Friday, September 03, 2010

Last week, we discussed the overall probability of a good September monsoon , with the forecast graph (which was put up then) showing good September rains.

Now, the situation remains hopeful, with Australian Bureau confirming the same. To reproduce from them :
"Over the past week, the MJO has progressed into Maritime Continent longitudes for the first time since May, and has contributed to the development of Tropical Storms "Mindulle", “Lionrock”, “Kompasu” and Namtheun” in the western North Pacific. Until now, tropical cyclone activity in the western North Pacific has been well below average; however the season lasts until December.
"The latest guidance suggests the active MJO will not spend long in the Maritime Continent region, but will begin to weaken over the coming week, and redevelop in the Indian Ocean towards mid-September. With this scenario, we would expect an increased risk of reinvigoration of the Indian Monsoon towards mid-September, and increased potential for tropical cyclone development in that region".

And, the SOI, an indicator of La Nina, was +17 as on 29th. August. A constantly +ve SOI , of more than +10 for 10/12 days indicates a La nIna phen
omena.


Coming to current situations, I would like to continue from where vagaries left off. As anticipated, rains (from the axis regions) have pushed into Delhi, Haryana and Utteranchal. Yesterday central M.P. saw good ranfall, and parts of western Gujarat/ Rajasthan too had rains (axis line regions). IMD picture shows rain distribution of 2 sept.

Due to an UAC formation (see map), clouds are seen of T.N. coast and South A.P. Chennai had some rains yesterday (Thursday), with the rain guage nothcing up 14.7 mms at Nungambakam and 5.7 at Meenambakam. Rains for not forecasted for T.N. coast earlier, as the UAC was expected to form off the A.P.coast. But on forming southwards, it has precipitated bonus rains in coastal T.N. Now, at least as per presumptions, the system will move North tomorrow and descend into a low off A.P. So, rains showers in Chennai till Saturday.
All models, including IMD forecasts now estimate a low to be formed in the Central Bay region tomorrow. The trough extending along the central bay, North to South, supports this cause (IMDMap).

Ok, the low should form and around 1000 mb should be of moderate intensity. Rainfall Friday/Saturday will be along the coast, near the system, and then move inland, into A.P.i estimate. But thence there are conflicting views of the system's movement. Some models predict rack into central India (M.P.) and fizzle out, in about 3/4 days. Others estimate not much deepening of the system, and fizzling out just on entering land in A.P./Orissa. ???

I would safely estimate the system to move inland upto west M.P. Rains along the path, as the system moves.
West coast will continue to receive the rainfall at the current levels, as the off shore trough "lives on" A little push from the system will increase rains in South Gujarat, near the gulf of Cambay, and adjoining Mah. coast on Saturday/Sunday.

Mumbai: The city received only 3 mms of rain on Friday. Periods of heavy showers on Saturday might increase the quantum to 15 mms/day on Saturday/Sunday. That is, a few more passing showers, with a cloudy day.

Rainfall total at Colaba: 3017 mms, short by 465 mms for target. Put in your opinion on poll.

4 comments:

  1. Anonymous2:39 PM

    Low is coming to Chennai!!! Thunder squall approaching..!

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  2. Dear Rajesh,
    Have you noticed the upper level easterlies[above 12.1km]which drags the convective clouds along its path. I have my own doubts whether it is moved along monsoon axis. But I have some fair confidence that clouds were dragged along the upper [divergent] wind.

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  3. Hi Kaneyen' Yes, systems are dragged by the jet streams, in fact most of the times. But a sea level system may track otherwise. I see the jet streams going westerly over Kashmir. Signs of monsoon retreating from northern most regions.

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  4. DEar Rajesh,
    You may be knowing the "Sub Tropical Westerly Jets[STWJ, or simply known as STJ]which is pronounced in Kashmir belt. That is different.
    My presumption is that the Bay branch of SWM 2010 is weak or WITHDRAWN.
    By the same time, mid tropospheric level westerly winds coming from Arabian Sea and blowing over the western ghats become weak and this weak winds create vortexes or circulation in mid tropospheric level [Called as MTC] in the Bay. Like that an axis from Himalayas to south West Bay is formed at MTC level. But this axis is not a straight line. Now if you go by axis easterlies will prevail North & Eastern sector of the axis and westerlies in southern and western sector of the axis.
    So it is SW monsoon withdrawal only.

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