Bengal Convective Outlooks
March-May 2007
Jonathan D. Finch

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Assessing Instability on the Front Range Without Upper Air Data
Potential Temperature and Mixing Ratio--Contributions to CAPE on Elevated Terrain

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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 available.  This program ingests data straight from the internet from sites listed on my links page such as
    College of Dupage or  Albany.

                                                          2007 Convective Outlooks

Radar archive

updated 2000 UTC  May 20  2007

The high-end severe weather season for Bangladesh and East India is over.  Convective outlooks will resume
in March 2008.


Storms did develop on April 10, but the environment was not conducive for tornadic storms.  



updated 0400 UTC  April 9  2007 (11 am BST April 9)

Day 1: April 09    moderate risk of high end severe storms
Day 2: April 10    moderate risk of high end severe storms
Day 3: April 11    slight risk of high end severe storms
Day 4-9 April 12 - April 17  no risk of high end severe storms

A thunderstorm developed near Jangipur, India(nw of BRP on the Caclutta radar) around 09 UTC April 8 or 3 pm
BST. This storm moved mainly east and was located near Natore, Bangladesh at 1148 UTC. Then the storm moved
east-southeast  about 20 mph to near Manikganj by 1448 UTC and still in progress. The base velocity image is here.
Another storm developed in northern Bangladesh after sunset. Also, storms have lined up along the Khasi Hills after dark
as is typical during stretches of active weather. Its is typical for Cherrapunji to get inundated by storms all  night in such
periods. This is part of the reason they receive 400 inches of rain per year.

Latest sfc map : April 8  2007  12 UTC   6 pm BST(past peak heating)

updated 1554 UTC April 08  2007 (954 pm BST April 8)

Day 1: April 08    moderate risk of high end severe storms
Day 2: April 09    moderate risk of high end severe storms
Day 3: April 10    moderate risk of high end severe storms
Day 4: April 11    slight risk of high end severe storms
Days 5-9: April 12-16 no risk of high end severe storms

Conditions will be ripe for high-end severe storms over the next few days. 500mb winds will incease to
40-50 kts on April 8, April 9 and April 10. Low level flow will be enhanced from the Bay of Bengal into
Bangladesh on April 8, April 9 and April 10. Also, the high level jet will be increasing to 100kts from
April 8 to April 9 to April 10. 500mb temps will be reasonably cold.  700mb winds will also be
fairly strong on April 8, April 9 and April 10 with 30-45 kts across northern Bangladesh.  Also, nocturnal
storms on the Khasi Hills near Cherrapunji and across northern Bangladesh will help maintain a low-
level baroclinic zone across northern Bangladesh.

April 9 and 10 look really stormy. But things will improve after that and tornado weather should be absent
through the 17th. Sometimes, like in 2003, another bad week can emerge in late April or May. But perhaps
this will be the worst week mother nature has to offer this year. We will see.

Severe storms injury many and kill a few on April 7


Severe storms injury many and kill a few on April 5 and 6

Day 1: April 07    moderate risk of high end severe storms (storms in progress)
Day 2: April 08    moderate risk of high end severe storms
Day 3: April 09    moderate risk of high end severe storms


updated 1539 UTC April 07  2007 (939 pm BDT April 7)

    Storms erupted in northern Bangladesh near sunset on April 6. By 1454 UTC, several storms were detected on
    Calcutta radar. These storms were about 300 to 350 km(180 to 220miles) northeast of Calcutta. The northernmost
    storms were just south of the Khasi Hills. It's too bad the USA lets politics, and the paranoia of a few, stand in the way
    of getting a .2 deg elevation doppler slice. 

updated 1630 UTC April 06  2007 (1030 pm BDT April 6)

    Storms erupted in northern Bangladesh near sunset on April 6. By 1454 UTC, several storms were detected on
    Calcutta radar. These storms were about 260 to 400 km(160 to 250miles) northeast of Calcutta. It's too bad the
    USA lets politics, and the paranoia of a few, stand in the way of getting a .2 deg elevation doppler slice. Storms
    were still ongoing as of 1754 Z(1154 pm BDT)..

   

updated 2330 UTC April 05  2007 (530 am BDT April 6)

Day 1: April 06    moderate risk of high end severe storms
Day 2: April 07    moderate risk of high end severe storms
Day 3: April 08    moderate risk of high end severe storms

    Discussion:

    **Dangerous weather will continue through April 10 or April 11.**

    The low to mid level flow is increasing across Bangladesh.  The high level jet will persist across central and
    southern Bangladesh on April 6 and April 7 and even backbuild across the area by April 8. Low level inflow
    from the Bay of Bengal will increase from April 6 to April 7 and April 8. The 500mb pattern will remain
    favorable for severe weather with windspeeds increasing from April 6 to April 7 to April 8.

    The ECMWF model indicates that the mid to high level flow will weaken dramatically across Bengal by April 11.
   
    The ECMWF  helped me identify the chance for severe weather several days in advance. But once the events are
    imminent, the UKMET model is superior and is used for most of my outlooks. Actually the UKMET  was
    exhibited superior performance over the NCEP models in last weeks tornado outbreak in the plains. 
   


updated 2210 UTC April 05  2007

   
Thunderstorms developed across western Bangladesh near the dryline on April 5. Other storms were noted
    later in the evening further east. These storms 
were probably severe. 




updated 2050 UTC April 03  2007

Day 1: April 04    no risk of high end severe storms
Day 2: April 05    no risk of high end severe storms
Day 3: April 06    moderate risk of high end severe storms





updated 2020 UTC April 01  2007

    ***Severe weather is possible across Bangladesh and far east India for the April 5 to April 12 period***

    The UKMET shows the high level jet settling southward across Bangladesh. By 72 hrs the high level jet will be located
    across southern Bangladesh and the northern Bay of Bengal. Low level moisture will surge  back from the Bay of Bengal.
    At 72 hours the UKMET shows the dryline near its climatological favored position across far east India with decent low
    level flow from the Bay of Bengal across Bangladesh. High level temperatures will be cold with the jet situated to the south.
    In fact the 300mb thermal gradient will be just south of Bengal. This is a favorable setup for extreme instability. The 500mb
    winds are progged to be between 30 and 40kts with 30kts at 700mb. This is sufficient for supercell storms with possible
    tornadoes. The UKMET is forecasting a convective cluster near the Khasi Kills the night before. So there could be an
    outflow boundary setting up across northern Bangladesh.
  


                                                            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.