Severe Cyclone Sidr is a potentially catastrophic tropical storm in the Bay of Bengal that will likely make landfall at the India/Bangladesh border in about 24 hours. At 14/18Z (1800 GMT on the 14th --it is 15/0130Z at this writing) it was packing 130 kt maximum winds (that's 149 mph, or a high end Category 4 on the Saffir Simpson scale). The northern end of the Bay of Bengal is basically a funnel, and water has nowhere to go but up the Ganges delta. The last time a storm of this magnitude hit here, over 130000 lost their lives. Even that pales compared to the terrible Bhola cyclone of 1970, which also hit the area and took at least 300000 lives and likely upwards of 500000.
This is an update to my previous posting here and FishOutOfWater's here.
I'm feeling a lot more pessimistic about this than I was yesterday. The most dangerous sector of a tropical cyclone is the right front quadrant. Here the wind speeds add to the storm speed, this is where the greatest number of tornadoes are found, and this is where storm surge is highest. The storm track puts the Ganges delta right in the danger zone.
(Image from the NRL Tropical cyclone page)
This storm is expected to weaken before landfall, although it is expected to weaken less than expected earlier (it is also currently stronger than expected). Sea surface temperatures remain high, so only high wind shear could slow Sidr down, and there is expected to be higher shear as it approaches the coast (figures below are from the U. of Wisconsin CIMSS page)
At 00Z it was under 25 kts shear -- basically nothing for a storm like this. It will not run into 50 kts until it makes landfall. But this high shear zone keeps backing up farther and farther inland:
This is the 24-hr shear tendency, and it is clear that the high shear that was earlier expected to weaken Sidr has fallen off dramatically in the northern Bay of Bengal.
I would be surprised if they dropped sondes in the cyclone (such as is routinely done for Atlantic hurricanes) so the 130 kt wind is an estimate. Some analyses have it at 140 knots. There are two things going for it: it is not terribly large, at 430 km diameter (gale force winds), and it is moving slowly. The latter is double edged -- surge will be reduced, but Sidr will drop immense amounts of rain inland and there will be massive flooding as all of that water drains back into the sea. Also, the storm is putting an immense amount of water into motion, and that will take some time to slow down even if the storm weakens. If the storm doesn't weaken basically right now (it is still expected to have 130 kt winds at 06Z) we'll be looking at very serious surge. It is currently running 40 ft max waves -- in the open ocean. At landfall expect extremely large waves.
Bangladesh is taking it seriously, ordering massive evacuation of the coastal districts. The place is pretty much dead flat (see here for elevation), so cyclone centers are basically artificial hills or large buildings on stilts with food and water where people and livestock can take refuge. But India does not seem to be paying it much attention.