Bengal Convective Outlooks
Jonathan D. Finch
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
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Forecasts will normally
be updated between 3 UTC and 6 UTC(0900-1200 BST) since surface based convective
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
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
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,
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.
Bangladeshi picking up hail
stones on April 25, 2005. This picture appeared in the "Daily Sangram".
Tornado hits northern Bangladesh killing 65
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
for March 20. Unfortunately, I apparently overwrote this file by mistake since I cannot find the March 20 forecast showing the
winds and lightning killed 7 people
in scattered locations across Bangladesh on March 30, 2005.
Thunderstorm winds and lightning kill
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.
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.
8 people were killed by lightning in 4 separate cases in India over the past few days(March 19-23, 2005).
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.
hit in northern Bangladesh on April 14, 2004, killing 75 people.
for details about the tornado in far eastern Bangladesh on May 4, 2003.