mumbai, Sept. 30 -- The Nifty 50 stock index is slated for its weakest monthly expiry in six months on Tuesday amid surging bond yields induced by persistently high crude oil prices. The bellwether index closed at 22,780.25 on Monday and dropped to a low of 22,569.65 in trading on Tuesday, the expiry day of the September series of derivatives. The lowest monthly expiry so far this year occurred on 30 March, when the Nifty 50 closed at 22,331.4, a month after the outbreak of the US-Iran conflict. However, the markets recovered from that low to close at 24,334.55 at last month's expiry on 25 August. NSE derivatives, from which investors take market cues, expire on the last Tuesday every month and weekly expiries happen every Tuesday. Though the intensity of the fighting in West Asia has reduced, recurrent blockades of chokepoints such as the Strait of Hormuz, which carries about a quarter of global seaborne oil trade and significant volumes of liquefied natural gas and fertilizers, and, more recently, the Red Sea, have driven a rally in crude oil prices from $90 a barrel at the end of August to $101 on Monday. Reflecting concerns of accelerating inflation, the 10-year US treasury yield surged 52 basis points to a two-decade high of 5.28% over the same period, according to financial portalinvesting.com. Consequently, foreign portfolio investors (FPIs) sold Indian shares worth Rs.34,609 crore in the Indian cash market so far this month after turning net buyers of Rs.24,098 crore in the two preceding months. "The markets are closely watching the surge in US treasury yields and if the same exceeds the two-decadal high of 5.3%, we could witness a crack in the Nifty," said Shai Coelho, founder of analytics firm VTrender. "The expiry shows that FPIs continue to remain bearish on India." While domestic institutional investors led by mutual funds absorbed this month's selling by FPIs by net buying Rs.57,806 crore of shares, according to BSE data, sales by other constituents such as retail direct investors and non-resident Indians have soured sentiment. Adding to the bearish sentiment in the cash market, FPIs initiated record-high cumulative short positions on Nifty and Bank Nifty futures at 319,669 contracts as of Monday. They continued to be cumulative net sellers of Nifty and Bank Nifty call options, underscoring their excessive pessimistic stand. Market analysts said unless the Nifty reverses course for a couple of days, the bearish undertone will persist. "Unless we see the momentum indicators reverse, I will be approaching the October series with caution," said Sahaj Agrawal, senior vice president of derivatives research at Kotak Securities. "The Nifty has been stuck in a range of around 22,200-24,500 for a while now and it's consolidating within this range with a bias to the downside. For this bias to change, the Nifty must give a positive closing for a couple of days before I can take a fresh view." Kruti Shah, a quant analyst at Equirus Securities, said globally induced uncertainty was impacting the Indian markets....