Wednesday, May 06, 2020

Monsoon Watch (MW) - 3... 2020...6th May 2020

These reports are the views, study and observations of Vagaries. Estimates and forecasts are also the calculation of Vagaries, and may/may not tally with any other estimates/nodels. This may not be used for commercial purposes. Vagaries of Weather is not responsible for any commercial loss from this article, or these series of articles.

The Monsoon developments, as they unfold, will be analysed , discussed and explained in Vagaries. 

Vagaries follows the developments(parameters) that emerge during the summer (Apr/May) which lead to the monsoon.
Vagaries believes the expertise to predict the monsoon in it's entirety during the  summer itself  is not feasable. The parameters themselves are developing in April and May. Quantum of rain forecasting in April for June right through September is an impossible task. 
😎Otherwise the charm of chasing the monsoon parameters as they unfold during the summer , is lost.

Further Analysis of the parameters as discussed in MW-1 ,MW-2... 

1. Cross Equatorial Winds from Southern Hemisphere and Mascarene high.
2. Pre monsoon showers / LWD.

3. Seasonal low over Indian subcontinent.
4. ENSO Status.

Additional parameters from MW-3 : 
5. Bay low.
6. Jet Streams (the winds at altitudes above 10 kms ).

Situation as on 5th May 2020

1. Cross Equatorial Winds and Mascarene high :

The winds on crossing the equator break into 2 branches. i) gushing into the Arabian Sea, and ii) moving into the Bay of Bengal. 

                           --Wind flow chart of 5 May 2020--

i) The cross equatorial wind flow, has improved compared to MW-2 in the Western sector of Southern Indian Ocean. SE Winds, are now striking the East African Coast at speeds of 40 kmph. The Northward movement of the ITCZ will surely hasten and strengthen the winds.

To get a defined Somali Current, we need a proper re-curving Northwards/North-East of the winds, on the Kenyan coast. The current SST off the Somali Coast is 28 c while the required SST in first week of June is 19/20c. (Required to form masses of Monsoon clouds over the warm central Arabian Sea).

ii) In the Eastern sector, in the Bay Branch, some Westward wind flow is observed towards the Sumatra Coast.This is an improvement over MW-2. Thunder clouds(Cb) developments are seen near the equator in eastern Indian ocean.

We prefer the Bay Branch to be better organised, as the SW Monsoon is normally expected there first (normal date 15 May at southernmost Nicobar island).

The low BB-1 is meandering over south Andaman sea from 2nd May 2020.But many parameters are now not favouring the system to strengthen.

ITCZ (Inter Tropical Convergence Zone)This Seasonal trough passes from the South Arabian Sea to Andaman sea after the withdrawal of SWM in Oct. Shifts further south in Dec. Moves North during South West Monsoon.

Now south of the equator (5S) over western Indian ocean and near the equator over the eastern Indian ocean as it was during MW-2.

--Pressure chart of 5 May 2020-- 

Mascarene highs - Highs of 1030 mb are seen in the south Indian ocean between Madagascar and Australia.
This is an improvement compared to the high reading of 1023 mb observed during MW-2. The power house of monsoon is almost on schedule as of now.
The sea surface temps in the region of Mascarene High formation are lower than normal .This should help in strengthening the High.
1.Indicator as on 5 May 2020 : Neutral .

2. Pre-monsoon showers / LWD (Line of Wind Discontinuity) 

A LWD in the central peninsula region "looks after" the moisture content in the interior areas, and prepares the atmosphere with the humidity required. In reality, this LWD remains till June, in variable phases, and finally merges with the Monsoon trough (axis)/ITCZ  when the ITCZ moves towards the Sub Continent. The seasonal low acts as a fulcrum to this Monsoon trough(axis) as it moves over the subcontinent .Also this trough helps to keep the seasonal low in place.

More than average thunderstorms over the Indian peninsula and Kalbaisakhis over East /North East India have been observed .Few violent thunderstorms / hailstorms have caused some destruction to property at many states (Eg. Pune , Maharashtra and many other states).

The rainfall has been excessive(blue colour) over most parts of the country during this summer, till now .

             LWD observed over the Indian peninsula on 5 May 2020

LWD has been active during this pre-monsoon season.

2.Indicator as on 5 May 2020 : Positive

3. Seasonal low over Indian subcontinent :

This summer there have been no significant heat waves over the Indian subcontinent till now !!
Churu (Rajasthan) recorded 45.2 c on 2nd May 2020 .But due to western disturbance temps dropped by 10 c to 35.3c the next day !! 

Min temp has touched 30 c at Barmer (Rajasthan) this summer. But the consistency of above average max and min temps over the core seasonal low formation region (Thar desert) is missing.

Jaisalmer has reported almost normal temps during the past week (29 April to 5 May).

In fact, temps have been significantly cooler over Northern / Eastern parts of the subcontinent during the past few weeks (as they were during MW-2) . 
East India (Bengal,Odisha) is experiencing one of it's coolest summers.

200 jet streams being in trough in the southwards direction ,are encouraging back to back western disturbances affecting the northern plains. This is causing delay in heating of the Northern regions.
No significant heat wave is expected over the core seasonal low formation region (Thar desert) during this week.

5 May 2020 -  Max /Min temps in c of some cities/towns :

Barmer.. 42 (0) / 26 (0)
Jaisalmer.. 43 (+1) / 26 (0) ..0.1 mms
Nagpur Airport.. 42 (0) / 26 (-1)
Delhi SFD..  37 (-1) / 23 (-2)
Angul (Odisha) .. 35 (-5) / 21 (-5)

Because of no significant heat, the seasonal low has not shown good deepening compared to MW-2. As of now,the seasonal low has a small core of lowest pressure at 1006 mb (it hould be 1000 to 1002 mb by this time) . Also no significant heat wave is expected over the Thar desert core seed region during this week. (The ultimate low in the core in June should be 994mb).

3.Indicator as on 5 May 2020 : Negative

4. ENSO Status : 

Map of NINO and IOD (DMI) regions

The tropical Pacific remains neutral with respect to the El Nino–Southern Oscillation (ENSO). Atmospheric and oceanic indicators of ENSO including the Southern Oscillation Index (SOI), trade winds, cloudiness near the Date Line, and sea surface and sub-surface temperatures in the tropical Pacific Ocean all continue to persist at levels consistent with neutral ENSO.

i) Multivariate ENSO Index (MEI)  is in neutral range. Latest available value is 0.2.
The bi-monthly Multivariate El Nino/Southern Oscillation (ENSO) index (MEI) based on five different variables (sea level pressure (SLP), sea surface temperature (SST), zonal and meridional components of the surface wind, and outgoing longwave radiation (OLR)) over the tropical Pacific basin (30°S-30°N and 100°E-70°W).

ii) The Oceanic Nino Index (ONI) is based on SST departures from average in the Nino 3.4 region, and is a principal measure for monitoring, assessing, and predicting ENSO. The last 3 overlapping seasons show towards establishment of a neutral eventONI latest available value is 0.5 ( 0.5 to -0.5 is neutral. Above 0.5 is El Nino, and below -0.5 is La Nina).

iii) Current NINO is in neutral range.

Persistent NINO3 or NINO3.4 values warmer than +0.8 °C are typical of El Nino, while persistent values cooler than −0.8 °C typically indicate La Nina.

iv) The 30 day Southern Oscillation Index (SOI) is calculated using the pressure differences between Tahiti and Darwin.It is in neutral range currently.
(sustained values below -7 indicates El Nino and above +7 indicates La Nina)

SOI values for 5 May, 2020
Average SOI for last 30 days0.79
Average SOI for last 90 days-2.70
Tahiti hpa - Darwin hpa (5 May 2020)1013.75 hpa - 1012.45 hpa

v) IOD (Indian Ocean Dipole) is in neutral range currently.
Latest weekly value is 0.08.
4. ENSO Indicator as on 5 May 2020 : Neutral 

5. Bay low :

Normally , a low pressure strengthens in the bay of Bengal around this time.It helps in heralding the SWM into the south Andaman sea.
This year a low BB-1 formed with positive signs on 2nd May. But many parameters are now not favouring the system to strengthen . 
Also, now the MJO is falling in amplitude (below 1) and moving away eastward over the Pacific ocean during the next week.
The 200 hpa jet streams are very much south in trough formation and the divergence is poor in them.The SST (sea surface temp) gradient is not favourable. Hence, we do not see the strengthening of BB-1.

A strong MJO would hasten the favourable formations and lows, with enhanced precipitation. The MJO is expected to reach the Indian Ocean after two weeks (the green line).

Another factor, which helps the winds gain strength, and bring more moisture into the Indian landmass, is the sea temperatures on both sides of the Peninsula. Optimum (warm) temperatures will hasten and create more clouds, and help in faster forming of the lows from the Bay.
Currently, the only sort of positive sign is the above normal temps (30-31 c) of the seas surrounding the peninsula , though the lack of required gradient in the SST temps of the bay of Bengal is one of the factors not helping the low BB-1 to strengthen.

5. Indicator as on 5 May 2020 : Negative

6. Jet Streams (the winds at altitudes above 10 kms ) :

The 200 hpa Jet streams over the subcontinent is the additional parameter to be watched from MW-3 .
The jet streams are to be followed as the westerly jet core shifts to the north of the Himalayas and easterly jet stream is formed. Over the course of late summer, the easterly jet stream forms progressively from south to north of the subcontinent , heralding the progress of south-west monsoon over the subcontinent.
This summer, the 200 jet streams being in trough in the southwards direction , are encouraging back to back western disturbances affecting the northern plains. This is causing delay in heating of the Northern regions.

Current Jet stream

Currently no easterly jet is observed over the Indian sub continent. 
They are required to strengthen to easterlies till 10 N latitudes in the coming days, for heralding the SWM to the South Andaman Sea.

6.Indicator as on 5 May 2020 : Negative

Summary as on 5 May 2020 :

Looking at the current parameters there seems to be a slight delay in the arrival of monsoons over Nicobar as of now.

But would like to wait for this week to see any change in parameters.
So, MW-3 Part 2 (brief write up) will be published with estimated dates of South West Monsoon arrival.

Strength/quantum will be discussed later in MW-4 .

Situations can change fast, and by the time we discuss the next MW, some factors can suddenly become +ve or -ve. So, MW discusses and follows the developments as they emerge, and discuss the parameters as is where is.

Please note  There are more parameters to be considered, as and when they are developed and to be taken into consideration in the subsequent Monsoon Watch (MW) series. 

To read MW-2 2020 article ,click on the link below :

To read MW-1 2020 article ,click on the link below :

See also Author's Page for previous Years.
Author's Note:  The MW this year compiled by Vagarian Rohit Aroskar. Perfectly and he has put in Maximum efforts for this tedious job...Rajesh.


NilaY Wankawala said...

Thanks Rohit for a wonderful follow up and in detailed analysis. Its a pleasure to read Rohit's work perfectly being followed in Rajesh sir's parameters every season.

Paresh said...

Thanks for this effort, Rohit! Everyone am sure is waiting to see how the monsoon shapes up this year.

Rohit Aroskar said...

Thanks Nilay

Rohit Aroskar said...

Thanks Paresh

sridu said...

thank you thank you thank you

emkay said...

Fantastic work Rohit. Would not have made out it is you and not our Guru penning this, if not for the footnote.

Vijayanand said...

Rohit great work. Eagerly look forward to this update every year.

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