Tuesday, October 30, 2007

The current system in the bay went upto a maximum capacity of a depression, and has now weakened and the clouding spread all over the peninsula upto Maharashtra(IMD satellite). The severity of the rain from the system is over, and the forecast for 30th. is in fact rain of sizable intensity only near the south A.P. coast, and the areas of south A.P. and north Interior Karnataka will get some rain, though scattered and not too heavy (map from "wetter").
A period of lesser rain is what is in store for the southern states for this week, at least till the 6th./7th. That is when GFS model has forecasted the next low for the bay.

Night temperatures in Central and Northern India are falling, but not at the pace required for this time of the year. The falling trend, which usually moves in westwards into N.India from Pakistan, has just begun to a small extent in Pakistan. In the hiily states, Kalpa in H.P. and Pahalgam have reached 1 c.
The nights can get cooler in N.India and central and N.Pakistan in this week, as no incursion of moisture is expected from the south, and the "high" can dominate the region.


Sunday, October 28, 2007

The well marked low in the Arabian Sea could now develop into a depression, after a day of stagnation. The JTWC announces the chances of it forming into a cyclone in the the next 24 hrs. as "good".

Meanwhile, on the eastern side, carrying on from yesterday's blog, the low is now centered east/south-east of Chennai, as on night of 27th. The rains have commenced along the south Tamil Nadu coast. The system may not gather too much strenght now, and may not go beyond a dperession level, as the low lies just south of an upper level ridge axis, and the minimum sea level pressure isestimated to be near 1005 mb. But the interiors of Tamil Nadu and Karnatak a will get rains from 29th. onwards.

The "power house" of the North-East monsoon is actually over North China/Siberia. This region is normally dominated by a large anti cyclone, or high pressure area. Similarity of this with the summer monsoon is that the "power house" of the summer monsoon is the Mascerenes High off Madagascar.
From these "power houses, the summer monsoon winds blow, and change direction on crossing the equator, and the North-East monsoon is fed by the strong winds blowing from the Siberia High, across the Bay.
During the North-East Monsoon, occasional strong flows of cold air from the Siberian High is associated with the development of low pressure systems over the regions of Sarawak and East Malaysia. Remnants of these low pressure systems that affect Malasia, cross into the bay, and move westwards to Tamil Nadu.
Also, the sub tropical westerly jet stream is a characteristic feature of the NE Monsoon.

Hence, as mentioned, we can expect some more low's to follow till the middle of November from the Bay.

Friday, October 26, 2007

Ok, the bay low, has now crossed over into the Arabian sea, and has merged with the existing low there.
Many models still predict a cyclone there in the Arabian Sea, but the course and direction of the cyclone "to form" is being changed with almost every update. Very uncertain and unpredictable ! But, how strong the system becomes,( I still call it a system, without grading it), remains to be seen, and the direction, is anybody's guess.

The low mentioned by me for the bay has formed,and should cross the south Tamil Nadu coast by the 28th. And the rains will follow, drenching the southern tip most. But later, rainfall should disperse northwards into inland Tamil Nadu and Karnataka.
The upper jet stream westerlies over north India, are much too deep in the south,falling to near 22N, and the jet stream easterlies are very weak for this time of the year,and near the southern tip of India. Hence, any low,or depression, is unable to cross the east coast higher than the 10N line.

The high over the central parts of northern India will keep the area dry, and cool at nights, for the next week, at least.


Wednesday, October 24, 2007

I would like to still cling to my estimated forecast of a low forming in the bay, and precipitating rainfall over South India, and dissipating over the peninsula.
It would have poured a fairly good amount of rain in the coastal areas( 28/29th.), and inland Tamil Nadu and south Kerala(29/30th.). Interior Karnatak can also get fair rains by 30th.
Several models like NOGAP and ECMWF predict the low to form into a depression, and cross the Tamil Nadu coast around the 28th.-29th. But I have my doubts of the same low reforming in the Arabian Sea. A low/depression in the bay is quite a normal occurance for this time, but the anti conditions of the western sector sea can prevent any major development there, as dry air is pushing from the mainland.
The coming system should cross the coast between 10N and 15N, due to the high pressure ridge in the northern parts.

But, like mentioned earliar, more systems could be expected till mid November atleast.


Tuesday, October 23, 2007

My apologies for not updating this blog for the last 10 days. Was away, and unable to keep in touch with the latest on the sub continent weather, and unable to answer to reader's comments.

Finally, this year's unpredictable south-west monsoon has widhrawn ! Since its setting, the monsoon has been erratic, and often proved the weatherman wrong, and quite a few times totally misled the predictions.

Today, the IMD has declared the widhrawal of the south-west monsoon from Goa southwards across the entire country, in one day. And "Simultaneously Northeast Monsoon rains have commenced over Tamil Nadu and adjoining states of south peninsula." Quote IMD.

The W.D. moving across the north, was the main culprit for the delay, as it produced stray showers in the east and north-east, and the prevailing south-westerlies prevented the monsoon from totally moving out. But, with the system moving away eastwards, the high pressure anticyclone has quickly taken control, and organised the north-east flow in a more systamatic pattern (IMD streamline).

Moving to the north-east monsoon, it is clear that the China Sea, is still in an active phase, and is expected to pulsate a low in the south bay around 24th. October. According to weather models, and Jim Andrews of Accuweather, this low could form into a depression and cross the TamilNadu coast around the weekend. Some other forecast models predict the system to cross the east coast and re-emerge into the Arabian Sea. GFS from COLA shows the system crossing the Tamil Nadu coast around the 26th. and the rain area spreading and subsequently covering the entire southern Peninsula as the system weakens. And in fact, the rains are seen reaching as far north as Mumbai around 27th./28th.(check at http://www.monsoondata.org/wx/india.vv.html).

Personally, I tend to agree with this, as the Arabian Sea may not be conducive to revive a system, as it may not be stronger than a depression at the time of crossing the east coast.
But, surely there will be enough heavy rains along the Tamil Nadu coast, and in the interoirs of the southern states. Kerala, can get heavy rain around middle of next week, as a vortex may form in the south Arabian Sea, as the depression crosses the coast.

With the West Pacific regoin still active, I expect more depressions to come along the bay path, and keep the north-east monsoon vigourous in phases for at least another month till mid November. After that , the feeding from bay systems can cease, and the focus will shift to the strenght and frequencies of the W.D's in the north, as that too will have a direct impact on the frequencies of occurances of the easterly waves from the bay.
Due to the IRI predictions of a strong La Nina during the Oct/Nov/Dec period, the rain forecasted for the same period is above normal for the southern Peninsula, hence indicating a weaker flow of W.D's in the northern regions of India and Pakistan.


Saturday, October 13, 2007

The IMD has announced that a "east -west shear zone " is establishing over the south peninsula by the weekend. This is the indication that the North -East monsoon is approaching. In fact, when a circulation gets embedded in a shear zone, it provides a good momentum to the monsoon to strenghten.

The ECMWF, has, infact predicted a formation of a circulation in the south-west bay, as indicated in my blog, yesterday. Another circulation is predicted in the south -east Arabian Sea. If they materialise, two circulations on either side of the shear zone can give a good momentum to the North-East monsoon.

IMD today announced the widhrawal line of the South West monsoon as passing through Aurangabad/Dahanu (20N), on the western side . The western side declaration seems to be slow, to me. The widhrawal is way below Maharashtra, I feel. The situation so demands, as 1. the north-east monsoon is gearing up. 2. the humidity levels are low right upto peninsula India(map) 3. the anticyclone has effectively pushed the north winds southwards and has wiped out the south westerlies. Dry winds in North and Central India,upto 15N, in this map 4. As far as Mumbai is concerned, there has been no rain since the last 13 days, and the day temperature was at 36c today !And the humidity was 54% !

Well, we chase and follow the North -East monsoon winds, which have already reached the eastern shores of the Bay. And we follow the temperatures dropping in North and Central India, and the"October heat" in Mumbai !


Thursday, October 11, 2007

It is clear now that the seasonal high over Pakistan and north-west India is more marked, hence the southward push of the monsoon widhrawal has now reached the southern parts of the peninsula.
And the weather focus is now moving towards south India. International models observe that a upper air circulation has formed off the Tamil Nadu/Sri Lanka coast. The winds which are now north to south inland along the south-east coast, are expected to become north-easterly along the south-east coast and inland in the next 2 days.This is expected with the anti cyclone pushing down and changing the wind direction. With these changes predicted, in confirmation with my last blog, south Tamil Nadu, Kerala and Sri Lanka should get the commencing showers of the monsoon in the next 2 days.

Meanwhile up north, the night temperatures have dropped, as estimated. The IMD colour map shows the temperature distribution and the cooler weather covering north India. Pakistan too is witnessing cooler nights with Quetta at 0 c.


Monday, October 08, 2007

The low over east-central India strayed towards the east, and as predicted, a new low has sprung up in the extreme north bay over the heated north bay waters. Today the low lies over Bangladesh,and is expected to move north-east. Further widhdrawal of the south-west monsoon got delayed beyond Maharashtra, again, due to the moisture incursion into the areas of Maharashtra and south peninsula.

For south India, and Tamil Nadu, the projected low from the typhoon activities in the China Sea/Pacific, is expected around mid week. The NCEP forecasts suggests a cyclonic circulation off the Tamil Nadu coast around the same time, and the ECMWF predicts a weak low in the same area. Hence, at the most, the low will remain weak, but, rainfall activity is likely to commence from mid week over south peninsula. The NCEP forecasts the "wet spell" to remain till 20th. Oct. But, the north-east monsoon does not set in till the south west monsoon widhraws completely from all the southern states, and then the winds are set to change course and direction.

However, during the north-east monsoon, a seasonal high stabilises over north China/Mongolia area, and is said to be the "power house" of the north-east winds blowing down the bay. This is slowly getting established. The related high of the south-west monsoon, which stations off the Madagscar coast, has almost gone.

The ECMWF gives a seasonal forecast for the tropics. In its Oct/Nov/Dec forecast, it predicts a slightly below normal rainfall for the north/central Tamil Nadu coastal region. The rest of the region will be around normal. The IRI forecast is for normal rain throughout south India.


Saturday, October 06, 2007

The northern areas of Pakistan,and the north, north-west and central regions of India, are very much toeing the post monsoon trend. The minimum temperatures have started their seasonal fall, as seen in the blue area of the IMD map, covering the major part of the region from where the monsoon has widhrawn.

The strong prevailing seasonal anti cyclone in the northern half of the sub continent, will drive any rain from the existing weak low away towards the eastern half of India.
Sea surface temperatures are still comparatively above normal in the northern most bay,and that is the area where the system will move ,and possibly strengthen again.

Typhoon Krosa, now in the East China Sea, has gained to a super typhoon strenght, and is now Category-4 (destructive). This is more likely to trigger off the southern based low I mentioned in my last blog. Late next week, a low is predicted off the southern coast , and bring to the south, the commencement of the north-east monsoon. But, after a brief wet spell, the system(low) may move northwards, maybe north-east, if the existing prevailing northwesterlies, now upto central India, push southwards to dominate the wind flow.


Thursday, October 04, 2007

The retreat of the monsoon continues southwards. The north dry air pushes its way south, and the widhrawal officially has taken place in north India, northwest India ,Gujarat, M.P. But, the rains have ceased now in Maharahtra and official widhrawal is expected soon. The rains are today restricted to the east coast and south as seen in this CPC map.

I expect, the night temperatures to fall in Kashmir, Punjab, Haryana, Delhi and Utteranchal by 2-4 c in the next week. That means towns in Punjab may reach a low of around 14-15 c , and Delhi - Haryana may touch a low of 16 c in the coming week. This woold mean clear skies as no systems are approaching the region.

But down south, as mentioned, the remnants of Typhoon "Lekima" is expected to spring up a low around the 8th. of this month in the bay. Some models predict this to gain upto the level of a depression. This development is bound to bring good rains along the coast of A.P. and interior south peninsula next week. The exact time forecast of the rains will depend on the speed of formation, development and location of the low.
If the low in the bay develops into a depression off the south A.P./north TamilNadu coast, the south west monsoon will be simultaneously replaced by the north east monsoon winds. The ITCZ will shift down south , with the central India seasonal low vanishing.