Mercury levels in N-W may relent from weekend

Vinson Kurian

Thiruvananthapuram, Feb. 25 The atmospheric pattern upstream of northwest India is witnessing a churn yet again, which is soon expected to help moderate the high mercury levels being experienced in the region.
A resident semi-permanent trough (of low pressure, rising air and convection) across the border is now in the process of being replaced by a counterpart semi-permanent ridge (of high pressure, sinking air and compression).
These seasonally alternating systems positioned roughly over the Pakistan-Afghanistan region have a substantive role in dictating weather downstream over northwest India – from the seasonal warm and moist to the cold and dry and vice versa.
WESTERLY TRAIN The resident trough has been sending in a relentless array of western disturbances into northwest India, which has been setting off snow/rainfall events even while keeping the mercury level above normal for sometime now.
But this trough had earlier broken the ‘jinx’ to bring western disturbances and resultant wet weather from February, after December and January proved dry like never before and had threatened Rabi crops.
‘Disconcertingly frequent’ westerly systems have been hitting the region back to back, not allowing any to stay anchored for a reasonable period of time (two to three days) and rain down the contents except over the hills.
Assent of the ridge is now expected to help switch off the western disturbances and make the way clear for cold but dry northwesterlies into the region. The mercury levels are expected to be brought down but without rain or snowfall events.
STILL HIGH On Tuesday, India Meteorological Department (IMD) said that both minimum and maximum temperatures continued to be above normal by 4- to 6 deg Celsius over many parts of northwest, west, central and east India.
But it also added that the current meteorological analysis indicated a slight fall in temperatures over these regions during the next three days.
Formation of the ridge would also mean that the band of northwesterlies would be split, with one part getting diverted and made to flow around the ridge to dip into a trough over the region adjoining Northeast India.

This is now expected to set off thunderstorm activity over the region over the next five days, which the IMD has been repeatedly mentioning in its forecast since the past two days.
On Tuesday, India Meteorological Department (IMD) said that both minimum and maximum temperatures continued to be above normal by 4-6 deg Celsius over many parts of northwest, west, central and east India.
But it also added that the current meteorological analysis indicated a slight fall in temperatures over these regions during the next three days.
Formation of the ridge would also mean that the band of northwesterlies would be split, with one part getting diverted and made to flow around the ridge to dip into a trough over the region adjoining Northeast India.
This is now expected to set off thunderstorm activity over the region over the next five days, which the IMD has been repeatedly mentioning in its forecast since the past two days.
On Tuesday, it reiterated that interpretation of numerical weather prediction models suggested scattered rain or thundershower activity over the Northeastern States.
Isolated rain or thundershowers is also likely over Sub-Himalayan West Bengal and Sikkim. Rain or thundershowers are likely at a few places over Arunachal Pradesh.
Rain or thundershowers have been forecast at isolated places over Assam, Meghalaya, Nagaland, Manipur, Mizoram and Tripura during the next two days.
Under the influence of the prevailing western disturbance over Jammu and Kashmir, isolated to scattered rain or snow is likely over the Western Himalayas on Wednesday and isolated thereafter. Rain or snow is likely at isolated places over Himachal Pradesh and Uttarakhand during the next two days.