Bangladesh Tornado Alley
Jonathan D. Finch  

                                                        Bangladeshi picking up hail

What are the causes of the Bangladesh severe weather alley?


An illustration of the Bangladesh/East Indian severe storm environment can be found here.

Land-Sea distribution and topography

                Elevated terrain of north India is EML source region

                            Ranchi, India

                            Albuquerque(June 7  1990)

                            Albuquerque(June 21  1960)

                The dryline separates the hot and dry EML source region and the most air over Bangladesh

                        April 14, 1969

April 14, 1986

                Tibetan Plateau enhances the mid and high level flow over south Asia

                Nocturnal storms over the Khasi Hills  near Cherrupunji leave outflow boundaries
             over northern Bangladesh. These nocturnal storms are probably caused by
             the low level jet impinging on the Khasi Hills of Meghalaya, India.
                        Average Rainfall for April
                        May 13  1996

                        Apr 14  2004

May 04  2003

            A front of sorts often stretches across central Bangladesh and into north India.

                        Apr 08  1995

Apr 14  2004

High level jet                           


The sounding

        The result is classic "loaded gun" type sounding that we see here in the plains and in South Africa

                May 22  1981  Oklahoma City

                June 02  1995   Friona, TX

                May 17  2000   North Platte

                December 19  1992    Durban, South Africa

                April 14  2004   Dhaka, Bangladesh

April 15  2004  Dhaka, Bangladesh

        Places in the world that have a pronounced severe weather season typically have elevated mixed layers.

                Sounding Comparison

The Timeline

Several factors lead to a very short but active severe weather season across Bengal.

        North and central India heats up and dries out in late March or early April. A deep, dry mixed
        layer develops.
 Low level  flow from the Bay of Bengal increases markedly during this time.

        Westerly mid-level flow around the Tibetan Plateau advects the Indian mixed layer over
        the Bengal moist tongue. This leads to the elevated mixed layer. Note that parts of the Indian
        desert are  "elevated"(1-3000ft) compared to Bangladesh which is near sea level.

        The mid level flow is still fairly strong in April with 30-50kt 700mb flow and 35 to 50 kt 500mb
        flow fairly common.   

        The high level jet is usually over or just north of the Bengal in April

        The southern branch of the polar jet often retreats north of the Tibetan Plateau by May, leaving
        light mid to
high level flow across the Bengal region.
 By June the high level flow is light.

 All these factors result in a tornado maximum in early to mid April. In short, vertical wind shear
        and instability are maximized and the jet is in a favorable position during this time.

Bangladesh and east Indian Tornado Documentation
        Previous tornado documentation 

                        Peterson and Mehta(1981 and 1995), Ono(1997), Qayyum and others
Individual tornado studies in journals

                           Peterson and Mehta (1981 and 1995) documented 36 tornadoes. (Most comprehensive)

                             Ono (1997) documented 28 spring events from 1990 to 1994 using a fairly liberal
                                set of criteria

                             Goldar(2001) documented 36 "possible tornadoes" which partially filled the gap
                                    in the 1890s and early 1900s. Again a liberal definition was used.                            

        Myself and Ashraf M. Dewan searched through old newspaper microfilm for March-May from
        the 1950s through the 1980s.  We came up with a comprehensive tornado climatology for Bangladesh
        which builds on the previous work by Peterson, Mehta and Ono.

          Thirty-three tornadoes were previously undocumented.

                All tornadoes

                30 or more deaths

                tornado paths

                tornado details

Tornado Cases 

        Surface and upper air charts for Bangladesh and east Indian tornadoes can be found here.

            April 14  2004

              March 20 2005
            May 04 2003

            Mar 23 2005    High winds and hail(possibly tornadoes)

May 13 1996



            200mb reanalysis

April 26  1989



            200mb reanalysis

Reanalysis Composites for all tornadoes

62 tornado cases from 1951 to 2005

        200mb wind  m/s

        250mb wind  m/s

        500mb wind  m/s

        700mb wind  m/s

        500mb height

700mb height

        300mb temps
400mb temps

        500mb temps

        700mb temps

850mb temps

        Sea Level Pressure

Reanalysis Composites for May tornado cases

14 cases
        200mb wind 

        250mb wind  m/s

        500mb wind  m/s
        500mb height

        300mb temp

        500mb temp

Implications for Tornado Forecasting

Look for nose of high level jet over Bangladesh

Watch out for outflow boundaries and trough/frontal positions

The dryline/front intersection is a favored area for storm initiation.

Optimal wind speeds at different levels for tornadoes:

        adequate low level flow from Bay of Bengal       
        700mb  260 to 290 at 40kts
        500mb  260 to 300 at 50 kts
        200mb  260 to 300 at 60 to 80 kts

Optimal morning sounding

April 14  2004

Surface dewpoints usually range from 74F  to 81F when tornadoes occur.