A look back at SWM 2018 and outlook for NEM 2018

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With almost 75% of the Southwest Monsoon (SWM) season gone by, we enter the last leg of SWM 2018. A lot to cherish about SWM 2018 but at the same time, the erratic pattern of SWM 2018 won't be forgotten (#Kerala floods, #Kodagu floods, #Bihar drought etc.).

The SWM 2018 made an early and powerful onset, but lost steam around end of July and August, bringing widespread fear about below normal monsoon. Current we are staring at a deficit of ~8%.  Had it not been for the very heavy rains in Kerala, the deficit would have been in double digits. 

August is going to end on a larger deficit, compared to June and July. Inspite of three back to back low pressure systems in the #BayofBengal, August underlined the “Vagaries of Monsoon” -  be it in terms of below average countrywide rains or circulation system positioning itself at one place for a week and delivering non-stop rains).

Overall, SWM 2018 couldn't keep up the pace with which it started. This is not surprising since monsoon pattern is known to be erratic with active and break phases. However, the point to note here is the prolonged dry spells in June, July, and August with flashes of intense wet spells. Such prolonged (dry) and intense (wet) spells do aid in bringing the countrywide R/F numbers near to its average value, but also comes with the burden of widespread damage.

Statistics - Till date, 8 systems formed in the BoB. Out of this, 2 were depressions and the rest were low pressure systems. In contrast, only 4 systems formed in the Arabian Sea and that too their strength was weak compared to those in BoB. All the systems in AS were weak-moderate circulation systems. So far, no depression formed in AS during the SWM 2018 season.  The AS contributes to 50% of rainfall during any monsoon season, however, this time AS has been somewhat dormant without any significant system (except the low pressure vortx system that formed in AS during the onset of monsoon on May 29th).

(A quick side note: People compare floods from different time periods (for e.g. Kerala floods of 1924, 1961, 2018) and make conclusions as to which one was the worst. Personally, I feel this is not a right thing to do. The point to remember is that just the R/F numbers do not give a complete picture -the intensity of rainfall is a significant parameter. 

Simple logic tells me that 480 mm R/F spread over 24 hours (with a constant 20 mm/hr) is probably better than 340 mm in 24 hours (but with a intensity of 70 mm/hr in 4 hours and then a meagre 3 mm/hr in 20 hours). Primarily because the drainage systems and the soil/vegetation have the capacity to withstand 20 mm/hr intensity (without eroding).

But again, both are extreme events, so it does not make sense to compare different time period and conclude that urbanization has lead to damage etc. We should also remember that over years the erratic nature of monsoon has increased with prolonged dry spells and intense wet spells. The reason for the change in the monsoon pattern is many, but not just urbanisation (which is only one of the many factors.)

 September month will also see some prolonged dry spells, but expecting some heavy rainfall events (due to activity in Bay of Bengal), which will bring the overall R/F average for that month to normal.

Overall, the SW monsoon 2018 should end up to be normal (i.e. less than (-)10% deficit).

Outlook for NEM 2018

Although SWM 2018 is still approximately a month away from withdrawal, but I guess it's okay to look at some scientific parameters and give an amateurish outlook to NEM 2018. 

As per my limited knowledge of dynamic meteorology, the factors that start to signal the withdrawal of SEM are: 

(a) Temperature increasing in the West Arabian Sea due to inhibition of upwelling process.
(b) Weakening of Somali Low Lying Jet (LLJ)
(c) Southward movement of Intertropical Convergence Zone (ITCZ).
(d) Wind reversal.
(e) Finally, MJO moving to a subdued phase.

The withdrawal of total SWM will take anywhere between 25-30 days. 

I expect the withdrawal process to start around Sep 20th (+/- 2 days), and the complete withdrawal will happen around October 17 (+/- 2 days). Based on this, my predicted onset of NEM is around October 18 (+/- 2 days).

The NEM is expected to be NORMAL with good rains in TamilNadu, Karnataka, and Kerala. However, due to the threat of looming Elnino, which generally starts to mature around November-December (before reaching a fully mature phase in February), there is a possibility of some extreme rainfall activity during the NEM 2018. Things will become clearer once the onset happens. More updates on NEM 2018 later.

You can also follow my twitter handle @SriGMFL

Disclaimer: The content given here are all personal views and the author will not be held responsible or held liable for any oversight or misinterpretation.





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