Arrival of South West Monsoon:
Subsequent to writing the MW-2, we are keeping a careful watch on the ENSO developments. El Niño is present and is weakening. During the last four weeks, equatorial SSTs were above average across most of the Pacific,except near S. America.However, Below-average subsurface anomalies are evident across most of the equatorial Pacific. Since mid-April, below-average SSTs are apparent near S. America.
But, (SOI) has become negative over the past two weeks, with values returning to neutral bounds. The latest 30-day SOI value to 26 April is - 20 ( Falling from -7.5 in 15 days).
The latest weekly SST
Niño 4 0.8ºC
Niño 3.4 1.1ºC
Niño 3 0.6ºC
Niño 1+2 -0.6ºC
The most recent ONI value is 2.0c...(El Nino is characterised by ONI values of 0.5 or above, La Nina is characterised by -0.5 or below).
Indicator: +ve...as Neutral Chances of ENSO in Early SWM Months..
2. Sub-Continent Seasonal Low:
We have a Heat Wave..Starting with the Eastern Region and Peninsula India. Heating trend started in mid-April though....The Heat Waves in NW Sub Continent has now started. Places in Sindh has recorded 47.5c, and Rajasthan with heat wave now records 46c. But things are taking shape slowly. The seasonal trough (eastern wing) could start forming in Northern Thailand in first week of May
The seasonal low in the Thar region is now very weak.The pressure around the Thar desert region is now 1002 mb (994mb required by June beginning). Proper central core is forming now. And ,as per the required scheduled development, a proper gradient should be in the making.
Day temperatures have been normal to above normal in the MW-2/MW-3 inter period !
Above normal day temperatures by as much as 6/7c had been recorded in West Bengal, Odisha and Telengana. The Heat now required is in Sindh Region (and Rajasthan). The highest on date in the Sindh areas is 47.5c. In India, it has reached 48.5c, in Odisha. Currently, last 2 days, the Rajasthan region is managing 45/46c.
An exception this year is the Vidharbh region. The temperatures in Vidharbha are just about managing 42/45c ( Highest recorded on 1st May), and in some cities just hovering around the low 40s.
Normally Vidharbh is a hotbed.
Nagpur trend in April....
2016: We may feel the heat in the day temperatures, but the minimums are not even near 30c, only on one night Delhi touched 29.5c, but since then its around 22/23c in Delhi. No 30c recorded yet in Sub Continent. The minimum anomaly is seen here. Most of the Sub Continent in the "Normal" range and North in below normal. In south and East we see above normal.
The max and min temperatures have to rise now in Sindh, Balochistan and North India. Trends of rising temperatures may be seen again in Vidharbh, Gujarat and adjoining regions of Sindh.
In 2015, the minimum temperatures touched 33c in Sibbi on 26th April, 31c in Rohri Pakistan on 30th. In India, a minimum of 30c or above was yet to be achieved.
In 2011, the minimum temperatures in the region were higher comparatively at this time. Minimum temperature touched 30c for the first time in 2011on 29th April at Kota (Rajasthan).
In 2010, 30c as minimum was recorded on 17th April.
True, the entire sub-continent region has to suffer a heat wave, but the gains are much more.
The line of dis-continuity, currently is located, as a weak trough, in the centre of Southern Peninsula, South from East MP thru Vidharbh and interior Karnataka into Telengana.
For a proper moisture incursion into the Peninsula, and for an ultimate strong Monsoon, LWD should strengthen, in the centre of the Southern Peninsula region. ( Will strengthen in the next 2 days).
Out breaks of pre- monsoon thunder showers are expected normally in the southern states and South Maharashtra. Though the showers in the South are scanty as on today. We need the thunder showers to commence and precipitate in Maharashtra and Interior Karnataka, indicating a proper flow of moisture into the peninsula region. This is seen developing from 1st week of May when we will see moisture from Arabian Sea and Bay converging in the Interior Peninsula.
A LWD in the central peninsula region "looks after" the moisture content in the interior areas, and prepares the atmosphere with the humidity required fro a proper and good rainfall in the Lee ward regions.
3. Bay Low:
Continuing from MW-2, "Bay of Bengal" low pressure parameter is a worrying factor. Normally, the Bay should host a pre-Monsoon low any time after 15th.April.
Like I mentioned in the previous MW, in 2011 we had a cyclone in the Bay by the third week, and in 2009 a cyclone crossed the Bay on 12th April.2012 hosted a low around 25th April (BB-1).
As on today, this region still maintains a luke warm response to the formation of a quick low. The pressure is anything but low, and the winds indicate a weak "high" trying to stabilise itself.
What is needed now is a pulse from the Far East, to start with, and break up the existing high pressure area. Currently no Storm or Low pressure in the Phillipines Seas.
MJO has entered a weak phase in our seas. MJO wave gets slightly positive in Bay around 18th May...seems suitable for the Monsoon there ! MJO may turn Neutral after 8th May in the Arabian Sea sector.
The shading indicates outgoing longwave radiation (OLR; W/m²).
Only +ve sign of a quick formation is that the SST is conducive. Map shows around 30c in the Bay and 31c near the Andaman Islands. Getting favourable temperature for low formation.
4. Cross Equatorial Winds :
The Southern high pressure region, the Power House of our Summer Monsoon,shows the development of a proper High at 1035 mb. Subsidiary Highs will soon form.
The Mascrene Highs have maintained their strength, and i would say gained to some extent... at 1035 mb.
The cross equatorial wind flow, has just about picked up in the Western sector Southern Hemisphere. Winds, have not yet achieved the required speeds, but just hitting the East African coast.To get a defined Somali Current, we need a proper re-curving Northwards/North-East of the winds, on the Kenyan coast. In fact, we see the SST off Somalia coast warmer than the surrounding Seas.(See SST map above)
The winds get organised as soon as the trough in the South Arabian Sea becomes weak.
However, The region below the equator in the Bay sector is almost in the same situation as MW-2. In the Bay Branch, the Cross equatorial winds are still haphazard and disorganised. But, in the Bay along India coast, , the winds turn Southerly, due to a weak High Pressure in the Bay.
The winds hitting Sumatra coast are not Westerly (as required), and also needs to gather strength soon.
Remember, the normal date for SWM to hit the South Andaman Sea is 15th. May.
ITCZ is now close to the Equator. If no further systems form, we see ITCZ moving North, the High in the Southern Indian Ocean has a fair chance of getting stronger and anchoring well.
5. The 200 Jet Streams too, show some awkward anomaly, in the last week.
The 200 hpa Jet Streams: core was located between Lat.19°N & Lat. 34°N with the wind speed varying between 65 kts & 128 kts around 200 hPa. The highest wind speed of 128 kts was recorded over Patiala on 21st April at 203 hPa
Conclusion: Indicators: 1) ENSO +ve 2) Seasonal Low -ve 3) Bay Low -ve 4) Cross equatorial -ve
I would put a slight delay in the South West Monsoon arrival dates for Andaman and Kerala as of now.
But would like to wait for another 4 days to see any changes in parameters, so will put up MW-3 Part 2 (Brief write up) on 5th May with proper dates of SWM arrival.
Then, we go into analysing the quantum of rainfall expected, in the subsequent MW-5 (which as usual will be in 2 parts), and will be ready for publication in Mid May.