Tropical cyclone Sidr in the North Indian Ocean is currently headed for NE India and Bangladesh. It is currently running 115 knot max winds, which would qualify as borderline Cat 4 on the Saffir-Simpson scale and is expected to strengthen slightly in the next day or so. Of course one thinks immediately of the Bhola cyclone 37 years ago yesterday, which struck with 120 mph winds and killed some 300000 to 500000 people in what was then called East Pakistan, the worst natural disaster in recent history. In 1991 a cyclone packing 155 mph winds struck the same area, taking over 130000 lives.
Sidr is a well-formed storm, beautiful and potentially lethal:
(Source is the excellent CIMSS website of the University of Wisconsin). For weather weenies, the UL divergence sustaining the system at the 100-250 mbar level is quite clear. Weather weenies are also encouraged to check out the CIMSS website (with which I am not associated) -- lots of good stuff there.
Here is the projected storm track.
(Source is another excellent website, hurricanezone.net)
Not exactly ideal: if it holds the center of the storm will make landfall pretty much at the India-Bangladesh border, putting the lowest-lying floodplains between Calcutta and Chittagong right in the path of peak storm surge, most of the tornadoes (if any are associated with this system), and highest winds of the system. A very large part of Bangladesh is within 1.5m above sea level (see also here). Also bad news for Calcutta, home to 14 million or so, which might take a direct hit.
This is peak season for cyclones in this part of the world. Like all tropical cyclones, Sidr is a giant heat engine powered by latent heat release of condensing water vapor and fuelled by warm sea surface temperatures. Here's a sea-surface temperature map from weather.unisys.com:
SSTs in the Bay of Bengal are well above the ~26.5o C threshold for tropical cyclone formation. Can't blame this one on global warming: the SST anomaly map shows that the Bay of Bengal is not particularly warm (or cool) right now. It's plenty warm already, and a big storm like this comes up every couple of decades or so.
Sidr is headed into a region of pretty strong wind shear (differing wind velocities at low and high levels, tends to rip TCs apart), and if it were just developing Sidr would likely fall apart shortly. However, once well-organized, a TC can take shrug off quite a bit of wind shear. Wilma (the biggest hurricane of the annus mirabilis of 2005) took on 40-50 kts of shear without weakening. The high shear may prevent Sidr from strengthening much more, but right now it looks like it will remain a powerful system. But always keep in mind that we don't have much skill in predicting where a TC will go or how it will strengthen or weaken.
Spare a thought for Bangladesh. Hopefully the authorities will evacuate well ahead of time if necessary -- but even in that case the property damage will be immense.