Convective Outllooks for Bangladesh and East India

Bengal Convective Outlooks
March-May 2006
Jonathan D. Finch

Links
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
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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

    Mar 28 2006    Day 1      no risk of high-end severe storms
    Mar 29 2006    Day 2    no risk of high-end severe storms
    Mar 30 2006    Day 3     no risk of high-end severe storms

Extended Forecast:

Apr 1-3 2006 Day 5-7 slight risk of high-end severe storms

Discussion:

    The Bengal severe weather season may begin by April 1. I recently performed a composite reanalysis of 62
    Bengal tornado cases. The synoptic situation for this coming April 1-3 is strikingly similar to this reanalysis.
    By days 5 and 6, surface pressures will begin to lower as a jet streak approaches north India. By day 7
    or April 3, the EC shows 850mb dewpoints around 15C in eastern Bangladesh. A majority of Bengal
    tornadoes occur in the exit region near the nose of the high level jet. This situation will be maturing by April 3
    with high-end severe storms possible.

    The following are the 00hr to 168 hr ECMWF model graphics for 200mb, 500mb and SLP/850mb dewpoint
    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.

Old forecast for April 1

The ECMWF 168 hr fcst valid 12 UTC Apr 1

Climatology

    The 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.

14th 15th 16th 17th

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.