Posted Friday afternoon:
It was discussed here that regions in Central Coastal TN will receive heavy showers on Thursday..
Nagapattinum measured  89 mms, Cuddalore ended  up with 87 mms and Karailak 67 mms in 24 hrs ended 8.30 am Monday.Chennai  received 12.6 mms
The Low off the Sri Lanka Coast persists. An UAC in the trough along Southern tip of India now exists off the Kerala coast. This will restrict the concentration of rains to Central and Southern coastal and interior TN. on Friday and Saturday.

Chennai:
Friday..Few Showers with bright spells. Rain around 10 mms.
Saturday. Rain frequency increasing by evening, and around 25-30 mms by Sunday morning.Very windy.
Sunday: Fairly frequent showers and intensity increasing. Around 40-50 mms expected by Monday morning.Rains stretch till Pondi and parts of interiors.
Monday (tentative) will also see occassional showers, some heavy. Amount of rain during the period can exceed 50 mms.

Posted Thursday 3rd Night:

The Low we were expecting (Wed Night Post) has formed off the South TN/Sri Lanka coast.Extended trough stretches to West coast.
On Thursday, rainfall was heavy due to this formation in the Karaikal and Cuddalore region as we had mentioned. Karaikal received  49 mms and Cuddalore 30 mms till 8.30 pm IST.

Chennai: A major let up in rains...with 1 mm recorded on Thursday 3rd (till 8.30 pm).
Few Showers on Friday 4th (5-10 mms..till Saturday 8.30am), mostly in the day. Very windy in the day.

Saturday 5th getting more frequent, showers late afternoon. 

Sunday 6th will see frequent showers, around 30-40 mms, till Monday 8.30 am.
Sunday rain stretches from Chennai to Pondi.

Mumbai this weekend, Mumbai sees no major weather change. Days will remain warm around 32/34c. Nights may see a small drop to 18/19c at Scruz and around 22c at Colaba.

In the Sub-Continent, no Major weather event this weekend.
Night temperatures dropping slightly in Pakistan Punjab and Upper Sindh regions.
Slight fall in Indian Punjab and Haryana and NCR.

"A Hot November in Goa" See the Latest Update on Vagaries Goa

Chennai Annual rainfall beats Surburban Mumbai and all Cities...see Rohit's Page

Thursday Night Update at 11 pm IST

Posted 2nd December 2015...Night:

The Vortex explained yesterday is in the form of an UAC over the North TN region on 2nd.
The trough off Southern TN coast and Sri Lanka will embed a Low over the Southern region.If this Low moves and tracks NW, we may see enhanced rainfall on TN coast from Sunday.
We may expect heavy rains on 3rd in South/central TN coast (Puducherry/Cuddalore).
Gusty winds from the NE will sweep the Chennai region from Friday thru Saturday.

Chennai saw 18 mms of rain in 12 hrs on Wednesday 2nd. With some showers at night, we may see around our estimated figure. 
Thursday 3rd will see some showers with sunny spells.Around 20-25 mms.
Chennai may get a partly sunny Friday, with a few showers. On Saturday, with intensity increasing to measure around 20-25 mms. (Rainfall amounts are very difficult to estimate and may vary. Given here just as indicators).

Heavy rains on 1st December has totally submerged Chennai.
Rainfall in 24 hrs ended 8.30 pm 2nd December in cms:

A brief Update on Chennai estimate tonite at 11 pm IST

Posted Wednesday 2nd December Morning...
 
Chennai Rains:
Meenambakkam 345 mms, Nungambakkam 294 mms and Pudducherry 218 mms....The 24 hrs rainfall in Chennai. 
The all time 24 hr highest ever (December) of 262 mms (NGMBKM) mms way back in 1901 is beaten !
 Meenambakkam 345 mms is also a 1 day record for December.

Rainfall given below compiled by Pradeep:
Chembarabakkam Lake - 475, Kattankolathur (near SRM University) - 445
Katupakkam (near SRM University) - 429
Thiruporur (near Kelambakkam) - 348, Chennai AP (Meenambakkam) - 345
Korattur Anaicut (Poonamalle) - 336, Puzhal Agro - 325
Redhills Lake - 320, Anna University - 319
Kolapakkam (near Chennai Airport) - 310, Taramani - 300
Puzhal - 294, Chennai City (Nungambakkam) - 294, Cholavaram Lake - 293
Poonamalle - 292, Cheyyur - 289
Palavedu (near Avadi) - 284, Tamaraipakkam - 281
Chithamur (beyond Madurantakkam) - 274, Avadi - 271
Hindustan Unviersity - 260, Madhavaram - 257
Minjur - 231, Ennore - 231
Uthukottai - 226, Poondi Lake - 189, Vembakkam (Near Kancheepuram) - 174
Sriperumbudur Agro - 171
Acharapakkam (beyond Madurantakkam) - 167
 
Chennai: Mostly cloudy on Wednesday with frequent showers. The intensity could be heavy, but shorter duration. The 24 hr rainfall could be around 50 mms. The sky may show some clearing by evening. Decrease in rainfall on Thursday.Rains may linger on throughout this week though.

Pradeep informs us :"Historic Rains comes to end. 30000 cusecs released from Poondi and 29000 cusecs released from Chembarabakkam"

The formation of the Vortex has weakened, or rather it has to an extent merged in the trough. The Vortex normally remains stagnant as explained earlier.
But, some charters prefer to show the formation as a Low (shown in a chart yesterday on this site).
 
With a rigid High pressure ridge in the North of the formation, and obstructing the trough, the whole system was "blocked " from moving North. The anti cyclone (again) played a part in the prolonged rainfall over Chennai

Heavy Rains lashed Chennai on Tuesday 1st December.
The Chennai Meenambakkam rain gauge measured 272 mms in 12 hrs ended 8.30 pm IST on Tuesday.
97 mms from 5.30 -8.30 pm, 
69 mms from 2.30-5.30 pm, 
33 mms from 11.30 am-2.30 pm and 
73 mms from 8.30 am-11.30 pm.

The other regions 14 hr amounts ( till 10 .30 pm IST) compiled by Pradeep is given in below:
Katankollathur (near SRM) 431 mms, Kattupakam 400 mms, Chembarabakkam 369 mms, Kolapakkam 281 mms, Chennai (Meena) 272 mms (10 hrs), Chennai (Nunga) 289 mms, Cheyuur 267 mms, Anna Uni 260 mms, Poonamalle 256 mms, Puzhal Agro 255 mms, Taramani 243 mms, Red Hills 213 mms, Madhavaram 192 mms, 

Why and How ? (This is my personal analysis of the event from my experience and not binding ):

Initially there was a divergence tendency in the 200 hp jet streams. Please, I repeat and stress upon the fact (which is hardly ever followed by fresh meteorologists) that one must always pay attention to the upper most jet streams..they are the weather makers. (This explains why in my Sunday post here I had mentioned rains on Monday and "more rains" for Tuesday.) But of course it turned out to be much much more !!

The divergence in the upper jet streams is shown here:

What is divergence ? 
Certain regions of jet streaks are more favourable for rising or sinking air. Where convergence occurs in the upper levels, sinking motion results. Where divergence (evacuation of mass) occurs in the upper levels, rising motion result. Convergence and divergence in a jet streak is caused by an imbalance of forces as a parcel accelerates into a jet streak then decelerates out of the jet streak. Air is less dense at 200 mb.Vorticity at 850 hp levels is also high.

Hence, obviously, air from lower level rises, and rises fast.This causes a UAC Vortex to form at the lower levels. Some charts may depict it as a Low with Localised pressure dropping to 1008 mb:

Hence, without a system, but embedded in a trough, this results in unfounded very heavy and constant rainfall.
Normally, these "Rapid" vorti can vanish in 3/4 hrs, or last till 24 hrs. (Mumbai downpour of 944 mms vortex lasted 10 hrs).

Chennai rains expected to decrease from late night Tuesday/early morning tomorrow, and will be reduced to 40-50 mms on Wednesday.

Pictorial Explanation of Mumbai Vortex of 2005 Record 944 mms.

Torrential Rains Lashing Chennai...Basic and simple explanation will be put up at 11 pm IST

Rain amounts around Chennai  from Pradeep's Tamilnadu weatherman:

in mm (till 6.30 pm from 8.30 am in last 10 hours )

Kattankolathur (near SRM University) - 368
Katupakkam (near SRM University) - 310
Chembarabakkam - 302
Thiruporur (near Kelambakkam) - 281
Cheyyur - 233
Kolapakkam - 216
Palavedu (near Avadi) - 211
Puzhal Agro - 193
Poonamalle - 193
Taramani - 180
Anna University - 177
Chennai AP (Meenambakkam) - 175 (till 5.30 pm)
Avadi - 174
Chennai City (Nungambakkam) - 165
Puzhal - 157
Redhills - 148
Hindustan Unviersity - 139
Madhavaram - 123
Minjur - 104

TAMILNADU RAINS .. NORTH EAST MONSOON (NEM) 2015

TAMILNADU STATE - NEM 2015 (1 Oct 2015 to 30 Nov 2015) -:  517.9 mms (+47 %) 
Best performing and highest rainfall district : Kancheepuram -: 1241 mms ( + 144 %) 
Worst performing district : Madurai -: 354 mms (-1 %) 
Lowest rainfall district : Namakkal -: 277 mms (+8 %) 

After a deficient October ,a vigorous NEM during November saw Chennai AP accumulate 118 cms ( +181 %) of rainfall during November 2015 nearing it's all time November monthly rain records.

This Mumbai type monsoon rains triggered unprecedented floods and waterlogging in Chennai .

Chennai Airport has recorded 208 cms of rain from 1 Jan to 1 Dec (till 2.30 pm) 2015 .It's on course to overtake 234 cms of rains recorded at Mumbai (Tulshi) this year , after  a deficient South West monsoon (SWM).

(CLICK AND ENLARGE FOR BETTER VIEW)Tamilnadu and surroundings (rain in mms)

Chennai: Overnight, Nungambakkam measured 39.6 mms and Meenambakkam 64 mms as at 8.30 am Tuesday (Last 24 hrs).
As forcasted (on Sunday post) in Vagaries, Chennai is seeing "More" rains on Tuesday , with Meenambakkam getting 106 mms in 6 hrs from 8.30 am.
With this Chennai AP has received 17 cms of rain in December 2015 ( nearing it's Dec average ).