Monsoon watchers appear to have heaved a sigh of relief on the weakening of cyclone 'Laila' as they believe it would now have little impact on the annual rainfall season.
"We are still going ahead with the initial prediction on monsoon. This season, monsoon will be normal and is expected to reach Kerala by May 30," Earth Sciences Minister Prithviraj Chavan told reporters here.
He said all the weather parameters have been factored in while arriving at the monsoon forecast.
A section of the meteorologists said that following the landfall, the cyclone will weaken further as supply of moisture to the weather system will be cut off after it hit the land.
But how the cyclone dubbed as "severe" by the weather office lose its sheen?
In fact, Laila began weakening significantly since last night as it travelled very close to the coast for a long time
but did not make a landfall. This could have been because of a sudden increase in the value of 'vertical wind shear' (VWR).
Wind shear relates to a difference in wind speed and direction over a relatively short distance in the atmosphere.
Wind shears are of two types -- vertical and horizontal.
"A cyclone gets energy from sea through evaporation of water from sea surface. This evaporated water is soaked by the winds. The moisture-laden winds flow into the cyclone's centre through a process called moisture convergence," said a top operational forecaster.
"When half of the circulation came over land, this process got cut off from western side leading to reduction of moisture supply as well as moisture convergence. This obviously has affected intensity of the cyclone," he said.
After churning across the Bay of Bengal, 'Laila' virtually came to a halt about 200 km south of the coastal city of Machilipatnam and remained there for quite a long time.
Similarly, shift in the mid and upper air high pressure area could have also played a major role as the intensity of a cyclone depends on the 'vertical wind shear'.
"Low value of vertical wind shear creates ventilation in the vertical column of cyclone so that the moisture laden flow enters into cyclone inner core, rises up and then gets out of the cyclone from top. This way a continuous supply of moisture is ensured," the forecaster said.
Since its formation, 'Laila' had benefitted from a low value of the vertical wind shear but suddenly Thursday, the high pressure area shifted slightly and the value of the wind shear started rising.
This shift is believed to might have caused some disruption in ventilation of air and hence resulted in weakening of the system.
However, this relief could be shortlived as there is a strong possibility of the cyclone moving along the coast for over 24 to 36 hours and re-emerging into Bay of Bengal only to
intensify further.
Experts believe that the re-newed cyclone may move in a North-North-East direction and move toward Bangladesh or Mynamar.
This weakening of Laila appears to have given IMD the hope that monsoon may not be disrupted.
Predictions suggest that the inter-tropical convergence zone (ITCZ), considered as the birth place of low pressure systems, in the Arabian Sea as also the Bay of Bengal may remain active even after the weakening of the cyclone.
An active ITCZ raises the hope for possible formation of an onset vortex like low pressure system over the Arabian Sea subsequent to the weakening of cyclone.
IMD Director General Ajit Tyagi also maintained that the weather office has not changed its forecast for monsoon onset.
The IMD last month forecast a normal rainfall for this year with a precipitation of 98 per cent of the Long Period Average (LPA), subject to a model error of plus or minus five
per cent for the entire season (from June to September).
Last year, the weather office had forecast a rainfall of 96 per cent plus or minus five per cent in April and then fine tuned it to 93 per cent plus or minus four per cent. But, the country received a rainfall of 77 per cent of the LPA only for the season.
The normal monsoon forecast is expected to bring cheers to over 235 million farmers who had faced drought last year due to failed monsoon.
A good monsoon could help in sowing of rice, sugarcane, soyabean and corn and lead to a rebound in the agricultural output.