Saturday, October 2, 2010

How Winters work in India

The 850mb wind map. Note the giant anticyclonic circulation in Bay of Bengal and the N-S winds

Akshay Deoras
Severe Weather Forecaster

The NE Monsoon or Monsoon withdrawal is on its half way! The 850mb GFS indicating that SW Monsoon has totally moved away from Northern India,Rajasthan, Gujarat,Western Madhya Pradesh,Weak in Uttar Pradesh,Madhya Pradesh East,West Bengal,Orrissa,
Monsoon is now moving away from Nagpur and surroundings and will take 48hrs more to withdraw from Nagpur.
(ALL AS ON 00Z,2ND OCT 2010)
Due to this, Rainfall already has started decreasing and mostly nil at the places where it withdrew.

The Minimum temperatures are falling over Central India,Madhya Pradesh as a result of the cool air incursion over the Central India from the north.
As happens during the onset of winter, A High Pressure area stars building over Tibet due to strong cooling of the land as as cool air doesn't rise much as compared to warm air given its mass and density. As this happens, a Sea Level High Pressure of 1022-1024mb forms over Tibet.
At the same time low pressure areas gets created in the seas around the earth since heating of sea water causes a low pressure area to form. Due to this, the low level winds or the 1000-700mb winds blow from High pressure over Tibet towards Central India and then southern India. A High pressure area over tibet also blocks the jet streams causing it to split into two paths. One branch directly digs in India while the other blows as westerlies
Thus cool air from Himalayas digs inland and causes winters over Central India,eastern India and southern India,along with non coastal areas in Western coast

Current conditions
The High pressure over Tibet 1022mb and two low pressures of 1006mb over southern India and the Arabian Sea (few kilometers offshore Karnataka). Due to this cold air already had swept across Nagpur and other parts of Vidarbha,Central India causing minimum temperatures nearing to 20C since the last 96hrs. The minimum temperature was recorded at Wardha on 1st Oct 2010 which was 16C. Nagpur recorded a temp of 20.3. 16.4C was found at Yavatmal

The 850mb winds are not aligning North-South over Central India since the forecast models now indicate the low pressure in Arabian to shift slightly west.
Temperatures (Min) will rise by 2-3 C in Maharashtra. However Central and Northern Madhya Pradesh will witness same temperatures at minimum

Temperatures in Central India will again start falling down from Wednesday as a New giant Low level circulation or LLC forms in a wide area in Bay of Bengal on Thursday. The winds will regulate North-south again and cause temperatures to fall by 2-3C again

The SW Monsoon will withdraw completely from India by 15th Oct 2010

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