Bengal Convective Outlooks
March-May 2006
Jonathan D. Finch

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    The purpose of this website is to forecast high-end severe storm episodes in Bangladesh and East India. High-end
    severe weather (1.75 inch or larger hail and destructive tornadoes) is usually produced by supercell thunderstorms,
    so supercell storms will be the main focus of these forecasts.

    Forecasts will normally be updated between 3 UTC and 6 UTC(0900-1200 BST) since surface based convective initiation
    is generally between 9 and 11 UTC. These are updated as needed through initiation time.  In particularly active periods
    I may update several times per day. If violent tornadoes are expected, I will try to issue tornado watch boxes before events
    unfold--time permitting.



    Surface maps, upper air charts and soundings may be posted whenever possible during active periods and time permitting.
    Please note that such maps can be easily generated with Digital Atmosphere. This is a fantastic graphical program that
    retrieves meteorological data from the internet and displays these data on customizable maps for any place on the globe
    where data is availale.  This program ingests data straight from the internet from sites listed on my links page such as
    College of Dupage or  Albany.


                                                          2006 Convective Outlooks
 

Severe weather alert!!  

updated 0520 UTC April 02  2006

Violent tornadoes, extremely large  hail and convective high wind events are possible in
the April 2 to April 5 time frame across Bangladesh and adjacent east India.


    
Apr 02   2006    Day 2     moderate risk of high-end severe storms
    Apr 03   2006    Day 3     high risk of high-end severe storms
    
Apr 04   2006    Day 4     moderate risk of high-end severe storms

Discussion: updated 0520 UTC April 02  2006

    The latest surface observations from Kurtimola showed 90F/76F with ssw wind at 12mph. Notcurnal storms
    rolled across Meghalaya, India near Dhubri and Cherrapunji all night, undoubtedly leaving an outflow boundary
    somewhere across northern Bangladesh. The jet streak that the models have been advertising for a week now
    is approaching western India and is not quite close enough to impact Bengal today. However, adequate mid to
    high level flow is in place for severe storms to develop. Unfortunately Dhaka is the only surface observation
    available for the last 3 days in Bangladesh and this makes forecasting very difficult. Hopefully the MODIS
    polar orbitor will make a close pass early this afternoon. 

    With 500 and 300mb temps around -13C, -40C across Bangladesh, the surface based lifted index at Dhaka
    was -14(500) and -18(300) at 11 am local time. Needless to say, explosive instability was in place. The main
    question is whether the low level shear will be adequate for violent tornadoes. The high level flow was favorable
    with 75 to 90kt flow over Calcutta.

Discussion: updated 0500 UTC March 30  2006

    The Bengal severe weather season may begin by April 2. I recently performed a composite reanalysis of 62
    Bengal tornado cases. The synoptic situation for this coming April 2-5 is strikingly similar to this reanalysis.
    Surface pressures lower across north India over the next few days. This pressure trough will deepen up to
    850mb and allow for deep low level moisture to surge north into central and eastern Bangladesh.
    A jet streak will approach by the April 4 and 5 time period.  The ECMWF shows 850mb dewpoints
    rising to around 16C with the nose of the high level jet impinging on Bengal. This is a great setup for severe
    and possibly tornadic storms.

    My composite reanalysis for 62 Bengal tornado cases can be found here. Note that tornadoes tend to
    occur when the exit region of the high level jet is over Bengal.

    The following are the 00hr to 168 hr ECMWF model graphics for 200mb, 500mb and SLP/850mb dewpoints
    and winds. Notice the surface pressure gradient increasing over Bangladesh and the Bay of Bengal by days 5 to 7.
    Also note that Bangladesh is near the nose of a jet streak by then. So the flow at all levels will be increasing, along
    with ample low level moisture. This will set the stage for severe storms. I will be looking  for the the annual return
    of nocturnal thunderstorm clusters over the     Khasi Hills of Meghalaya India near  Cherrapunji, IN. Once
    this occurs, outflow boundaries will be present across northern or central Bangladesh on many days in April.
    Note that by day 6 and 7, the 500mb heights and slp contours are perpindicular. The 500mb winds will be
    increasing to around 50kts.


ECMWF day 5 200mb and 500mb from 12 UTC March 29 and valid at 12 UTC April 03  2006
ECMWF day 7 200mb and 500mb from 12 UTC March 29 and valid at 12 UTC April 05  2006

GFS 114hr 500mb hgt and sfc wind from 18 UTC March 29 and valid at 12 UTC April 03  2006
GFS 138hr 500mb hgt and sfc wind from 18 UTC March 29 and valid at 12 UTC April 04  2006
GFS 162hr 500mb hgt and sfc wind from 18 UTC March 29 and valid at 12 UTC April 05  2006

ECMWF day 5 SLP and 500hgt from 12 UTC March 29 and valid at 12 UTC April 03  2006
ECMWF day 6 SLP and 500hgt from 12 UTC March 29 and valid at 12 UTC April 04  2006
ECMWF day 7 SLP and 500hgt from 12 UTC March 29 and valid at 12 UTC April 05  2006

  
      Climatology

    The climatological tornado peak is April 10-14. There is a sharp decline in  violent tornadoes after April 19. There have
    been a few notable tornadoes in May. However, 2 of the most violent tornadoes on record occurred outside of the peak
    period on April 26 and May 13, with 1300 and 700 fatalities respectively. The April 26th tornado path was 8 miles long
    but up to a mile wide. Interestingly, the violent tornado on March 20, 2005 that killed 65 people was very early. Very
    few violent tornadoes have occurred so early in the spring. The exceptions were March 19, 1961 when over 200
    people were killed by a single tornado and March 13, 1953 when over 20 were killed. Only 2 violent  tornadoes have
    occurred after May 13.

    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.

       


                       Severe storms or tornado hits Bangladesh killing over 20

    Over 20 people have been killed in a tornado or severe thunderstorm on May 17, 2005.
 


                                            Severe storms hit central Bangladesh
 

    Bangladeshi picking up hail stones on April 25, 2005. This picture appeared in the "Daily Sangram".



 

                                         Tornado hits northern Bangladesh killing 65

    A tornado killed at least 65 people in northern Bangladesh on Sunday March 20, 2005. This tornado hit Sadullahpur and Sudarganj
    upazilas of Gaibandha and Mithapukur upazila of Rangpur.  I am currently preparing a case study with surface and upper air charts.

    On March 18, before leaving town on a 2 day trip, I issued a slight risk for March 19 and the first moderate risk of the year
    for March 20. Unfortunately, I apparently overwrote this file by mistake since I cannot find the March 20 forecast showing the
    moderate risk.



                                           Storms kill 7

    High winds and lightning killed 7 people in scattered locations across Bangladesh on March 30, 2005.


                     Thunderstorm winds and lightning kill 20
 

    Storms developed on March 23, 2005 in a tornado watch. At least 16 people were killed across Bangladesh. The deaths were
    scattered in nature and were the result of strong straight line winds and lightning. Lightning killed 4 people working in a field.
    An outflow boundary can be seen on the 06 UTC  Mar 23 surface map. Surface dewpoints south of this boundary were in the
    mid to upper 70s(24 to 25C) and surface based lifted indices ranged from -9 to -12 along and south of this boundary. Some of
    these deaths may have been from storms on March 22.

    A jet streak was approaching Bengal as seen from the UKMET 250mb initialization. The 500mb chart at 12 UTC March 23
    showed a shortwave trough over western India. I analyzed this map despite the very poor data quality over India, and the paucity
    of upstream data. The large view and small view  12UTC March 23 UKMET initialization 500mb maps are also available.



                               Lightning kills 8 in India

    8 people were killed by lightning in 4 separate cases in India over the past few days(March 19-23, 2005).
 



                                                  Storm kills 2 in Meghalaya

    A late evening thunderstorm on Marh 19, 2005 killed a couple on the western end of the Khasi Hills in the Meghalaya state of India.


    Morning(6am BST) soundings from Dhaka--good  job BMD(Bangladesh Meteorological Department). These are
    high quality soundings(especially the T/Td data)--a dramatic improvement from years past. These are from April 2004.

    14th15th16th17th



                                            Killer tornado on April 14  2004

    A tornado hit in northern Bangladesh on April 14, 2004, killing 75 people.



                                            Killer tornado on May 4  2003

    Click here for details about the tornado in far eastern Bangladesh on May 4, 2003.