Wednesday, October 24, 2018

Posted Thursday 24th October 2018 Evening:

An East West trough at 925 hpa level exists from Sri Lanka to 95E. This Easterly Wave could deepen a bit and heralds in the NEM ( as a weak current) over Tamil Nadu by 25th Evening. It may culminate to a UAC at around 850 hpa, over SriLanka/South Tamil Nadu.This will strengthen the NE winds upto 700 hpa levels into Tamil Nadu.

With the 2018 rainy seasons initially hesitating to change between the SWM and NEM, a brief Summary of the NEM.

-The SWM onset is well defined, and follows a well coursed out Northward progress. And can be discerned with reasonable accuracy on charts.
For the NEM, it is not so.
In fact, on many occasions, there is no clear indication between the withdrawing SWM and setting NEM. Often one tends to merge into another.
So, setting a date for the commencement of NEM is difficult, and sometimes not possible.

-The SWM has its origin, its "Power House", is at the Mascrene Highs off Madagascar. The NEM has its origin round a large anti-cyclone over Siberia.

-The Upper winds during the NEM across the Bay are in a clockwise direction.That is an anti-cyclone is formed. At a level of 500 mb, the centre of the anticyclone shifts to central Burma region.
-The Upper Air temperatures show a Northward gradient in the NEM.Near 10N it is normally warmest, and dropping by 8c at 30N.

-Jet streams at 200 hpa are common to both the monsoons.
In the SWM, the Tropical Easterly Jet stream dominates the Peninsula, while the Sub-tropical Westerly Jet Stream is the feature of the NEM.

-The normal Sea Level Pressure during the NEM is a large system of low pressure over the Central Bay. It can extend into the Indian Peninsula as a trough.Towards the end of November,there is a shift southwards of this extended low.
-The general variations and fluctuations in intensity of this low pressure, governs the rainfall. When the trough is well defined, and the low is well marked, rainfall over the southern peninsula is good.
-During the NEM, an occasional burst of cold air from the Siberian High develops a low pressure systems over the Equatorial regions of the South China Seas. These systems move slowly westwards thru Sarawak/Eastern Malaysia coast, and remnants of these cross over into the Bay and form depressions.

Most of the NEM rains depend on depressions and cyclones from the Bay towards the East Coast of India.
Excerpts from Bose's book on Indian Monsoon.

The advent of the NEM is getting  delayed this year.
Vagaries predicts its advance around the 25th. taking into considerations the withdrawal of the SWM. 

The overall delayed actions, beyond forecast estimates, is mainly attributed to the stubborn SWM, and to some extent the weak MJO not "supporting" the fast formation of NEM currents in the Bay. Cooler SSTs are long overdue west of Java and Sumatra.

But, the upper winds are showing signs of "resigning" and changing. The Mascrene highs have lost their grip, and are seasonal and weaker, and the Tibetan/Siberian High is taking shape ,though late.

Image from Vagaries of 24th October 2017..



Some cooling effect is seen in Interior Maharashtra, as the minimum Temperatures fell to 14c at Nasik, 14.4c at Ahmednagar,  Aurangabad saw 15.2c, 15.4c at Osmanabd and 16.5c in Pune & Nagpur.

Mumbai: Santacruz saw a low temperature of 20.2c (But a high of 37.6c !) and Colaba's low was 24c on Wednesday Morning.
Thursday will be partly cloudy in Mumbai, with the Eastern Regions possibily getting light drizzles in some parts. 
Friday will be clear and the weekend will see a further 2c fall in Night Conditions in Mumbai.

2 comments:

Cumulus Arjun said...

Rajesh sir but when Mumbai will get rid of daytime heat?

sset said...

Thanks for excellent technical explanation on NEM by vagaries. NEM is lifeline for entire south east India - without which desertification will follow. Unfortunately NEM is failing year after year. Wondering even with +ve IOD and el-nino still ocean waters are cold over Sumatra? Will NEM 2018 fail?

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