Thursday, May 05, 2011

Monsoon Watch 5- Part 1( Arrival Time).

As always explained, the MW parameters are showing the progress of the developments in the Monsoon build up. On as - is - where - is basis. Vagaries calculates and forecasts the arrival date of the Monsoon. These parameters are not for the quantum of rainfall.

Vagaries will now, for the 1st time, attempt at forecasting the quantum of Monsoon rainfall. This is in the Part 2.

Bay factor: In the Bay sector, the cross-equatorial wind is building up across the East Indian Ocean with clouds massing west of the Bay Islands. Strong South-West winds have suddenly developed in the South Andaman Sea, and SW winds are now clocking 30 knts south and SE of Sri Lanka. Gauging from the the wind speed, we can expect the Monsoon to strike the South Andamans Seas by 13th, and the Middle Andamans by the 18th. ( Almost on schedule).

As stated, the first South-West Monsoon rains normally hit the South Andaman Sea on 12th.May. And proceed to the Middle Andamans by the 15th. By the 20th. it covers the Islands totally.

As the South-West winds have geared up, and the moisture inflow has picked up in the region,since the last 2 days, by the 10th, we should see a feeble Monsoon current in the South Seas.

Onward Monsoon progress Northwards, will have to be monitored.

In this sequence, we now have a weak low. A weak low has formed in the South West Bay on the 5th. viz.96B. I would not consider this low as a pre-monsoon "bay" low, but just an embeddment in the MJO.

The latest winds in the Bay continues to show strong winds rushing Northwards along the East coast of India, and re-curving into a Westerly flow of winds at the South Myanmar coast.

A lone pre-Monsoon low has appeared in the region (end April) this year. SST in the Bay is now very conducive to host a low anytime, but in the South Andaman Seas.(See SST Map). Indicator: +ve

The cross equatorial flow, off the African Coast, in the Western Indian Ocean, a bit "disrupted" previously, has now re-formed itself fast, and is strong off the African Coast South of the Equator.The Somali Current , is getting defined,and blowing strong ! We will wait for the Somali coastal SST to drop further. (See SST map).

The previously weak factor,the equatorial cross flow South of the Arabian Sea, is getting stronger.

The Mascarene highs are established, are are now "pushing" the South-Easterlies from below the Equator.

However, I feel, a good booster required for the Arabian Sea branch of the Monsoon, is still wanting, and is at least a week away. Indicator: Improved since MW4

Seasonal Low: With the heat factor still lacking absolute "full strength, in the Northern regions, the Seasonal Low core in the Thar Desert is now somewhat "unsteady", with a core pressure at 1000 mb, and requires to spread more westwards, rather than east. This (pulling Eastwards) is happening due to the W.D. M1.

A North-South trough indicates the line of wind dis-continuity is getting favourably tilted towards the east in the Peninsula region. Normally, a perpendicular N-S trough is a pre monsoon formation, and is a fore-runner for the seasonal low to stretch its presence over central India.(See Thai Met Dept Map).

As mentioned in my MW prevoiusly, the creation of a good gradiant between Kerala pressure and Rajasthan/Sindh pressure is an absolute requirment. And that is a must for creating a pull Northwards for the Monsoon. Overall position in this parameter is improved since MW 4. Indicator: +ve

The 200 hpa jet stream. Normally, they should be Easterlies right upto 20N by May end. This would facilitate the Northward movement of the Monsoon Current. Today, the Easterly jet stream is developed upto 5N.These 200 hpa jets winds need to come upto 8N by 15th.of May at least, and then move upto 10N before 20th.May,to be in time for the Andaman Sea Monsoon arrival. But forecasting maps are slow to show this, and indicate a slow movement Northwards, more so in the Eastern (Bay) sector. Indicator: Slow hence -ve

Arrival Time Conclusion:

SWM could reach the South Andaman Sea Islands, the Southern most tip, by 13th May. Progress into the Middle Andamans will be within 2 days, and should cover the entire Islands by the 18th.

Considering all the other parameters, on today's situation, SWM should hit Kerala a few days behind scheduled date, by the 3/4 th June. Northwards into coastal Karnataka should be smooth (nothing adverse) and arrive there by 5/7 th June.

Monsoon Watch - 5 Part -2 will be published on tomorrow (Friday) nite.

(This Monsoon chase is my personal view, and my personal analysis and deciphering is done as per my assumptions).


Anonymous said...

Hello Rajesh,

Had a basic question, does WDs exert an influence on monsoon onset, or is its influence limited to flows across the northen plains & affect the seasonal low.

Thank you, Regards, Ananth

Anonymous said...

Your forecast for Karachi (monsoon)
Thanks, from Pak Weather Portal(PWP)

Rajesh said...

Ananth, of-course, prolonged activity of fairly strong W.D.s do effect the monsoon arrival.Could delay the formation of the seasonal low, and also push the Bay sysytems if any from forming.
W.Ds in the upper most regions, above Kashmir are acceptable and do occur.

@PWP. very difficult. As per my assumptions, Karachi depends on systems from the Bay to travel thru India and reach Gujarat and then Karachi. Only UAC formations in the Gujarat regions will have strong effect on your city.
Systems from the arabian Sea, in early June can precipitate rain.When monsoon trough in June /July passes thru central regions of India, the western end of the trough can linger around lower Sindh, and produce rains along the coast.
I will study and give a 2 month presonal estimate in blog tomorrow.

25th September Evening Post: South West Monsoon likely to bid goodbye from Punjab, Haryana, Delhi and Rajasthan by Thursday 29th/ Friday 30t...