Thursday, April 28, 2011

Monsoon Watch 4

Seasonal Low: Indicator: -ve

The days have started heating up in the Sub-continent since the last couple of days. 47c at Larkana in Pakistan 2 days back, and consistent 45s in the Rajasthan and Sindh regions (Pakistan) assure us of the seasonal low forming soon. But, what we still require are the severe heat waves, which are still missing.

The formation of the low is behind schedule, with the core region in the Thar desert still to form and lacks consistency. The MSLP in the Pakistan Sindh region is 1002 mb as of today. Lower since MW 3. But we must remember, that a series of W.D.s have been constantly keeping the temperatures down by as much as 5-7c below the normal.

As on end April, a pressure of 1000 mb in the central core region around Barmer is normal.

Bay Sector: Indicator: -ve

At least the high pressure mentioned in MW 3 is now dissolving, with the help of Southerly winds. Many international forecasting models indicate the formation of a low in the Bay by the 1st. of May. Pre- Monsoon low formation seen in the next week's time frame.

Previous MWs have stressed upon the importance of a low in the Bay by end April. Once a pre-monsoon low is formed, it increases the speed of the SW winds in the Bay sector. An ideal wind speed of 25-30 knts hastens the cloud formations.

Cross Equatorial winds: Indicator: -ve

Good and +ve development here since MW 3. In the western sector, the Arabian Sea branch, the winds below the equator are much better organised now, and have started curving near the equator. A Northerly gush is seen off the Somali coast. This ultimately leads to the formation of the "Somali Current" an essential formation required to accelarate the cumulus cloud formation in central Arabian sea.

Somali Currents are very strong S-N winds off the Somali coast, and this strong wind brings the SST off the coast down (by end May) to almost 18c ! With a warmer central Arabian Sea, cloud masses in huge volumes are formed in the Central area. (Shall discuss in detail in later MWs).

For now, it is sufficient to understand that the winds have turned Northwards, and have re-curved.

In the Bay sector, SW winds are organised South-West off Sri Lanka, and the SE winds from below the equator are pushing North in the Bay region. Signs of getting orgainised. SW winds just South of Sri Lanka have gained speed and are now clocking 25 ktns.

But, in the Arabian Sea and remaining sectors, the winds are still to gain speed. They could be considered weak.

With no cyclone in the Southern Hemisphere, the ITCZ could be forecasted to move Northwards. Currently it is around 5S. ITCZ movement discussed in MW 3.

200 hpa Jet Streams:Indicator:Neutral

Another parameter to observe now are 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 jet stream is developing just below the Equator. Forecasts on models show these winds getting organised around the first week of May. Should march up to the 10N by middle of May, to be in time for the Andaman Sea Monsoon arrival.

La-Nina: Indicator: +ve

Sea surface temperatures in the equatorial Pacific Ocean have shown very little change. The weekly SST anomaly map for the week ending 24 April shows weak cool anomalies along the equator. Still, a few areas of SST anomalies of more than 1 °C cooler than normal for this time of the year now remain.

However, atmospheric indicators continue to be at odds, and remain consistent with a well developed La Niña event. The latest 30-day SOI value is +30.2. This is alomost touching the the highest April monthly value on record +31.7, recorded in 1904, and has remained consistently high throughout the event.

Cloudiness near the date-line remains below normal while trade winds continue to be stronger than normal.

Untill these indicators turn towards normal conditions, a fully neutral ENSO condition cannot be established. La-Nina prevails till then. And seems it could well slip into June.

Conclusion: Though there is some improvement in most of the parameters, there is still much to be achieved, and most parameters are still lagging behind time, and are -ve.

Based on these calculations, Vagaries would still maintain the Monsoon to reach Kerala by the 4/5th June. SWM should hit South Andaman Sea around 18th. May.

First half of June should see some deficient rains in the Peninsula regions.

MW 5 will be up on 7th May.


Anonymous said...

What is your forecast for monsoon rains in Karachi??Its really hard to predict rainfall of this city..most weather systems die while reaching karachi..but most weather system cause unforcasted rainfall in the city like in July 2009..60mm rainfall was forecasted but 245mm rainfall occurred and again in August 2009 sunny weather was forecasted but in real 147mm rainfall occurred...Thanks (from Pak weather Portal)

Rajesh said...

as i am away,shall discuss and put my views on 5th May. Thanks.

Owais hyder said...

Nice and interesting Weather blog..
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