Extreme rainfall event in the Darjeeling-Sikkim Himalayas: 04/05Oct20205 (Part I - 'The Rainfall')

 Thus far, in the SW monsoons of 2025 our region saw a deficiency of rainfall:

Sept2025 was especially dry in Kalimpong and with the monsoons expected to exit our region on 10Oct2025, one did not expect a major rainfall event.
What has been happening, however is that the Bay of Bengal has been inordinately active with bumper to bumper low pressure areas forming and many of them, after moving over land in a north westerly direction reaching as far as Rajasthan leading to excess rain in a normally arid area.

Origin of the extreme rainfall event.
Around the 29Sep2025, I posted a notice in our What's App group 'Hazard Alerts  or HA' stating a LOW was expected (by IMD) in the Bay on 01Oct2025:

The LOW pressure formed as forecast and quickly intensified into a depression by 02Oct2025

The above image was posted in our Whats App group (HA) on 02Oct2025,showing location of the depression, the forecast track and warnings issued by IMD.
An update to the forecast track of the 'deep depression' now showed the storm moving in a 'nearly northward direction' instead of the normally NW direction.

As forecast, the storm (now a well marked LOW) on 03Oct2025, reached parts of east UP (see below) and was almost stationery there until 04Oct2025.

The storm finally started moving as predicted in a NNE direction, heading directly towards Bihar, SHWB (Sub Himalayan W Bengal) and Sikkim at approx 3pm on 04Oct2025 (yesterday), below:

In the top image, do note the density of lightning strikes which were taking place.

Rainfall data
03Oct2025: Despite having a RED warning for heavy rain, our entire region received very little rain. I distinctly recollect that in Kalimpong (my home), it was a densely overcast day which turned stormy around midday but except for some drizzle, there was no precipitation.

 04Oct2025: I was monitoring the Davis Automatic Weather stations in our area and we had light rain from  around midday. At Kalimpong, it started thundering in the late evening and by 7.30pm, the thunder had intensified. Thereafter, the rainfall, lightning and thunder continued thru the night with little or no break

windy.com image of 11pm/04Oct, just when lightning and rain was increasing over our area. The storm had moved from its earlier location and was approaching us from the SW.

Rainfall Intensity at Kalimpong

The above graph showing the rainfall intensity at Kalimpong on 04Oct2025, also shows the duration of the rainfall. Peak intensity of rainfall almost hit 150mm/hr and almost the entire 158mm of rain came down between 8pm and 4am ie 8hrs.

Lightning strikes at 3.30am/05Oct2025. The storm exited our area and continued NE wards about an hour later having killed 23 people.

Rainfall readout from our Davis AWS at Kurseong at 4.13am on 05Oct2025 (today). Kurseong is known to have heavy rainfall and has some very large landslides there. The rainfall from the extreme event was 393mm.

Praful Rao
SaveTheHills
savethehills@gmail.com
9475033744

We will cover the casualties as well as the damage done by one day's rainfall on 04/05Oct2025 in Part II to this post.

Our adverse weather early warning work on What'sApp groups in the Darjeeling- SIkkim region: Thunderstorms of 11/12Apr2024 in Kalimpong

After the Teesta Valley GLOF of 04Oct2023, our region has been extraordinarily dry, with the erratic western disturbance activity in the winter months, robbing us of all our winter rain. In Kalimpong we had the first showers after Oct2023, between 19 & 27Mar2024 where we clocked 112.6mm. Thereafter, it has been dry with forest fires again starting in the Teesta Valley.
With regard to early warnings on adverse weather events and disbursement of information on landslides, road blocks etc, SaveTheHills has been running several very active and large What'sApp groups in the Darjeeling-Sikkim region called 'Hazard Alerts or HA' for many years now and the following cloud image on approaching adverse weather was posted in the group on 11Apr2024:

This image was posted on 'Hazard Alerts' WhatsApp group at 1.31pm on 11Apr2024 along with lightning warnings from the IMD 'Damini' (lightning warning) app. Shortly after this was done, the first storm hit our region at around 2.30pm. Wind speeds in gusts reached upto 58kmph.

This storm died down momentarily at around 4.30pm only to pick up again quickly. The following cloud image along with NOWCASTs from IMD were posted to members in the 'Hazard Alerts' What'sApp groups.

Warning posted at 5.36pm to our WhatsApp groups alerting them about another storm which was approaching.

Subsequent cloud images which show the progress of the storm from 6.45pm to 10.45pm on 11April2024

The thunderstorm which hit Kalimpong at 2.30pm on 11April2024, raged right thru the night and only ceased at around 8.00am on 12Apil2024. Surprisingly, we had only 2mm of rain however strong, gusty, westerly winds from Nepal along with lightning prevailed for more than 12hrs. The graphical analysis of the windspeed from our Davis Vantage Pro AWS is placed below:

Though there were no casualties or damage from the storm, what was unusual was its duration considering there was no major weather system (such as a depression/cyclone) near by.
Also what is possible today is to get reliable and accurate information of such weather events in advance and to disburse the same quickly to a large number of people. We have been doing this regularly in WhatsApp since Jun 2016.

Praful Rao
Kalimpong district
Darjeeling-Sikkim Himalaya

Rainfall of 26/27Jun2023 -106mm in Kalimpong over 24hours

 Convective buildup is seen west of Kalimpong in the IR satellite image at 8.15pm on 26Jun2023. Cloud movement is approx west to east.
It started raining heavily at around 7 pm on 26Jun and continued ON and OFF thru the night. Late at  night or in the early morning, there was lightning and thunder heard for sometime.
It rained thru the night in Kalimpong and its vicinity:
Darjeeling: 52.4mm
Gangtok:18.2mm
The rainfall seems to be totally localized affair - with Kalimpong bearing the brunt of it.
Bagrakot, Jalpaiguri, Cooch Behar which lie in the Dooars east of Kalimpong received only a  trace of rainfall.

 

Rainfall persists in Kalimpong at 10.30am on 27Jun2023.

Praful Rao
Kalimpong district,
Darjeeling-Sikkim Himalaya

The sudden and heavy downpour of 17Jun2023 in Kalimpong.

Kalimpong experienced its first really heavy, showers in a thunderstorm which lasted for almost 90min from 7.00pm on 17Jun2023.
I checked the IR imagery and lightning at 5.30pm on 17Jun with a group of IIT (Mandi) interns and mentioned to them we had no lightning nor convective clouds in our vicinity. There was absolutely no indication that a severe thunderstorm was building up and the NOWCAST warnings for heavy rain also do not indicate this.

The timeline of satellite images below indicate how CB clouds developed and moved eastwards over Kalimpong from the hills of Nepal.

The rainfall intensity graph from the DAVIS AWS at my home shows that we had heavy rain (ie above 40mm/hr) for an hour at least and that we received almost 60mm of rain during this period.

Videos sent to our WhatsApp group by members show the severity of the downpour in Kalimpong on 17Jun2023 evening.         

                           

Update on 19Jun2023

My thanks to HA Members who shared the videos.

We had 101mm of rain in the last 24hrs (all of it from approx 7pm last evening till 4am on 18Jun2023)

Praful Rao
Kalimpong district
Darjeeling-Sikkim Himalaya
savethehills@gmail.com
9475033744