Saturday, August 25, 2012

"Mysterious Clouding in the Southern Arabian Sea"??

Akshay Deoras
Severe Weather Forecaster


I received a few questions today about the "mysterious" clouding in the Southern Arabian Sea. The attached image is of the DUNDEE Satellite Imagery (Meteostat) VISIBLE of  0.45 - 1.00 ┬Ám RANGE.

It clearly shows some continuous clouding stretching from Offshore South-Western Coast to South Central Arabian Sea.

The above IMD Cloud top temperature image indicates the cloud tops having a peak temperature as significant as -40C. More negative cloud top temperatures indicate that the convection happening in the system is very significant. 

But what is causing such clouding???

Let us check the 12z,25th August 2012 GFS forecast models for Mid Troposphere ( 500mb). 

The above is the GFS Analysis at 12z,25th Aug 500mb. One can see some red shaded regions EXACTLY AT THE SAME PLACE WHERE CLOUDING IS NOTED IN THE ARABIAN SEA.

Such red shaded regions indicate Positive Vorticity in the Northern Hemisphere. Positive Vorticity basically deals with the anti-clockwise flow or the movement of the air ( in Northern hemisphere) which is often noted at the base of troughs ( region with short geo-potential heights). Thus there is a PVA or Positive Vorticity Advection happening due to which the lifting of the air is boosted which in turn is responsible for such clouding in the Arabian Sea.

This is also confirmed by the 500mb Geopotential Height which is 585dam in that region as compared to the higher values in the same latitude around.

So far, at the surface level, no low pressure region has been reported in that area and hence following the methodology, one can't call this region as a depression!


There is another Bay system approaching India from the East. The above satellite images followed by the 500mb analysis and forecast suggests that the system has started its movement into India and the 850mb anti-clockwise circulation at the moment is over Odisha region with the MSLP of 1000mb as per the Thai Maps.

This system is expected to pass through ( the PVA region) Maharashtra-Andhra Pradesh-Chattisgarh border and head NW by 72hrs. The convection seems strong in the Northern periphery of this system and hence considering its path, Southern,Cent Chattisgarh,Eastern Maharashtra including Nagpur will receive rains though accumulation will be more towards Northern AP and Southern Chattisgarh till Tuesday.

Rainfall is now expected to decrease in Northern India considering no approaching system and arriving DRY phase of the MJO.

ENSO update- 

The SST in the Nino 3.4 region has reached +0.6C ( Anomaly) which is a classic indicator of the El Nino conditions but only from the point of view of SST anomaly. The atmospheric circulation in the region remains neutral or average and is slightly turning towards the Positive phase ( El Nino).

It can be expected that the ONI values for the period of JJA to be at the El Nino threshold and by Mid September to September end, the phenomena establishes in the Pacific Ocean.

I shall update later on that

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