Mumbai, June 22 -- Light to moderate rain lashed parts of Mumbai on Sunday morning, offering respite from the sweltering heat even as the city continues to grapple with an acute water shortage. With reservoir levels at just 8.68% of capacity, the India Meteorological Department (IMD) has forecast an increase in rainfall activity from Monday and said the southwest monsoon is likely to reach Mumbai by June 23. The IMD has issued a yellow alert for Mumbai, Thane and Raigad for the next three days, forecasting moderate rain accompanied by thunderstorms, lightning and gusty winds of 40-50 kmph. An IMD scientist, who did not wish to be named, clarified that Sunday's showers were a localised weather event and not the formal onset of the southwest monsoon. "High humidity over the city led to cloud formation. The prevailing heat helped the atmosphere hold large amounts of moisture, which then released as showers," the scientist said. According to Brihanmumbai Municipal Corporation (BMC) data, Ramabai Municipal School in Ghatkopar recorded the highest rainfall between 6 am and 7 am at 24 mm, followed by Chembur Fire Station, 20 mm and Mankhurd Fire Station, 16 mm. Between 7 am and 8 am, Worli Seaface Municipal School and Savitribai Phule Municipal School recorded the highest rainfall at 25 mm each. The G-South ward office in Lower Parel received 21 mm. The delayed monsoon rains have pushed reservoir levels to worrying lows. A recent citywide water tanker strike further exposed the vulnerability of the city's water supply, disrupting deliveries to housing societies, commercial establishments and construction sites. BMC data released on Sunday showed that the combined water stock in the seven reservoirs stood at 125,585 million litres, representing just 8.68% of their total storage capacity of 1,447,363 million litres, significantly lower than the 25.87% storage recorded last year. Among the reservoirs, Upper Vaitarna has already run dry. Modak Sagar is at 23.8% of capacity, while Bhatsa, Mumbai's largest source of drinking water, is at just 7.99%. Hydraulic engineer Dileep Patil said, "The first shower does not have much effect on the catchment areas as the water gets absorbed due to humidity. The second spell of rain will provide a better idea." The BMC has already imposed a 20% cut in water supply to industrial, commercial and sports establishments while awaiting stronger rain activity....