Bengal Convective Outlooks
March-May 2008
Jonathan D. Finch

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Main Page
Elevated Mixed Layer
Special Cases for the United States
Bengal Tornadoes--background information
Historical Tornado Tracks for East India and Bangladesh
Meteorological Charts for Historical Tornado Cases for Bengal
Latitudinal Comparison of the Geostrophic Wind Approximation
Assessing Instability on the Front Range Without Upper Air Data
Potential Temperature and Mixing Ratio--Contributions to CAPE on Elevated Terrain

    The products on this website are provided on a voluntary basis. The forecaster(author of this website) is not
    responsible for the correctness or availability of products on this website.



    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.

                                                          2008 Convective Outlooks

updated 1945 UTC  Apr 11  2008

Days  1      April 12             no risk of high end severe storms
Day   2      April 13             slight risk of high end severe storms
Day   3      April 14             moderate risk of high end severe storms
Days  4-5  April 15-16       no risk of high end severe storms

I am traveling to Peru on April 12 to bring my fiancee back to the United States. I will be back on April 16. Forecasts will
not be updated
until then.

Discussion:

The Bangladesh tornado season is at it's climatological peak (April 10-14). A jet streak centered near 28N, 55E(southern Iran)
will propagate eastward and approach the Bengal region on April 13 and 14. In a personal conversation with Edward Berry--
Science Operations Officer at WFO Dodge City, the source of this jet is tropical convection over Africa. He pointed me to
this water vapor loop which shows a jet streak propagating away from equatorial east Africa and toward southern Asia.
Northern Bangladesh will be in the exit region of this jet. As the jet approaches, a southerly low level jet will develop over
Bangladesh. 925mb dewpoints will rise to over 20C. The winds at all levels will increase. The vertical wind shear profile will
become much more favorable for severe storms with 20-25kt winds at 925mb from the south and 35-40kt 700mb winds from
the west. High level winds should reach 50-60 kts in the exit region of the jet. Severe storms will be possible on April 13 and
likely on April 14.

The following are loops from the 00 UTC April 11 run of the ECMWF. These loops are upside down unfortunately. I did not have
time to rotate the images. The locations of Dhaka and Calcutta are shown.

925 dewpoints/winds and 700mb winds overlaid    location of Dhaka(VGTJ) and Calcutta(VECC) are shown
500mb heights and 250mb winds overlaid  location of Dhaka (VGTJ) shown




updated 0900 UTC  Mar 26  2008

Discussion:

The high resolution ECMWF is available this year for the first time. The 00 UTC March 26 run of the ECMWF is showing a pattern
favorable for high-end severe storms across the Bengal region. 

The following are loops of the ECMWF model out to 00 UTC April 2. The dates are inverted and at the top of the images. These
images were made with very limited time. Better loops will be prepared in later outlooks. The locations of Calcutta(VECC) and
Dhaka(VGTJ) are plotted on the map upside down.

250mb winds and 500mb temperature
250mb winds and 300mb temperature
925mb wind and dewpoint

At 00 UTC March 26, 2008, a strong jet was roaring across the southern Tibetan plateau. South of this jet axis over the Bengal region,
500-300mb temmperatures were very balmy(-6C and -32C respectively). Ovet the next few days, this jet will sag south while progressing
east. The bengal region will be in the right rear quadrant of this jet by 12 UTC March 29. Strong 500mb to 300mb cooling will take place
along with increase low level flow from the Bay of Bengal. This will lead to increasing insatbility and shear across the Bengal region.



                                                            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.