Ik heb wel eens gelezen dat die regio (Bangladesh + aangrenzende regio) de hoogste concentratie tornado's ter wereld heeft. De meest dodelijke tornado's ter wereld komen daar voor (300+ doden is geen uitzondering). Ik vraag me dus af of dit geen doden zijn die veroorzaakt zijn door een tornado producerende onweersbui in plaats van een 'gewone onweersbui'.
Het tornadoseizoen duurt echter maar kort, doordat de jaarlijkse verschuiving van stromingspatronen (moesson) lokaal tijdelijk extreem gunstige windschering veroorzaakt door de aanwezigheid van de Himalaya's. Afgezien daarvan is de Golf van Bengalen qua vocht en warmte natuurlijk de grote broer van de Golf van Mexico. Ik heb wel eens dauwpunten van 29-31 graden celcius voorbij zien komen in de regio deze tijd van het jaar.
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Although East India and Bangladesh are positioned far to the south(21-26N), the regional topography of south Asia makes Bengal more susceptible to violent thunderstorms. The mid-upper tropospheric flow splits around the Tibetan Plateau, allowing the westerlies to be stronger than the zonal average at 24N. Occasionally in the spring, strong westerlies roar around the Plateau, with a 500mb maximum from New Delhi to Lucknow to Patna to northern and central Bangladesh. Mid-upper level cold pools are steered around the plateau across north India. North India is extremely dry in the winter and spring and a deep mixed layer from the sfc to above 600mb develops south of the Ganges river valley. Daily low-level downslope flow off the mountains of Afghanistan and Pakistan leads to desert conditions during the spring with surface high temperatures ranging from 95-104F on the higher terrain of central India(1500-3000ft) from Bhopal to Ranchi, to 105-120F at low elevations just west of the surface dryline, along with negligible rainfall. When mid-upper level cold pools move across north India, extreme lapse rates are the result. A lee surface low is a daily occurrence on the eastern edge of the 1500-3000ft escarpment around 23-25N and 87E. On some days this low is further south near Jamshedpur while on other days the low is near the base of the Himalayas. North of the low, easterly flow transports moisture westward along the Ganges river sometimes as far west as New Delhi. An excellent example of this is March 20, 1968 when 20 people were killed by a hailstorm in northern India. To the east of this low pressure area, the low-level flow is out of the south starting in March, bringing moisture laden air from the Bay of Bengal. The sea surface temperatures in the Bay of Bengal remain in the 80-85F range year-round. The southerly flow generally accelerates from March into May. This moisture underruns high lapse rates, leading to explosive instability with classic loaded-gun type soundings. Occasionally the mid-level flow becomes strong with 700mb winds from 40 to 50 kts and 500mb winds from 40 to 70kts. Most tornado events in the Bengal region occur with at 50-100kt jet-level winds, 40kt west to west-northwest winds at 700mb, 40-60kt west to west northwest winds at 500mb, along with south to south-southwest flow in the boundary layer and south or east surface winds. There are a few notable exceptions when disastrous tornados occurred with very weak mid-level flow (4-11-64 and 4-13-70). Although the sfc winds increase from March into May, the upper level winds decrease during this time, resulting in a maximum in vertical wind shear in early to mid-April. Surface based capes generally range from 4000-8000j/kg on tornado days and can reach as high as 9000j/kg. Intense thunderstorm activity is almost a nightly occurrence on the Khasi Hills near and around Cherrapunji, and across northeast Bangladesh as the low-level jet impinges on the higher terrain, often resulting in an outflow boundary across northern or central BD. The surface dryline is usually positioned in far east India or western Bangladesh in the spring, and often stretches from northeast to southwest from Rajshahi, BD to Nabadwip, IN to Jhargram, IN to Baripada, IN. Occasionally the dryline pushes east of Calcutta. The sea breeze front along the coast of India from near Balasore to Bhubaneswar is also an important player. A simplified version of the typical, large scale, severe weather pattern for the Bengal region can be found here.
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