This has been one of the wettest Septembers for Karachi, probably surpassing the year 2011. Around 200+ mm rain was recorded during the four-day rains in early September in the city (when taken the highest 24 hour stats across city’s stations) while the 2nd spell in September has caused 70+ mm rain so far. Now eyes should be on a third spell!
There are chances of ‘Lu’ Winds (strong warm winds) in Karachi due to the expected formation of a depression in the Bay of Bengal which is very likely to move towards central India after that its track is disputed. This will be 2021’s most strongest monsoon system. Since wind flows of strong weather systems are larger hence they impact the prevailing wind patterns in the area, therefore;
- In the coming week, Karachi’s temperature will range from 35°C to 36°, peaking at 37°C to 39°C during the mid of the week (+1°C error).
We have been keeping an eye on the probability of this system since Sept 1 and are currently maintaining its chance at 40% for south Pakistan. This will be a significant weather system to watch. If it comes to South Pakistan, there can be widespread very heavy to extremely heavy rains in many areas of Sindh. Since the conditions are unfavorable due to -IOD, dry winds and SST, there is a huge variation between GFS, ICON and ECMWF. Such unstable runs by numerical model makes it difficult to determine the exact situation. It is expected by September 14 to have some clarity. But for now, its confirmed that a depression is developing in the Bay of Bengal.
Update – September 26
70 mm rainfall was recorded due to this system. The depression has stalled over central India due to westerlies but after much delay its remnants were able to cause thunderstorms in Karachi.
Pakistan Weather Portal (PWP) 
IOD index has increased significantly and is currently in the neutral range and has been for a couple of weeks now, according to the bureau of meteorology, Australia. SSTs are also stable at around 28°C which is decent enough to support the upcoming system. If ridge is concerned then ridges are easily pushed away by systems as strong as the upcoming one.
Off the coast of East Africa, the temperature remains towards the cooler side extending till the Balochistan coast, coupled with dry air, we haven’t seen much precipitation crossing west. Hence, at least for Karachi the rain remained hit-or-miss as observed especially during the 2nd spell where it has been infrequent. BoM has maintained -IOD as of Sept 2 and I have checked the NOAA maps, the trend is towards the cooler side. Unlike in 2020 when Arabian Sea was warmer with back-to-back circulations, this year the waters of the Bay of Bengal are unusually warm (we didnt check the Bay of Bengal’s conditions when issuing the 1st withdrawal dates as our primary focus remained on the Arabian Sea), the westerlies are also weak in this month and the monsoon axis is no doubt allowing thunderclouds to cross into SE Sindh beyond that it becomes hit-or-miss.