Monday, August 19, 2013

Note on Withdrawal of SWM this Year:

Several models are recently showing absolute dry and "no precipitation" conditions in major parts of Pakistan and adjoining Rajasthan.
And, consequently, I have received several queries on this forecast, such as "is it an early withdrawal of the SWM "?
Does it necessarily mean or indicate a monsoon retreat ?

Let us first study , and understand for ourselves what actually are the parameters and conditions for SWM withdrawal. On monitoring, and following these, we can ourselves decipher and judge whether it is a withdrawal.

Parameters for withdrawal from Pakistan and Rajasthan region:

-1. Let us initially keep a watch on the 200 hp jet streams. During the monsoon , they are Easterlies approximately below the 30N line.This is the Southern periphery of an anticyclone at 200 hp, with the core between 30N and 35N at 150 hp. Peak winds at core exceed 100 knots. Westerlies are to the North of the Himalayas. 
Now, when the withdrawal process starts, we may see a deviation, gradually  from the Northern regions. The anticyclone in the Northern most regions vanishes, and westerlies start taking over below the 35N.
-2. A trough, in the westerlies, in the Afghanistan/ North Kashmir region, would mean the movements of WDs in the upper atmospheres. The frequent movements of these troughs and WDs would hasten a withdrawal.
-3. High temperatures prevail over Tibet till August End. This normally provides the ascent for the Hadley Cell. Falling temperatures over Tibet weaken the Heat source for the Hadley Cell.
-4. The Seasonal Low starts weakening with the formation of a high pressure in the Pakistan /Rajasthan region at the 850 hp level, or below. This would start "crippling" the axis, and its strengthening would regulate how fast the axis moves Southwards. The Southward moving axis, shifts and takes the upper limit of the Monsoon southwards with it.
As the axis reaches Karnataka and TN, the NEM conditions like wind and humidity get established  So, a direct link between SWM withdrawal and NEM setting.
-5. Relative Humidity reduction is observed in the NW parts of India. As the winds change direction, we see reduction in SW winds in the NW regions, thus reducing Humidity.
-6. UTH shows a marked and steady reduction.

Now, where are we placed this year as on 18th August ?

Currently, we do not see any of these parameters showing a withdrawal signs.
AN UAC/Low will be tracking thru Central India next few days. I would rule out any indication or parameter of withdrawal revival till the life span of this system. 

Possibly, this system may push in extreme Coastal and Eastern Sindh region around 24th/25th August.Rainfall. whatever amounts, is possible from Central India thru N/NW India from this system from 20th August-24th August. Good rainfall possible next 2 days in Northern Pakistan.
In fact, Northern India can get more rains for a couple of days after 30th August. (This is not a detailed Forecast report, but just a hint that withdrawal is not immediate).

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Specific Forecast for Monday/Tuesday/Wednesday:

Chennai has a chance of a regional /local thundershower popping up on Monday evening, in some parts. At the most a remote chance on Tuesday with a local area getting a convective uprising shower. Later, as the trough weakens, Tuesday onwards, the chances of rain are negligible. Warm days later.

Mumbai sees the 2/3 passing showers reminding of the monsoon season on Monday. upto 10 mms possible.
A slight increasing trend from Tuesday night, will possibly see a sharp shower on Tuesday night, and some frequent rains on Wednesday. 20 mms/day.

Delhi NCR will continue to receive the spotted regional showers it is receiving. Regional or parts of the region will get showers next 3 days , with some downpours amounting to upto 30 mms on a day.


Jayeshbhai sends Last years Vagaries Lake Levels comparison (as put on vagaries with today's

See Pradeep's Page for...
Super Heavy 10 day Spells This Monsoon...And What Spells ! 
Also Pradeep's effort in working out Normal Goa Averages.

12 comments:

Neeraj said...

two tropical storms in western pacific region - pewa and unala. their forecast tracks seem to crisscross . isn't it a bit unusual ?

Unknown said...

Rajeshbhai, Your estimate and BBC Mid week forecast for Western Vidarbha / adjoining areas of MP is showing extreme rain... can you pls elaborate, I think we need to watch this closely ....Potey

sam khan said...

Rajesh sir,
is it possible for a cyclone to form in the Arabian sea in the month of september.

rohitaros said...

superb explanation of monsoon withdrawal parameters

sset said...

Great power in Rajesh sirs explanations (monsoon watch series, withdrawl parameters) no blogs can match - it is a masterpiece on its own.
Now during weekends I need to google each term and try to piece the puzzle-- jet streams...2012 monsoon was stubborn refused to come to south..till end October...

As per past records cyclones can form other side Arabian/Bay any time of the year.
For instance - pre monsoon -> Gonu(Arabian), Laila (Bay)
Post monsoon -> Phyan(Arabian), Nisha(Bay). But recent years probability of number of cyclones over Bay during NE monsoon has reduced considerably. Infact 2012 only 1 cyclone came "Neelam" during end of SW monsoon - 2011 cyclone "Jal" did not produce any meaningful rain..for TN/Rayalseema this is the only means of water - depressions/cyclones are very important for water to reach SE India.

sset said...

Pradeepsji 10day rain storms - amazing rains -this shows much of intense rain areas are moving towards MAHA-south -> north coast...gradually..Augumbe has lost its steam (much due to mining and deforestation)

rajesh said...

Neeraj: Not very unusual for this time. The near neutral conditions (ENSO) has anticipated these TS in the region..
sam: Normally cyclones are seen in October..there has been the odd storm in end of September.Normally October storms are evolved in Bay, and after crossing S.India, emerge as DD or CS in Arabian Sea..

sset: You are right...axis refused to move down in 2012...Why ? can you remember ? You may refer to vagaries...:-)

Atul Naik said...

Rajeshbhai, great post on SWM withdrawal. Thanks!

Mohsin mulla said...

sw monsoon not withdrawal immediatly... It is a break or weak phase for west coast..but its peak once again in first week of septmber..

sam khan said...


Monsoon might pick up momentum again on the west coast either during the 1st week or 2nd week of September for sure.

Neeraj said...

I was not talking about the tropical storms themselves but the tracks. The tracks of the two storms crossed each other twice as per the forecast at that time. But now both the storms are gone- in fact Pewa and Unala cancelled each other out, it seems.

parul nimbekar said...

Thanks for sharing such valuable information.
Damdama Lake