South West Monsoon Criteria for Withdrawal as per IMD:
Withdrawal of SW Monsoon
a) Withdrawal from extreme north-western parts of the country is not attempted before 1st September.
b) After 1st September:
The following major synoptic features are considered for the first withdrawal from the western parts of NW India.
i) Cessation of rainfall activity over the area for continuous 5 days.
ii) Establishment of anticyclone in the lower troposphere (850 hPa and below)
iii) Considerable reduction in moisture content as inferred from satellite water vapour imageries and tephigrams.
Parameters for Withdrawal considered by Vagaries:
a)*A Primary factor is the 200 hp jet stream. This is one of the main and assertive parameter. The Easterly winds at 200 hp, gradually become Westerly starting from the Northern regions as the Monsoon withdrawal line moves South.An anticyclone High forms in the jet streams around the Sindh coast by 1st week of September, thereby creating Westerly winds almost throughout the region of Pakistan and Rajasthan. In fact, these winds bring down the axis and the withdrawal line.
b)**As the Monsoon withdrawal commences, the seasonal Low weakens, and the core (then becoming around 1000 mb during initial withdrawal ) moves to North Pakistan region.
c) Another factor, connected is the Outgoing Longwave Radiation (OLR). As the norms touch 290 W/m2 or rise above ...
d)***Also, the Upper Troposphere Humidity plays an important part in supporting and "holding " the Monsoon. The current humidity position indicates a weak situation for Monsoon sustenance in North/North-West.(Below 10%).
Position Parameter wise as on Sunday 18th September
* The West winds (at 200 level) are firmly above the 30N line. An Anti Cyclone will form along the 27N line, roughly in the Sindh/Rajasthan region in a couple of days.Jet Streams North of the Anti Cyclone will be firmly West.
**The Seasonal Low is weakening, with the core around the Central Sindh Region at 1002 mb. Will move and merge with the other Low in North Pakistan, also at 1002 mb.
***OLR and UTH Map shows the immediate withdrawal possible from Rajasthan and Punjab.( From 19th as mentioned in yesterday's Post).
At the 850 level, a weak anti Cyclones prevails over NW India and adjoining Regions. But conditions are apt for a Ridge formation over the Region in next 2 days.