Bangladesh Tornado Alley
Jonathan D. Finch  



                                                        Bangladeshi picking up hail


                                                                                        Map of the region


What are the causes of the Bangladesh severe weather alley?


                           


Land-Sea distribution and topography

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

                Elevated terrain of north India is EML source region.

                            Ranchi, India
                         
                            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 level flow over north India and Bangladesh.

                        April 6  2006    12 hr UKMET  700mb winds

            
                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
                                               
                                        April 5  2006  Aqua MODIS polar orbiting shot shows outflow boundary.
                                Dryline is evident on edge of vegetation.
     


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

                                          
                        Mar 20  1968

                       
Apr 14  2004
                       

                      
            Mean position of dryline and other boundaries in early April severe weather regime.





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 extremely large hail and tornadoes 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 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 spring 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-six tornadoes were previously undocumented for a total of 89 tornadoes through 2005.


            Tornado Criteria
                    

1. Specifics such as path length, path width, sharp gradients in damage or damage intensity or description of funnel.

2. Heavy objects,  people or animals were thrown long distances.

3. Flying debris such as corrugated iron sheets caused lacerations, decapitation, or loss of limbs.

 4. The tornado caused catastrophic damage (for example, entire villages reduced to rubble and/or photos showing
tornadic damage).

5. At least 15 deaths occurred inland, that can be shown to be unrelated to tropical cyclones, single building collapse,
or capsizing of boats. The evidence of widespread straight-line winds was lacking.

6. There was a very short duration of violent winds.

     

                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
       
           
            Surface

      
            500mb


            200mb

            200mb reanalysis


April 26  1989
                       
     

            Surface


            500mb
       
            200mb

     
            200mb reanalysis






Recent severe storm outbreak

April 3  2006 severe weather outbreak

Severe storms exploded along an outflow boundary and
along the dryline.


March 31 outlook

120 hr ECMWF forecast from March 29

day 2 high risk forecast


tornado watch


00 UTC surface


09 UTC surface

0908 UTC radar image

250mb

500mb

700mb



Reanalysis Composites for all tornadoes

62 tornado cases from 1951 to 2005

        200mb wind  m/s
                        climatology
                        anomaly

        250mb wind  m/s
                       
climatology
                                       
anomaly

        500mb wind  m/s
                       
climatology
                       
anomaly

        700mb wind  m/s
                       
climatology
                       
anomaly

        500mb height
                       
anomaly

       
700mb height
                  
                        

        300mb temps
                 
      anomaly
     
       
400mb temps
                    
  anomaly      

        500mb temps
                 
      anomaly     

        700mb temps
                   

       
850mb temps


        Sea Level Pressure
                       
anomaly




Reanalysis Composites for May tornado cases
       
14 cases
       
          
        200mb wind 
m/s
                climatology
                anomaly

        250mb wind  m/s
                climatology
                
anomaly

        500mb wind  m/s
                
climatology
               
anomaly
       
        500mb height
                
climatology
               
anomaly

        300mb temp
                
climatology
               
anomaly

        500mb temp
                
climatology
               
anomaly



Tornado Forecasting -- things to keep in mind

Look for nose of high level jet over Bangladesh.

Watch out for outflow boundaries and trough/frontal positions.

The dryline/front(trough) intersection 

wind speeds at different levels favorable for tornadoes in Bangladesh:

        adequate low level flow from Bay of Bengal       
        700mb  270 to 290 at 35 to 50kts
        500mb  270 to 300 at 40 to 55 kts
        200mb  270 to 300 at 60 to 80 kts


Surface dewpoints usually range from 73F  to 81F when tornadoes occur.