthening through the ESCS Mocha that made landfall over Myanmar coast a few days before. Subsequently the progress stalled for more than 10 days with rest of the Andaman & Nicobar islands coming under monsoon on 30th May. Sri Lanka which sees monsoon check in during the 3rd week of May has still not seen onset yet with even parts of Maldives also awaiting the onset of Monsoon.
To give a perspective last year Monsoon made onset over Maldives on 8th May while this year the Maldives Met announced onset on 3rd May while IMD has announced further progress over Maldives in the next couple of days. This pretty much sums up how Southwest Monsoon progress has slowed down considerably over the past couple of weeks. One of the reasons that could be attributed to this lull is unfavorable MJO influence which had parked itself over West Pacific for many days and has now started to cross from Western Hemisphere to Eastern Hemisphere. The arrival of MJO over Indian Ocean is an important factor in proper onset and progress of Monsoon as often supportive MJO has helped with Monsoon progress over the Indian Sub Continent.
A look at the prevailing wind conditions indicate at least for the next few days it is unlikely Southwest Monsoon may make onset over Kerala though IMD has indicated 4th June as the potential onset date. IMD has a well laid out onset criteria with strength and depth of westerlies a key factor in declaring onset. Currently the lower level westerlies are weak, indicated by the sub par Somali Jet index (an indicator of monsoon strength). Additionally looking at forecast from weather models depth of Westerlies also remain a question mark during 1st week of June with easterlies seen North of 5N latitude even on 5th June. This is also confirmed by the weekly rainfall anomaly chart which indicates below average rainfall for Peninsular West Coast up to 5th June.
While the arrival of MJO is expected to give an impetus to Monsoon onset conditions, there is a potential banana skin that could develop in the form of a tropical disturbance in Arabian Sea influenced by the West to East arriving MJO. While IMD would refrain from announcing Monsoon Onset / Progress if there is a disturbance in Bay of Bengal or Arabian Sea the larger worry is the disruption in westerly wind pattern that could be brought by this disturbance if it moves in a WNW direction. While a movement along Peninsular coast may be beneficial by dragging the onset along with it, a pronounced WNW movement may become counterproductive for the onset and progress.
All in all it is going to be a tense few days in order to understand on Monson onset and progress. It is pertinent to point out this delayed onset is not going to relate directly to the rainfall prospects for rest of June and it is important to look only at the first milestone, Onset over Kerala, in this year’s monsoon journey,