Risk of severe thunderstorms in the Bangladesh region

[Paul Knightley]

At this time of year a strong westerly jet stream often develops to the south of the Himalayas. Low pressure across the region, especially around Bangladesh, draws extremely moist air northwards from the Bay of Bengal, whilst dry air from the higher ground to the west advects eastwards over the moist air - an 'elevated mixed layer', or EML. This creates extreme instability in the area, although it can also be strongly capped (meaning no storms). The combination of extreme instability and strong vertical wind shear mean that supercells can develop, and often produce giant hail and sometimes tornadoes, as well as strong 'straight line' winds. These charts from the ECMWF model show that the current situation has fairly strong winds aloft (right hand chart, the wind barbs show 35-40 knot westerly winds at the 500 mb level), extreme instability (CAPE values of over 4000 J/Kg), moist air (lef chart, the shading shows surface dewpoints of 23-24C) - the left chart also shows the surface flow, with the wind barbs showing northwards advection of the moist air.

A strong westerly jet is indicated to continue for the next 10 days - minor perturbations in the flow, as well as low-level forcing mechanisms which are not well understood should allow the development of supercells at times, with an attendant risk of severe weather including tornadoes.

 

[Bram]

I took a look at a radar in the eastern part of India, and there were some nice storms visible. Here are 2 animations I saved from the radar in Kolkata, which is located at a nice place in the eastern part of India. The link to this radar (and others) is this one: http://www.imd.gov.in/section/dwr/dynamic/dwr.htm.
I've also included 2 VVPs, which show a nice amount of shear.


And here's a current image, which also shows a nice storm:

 

[Paul Knightley]

Thanks, Bram.

Sadly, there were some nasty storms - http://www.skynews.com.au/news/worl...angladesh-storms-kill-35--flatten-houses.html

 

[Paul Knightley]

More severe storms are developing in the area today, mainly southern Bangladesh and eastern India, near Kolkata. On the radar site posted above by Bram there are a couple of storms which look like supercells - one showed evidence of splitting as it migrated off the higher ground to the west. Current obs show air temperatures around 32C in the area, with dewpoints of 23-26C. Surface winds are easterly immediately ahead of the storms. The Dhaka sounding for 00Z showed a cap at the time but with around 2200J/Kg of CAPE - modifying this sounding shows a sizeable area of SBCAPE.

 

[Isaac Pato]

I made some EHI maps using GFS data with IDV. Unfortunately it appears that the ucar data server has been down for the last day or so.

 

[Paul Knightley]

Several clusters of storms have formed today in eastern India, close to the border with southern Bangladesh. Radar suggests supercell behaviour, with splits and mergers taking place.

 

[Bram]

And here's an animation from today. Unfortunately, there is no data from after 13.54z available.

 

[Bram]

And today was another day with strong convection in eastern India and Bangladesh. Initially discrete storms, with several cell-splits, growed rapidly upscale into a MCS. I've included 3 animations, with different zoom-levels. At the last one you can see a nice example of a cell-split, after which the rightmover exhibited a classical supercell signature for some time, until it was absorbed by the MCS. Unfortunately, there was no data from between 12.35z and 13.57z.



And here are 2 VVP's: