Bay of Bengal Tropical Storm Mahasen remains a dangerous threat

It's always a nervous time when a tropical cyclone with the potential to intensify marches through the Indian Ocean's Bay of Bengal. That's because fifteen of the twenty deadliest tropical cyclones in world history have been Bay of Bengal storms that have hit Bangladesh, India, or Myanmar. The most recent of these horrifying storms was 2008's Cyclone Nargis, which killed 146,000 people in Myanmar. The Bay of Bengal's notorious history is why hurricane forecasters are watching Tropical Cyclone Mahasen a little nervously today. Even though there has been little change to the 55 mph tropical storm over the past two days, the storm remains a potential threat to undergo rapid intensification into a dangerous major hurricane. The 11 am EDT Monday advisory from the Joint Typhoon Warning Center put Mahasen's top sustained winds at 55 mph, with a motion northwest at 11 mph towards India. Satellite loops show that Mahasen has a large area of intense thunderstorms with very cold cloud tops that reach high into the atmosphere. The cloud pattern is not well-organized, with little spiral banding. This lack of organization is also apparent on radar out of Chennai. However, the cyclone has developed a respectable upper-level outflow channel to the northwest. Wind shear has decreased to a moderate 10 - 20 knots, and is continuing to decrease. Ocean waters that are an exceptionally warm 31°C (88°F), about 1°C warmer than average for this time of year. The warm ocean waters extend to great depth, and the Tropical Cyclone Heat Potential (TCHP) is over 90 J/kg/cm^2, which is favorable for rapid intensification.


Figure 1. MODIS image of Tropical Cyclone Mahasen taken at 07:55 UTC Monday May 13, 2013. At the time, Mahasen was a tropical storm with 55 mph winds. Image credit: NASA.


Figure 2. Storm-total rainfall from Tropical Cyclone Mahasen as predicted by the 00 UTC May 13, 2013 run of the HWRF model. Rainfall amounts of 16 - 30 cm (6 - 12") are expected along a wide swath just to the right of where the storm makes landfall. Bangladesh's two largest cities, Dhaka and Chittagong, are shown. If Mahasen's track occurs farther to the left, as suggested by some models, these two cities will receive Mahasen's heaviest rains. Image credit: India Meteorological Department.

Forecast for Mahasen
The official forecast brings Mahasen to Category 1 strength before landfall occurs in Bangladesh near the Myanmar border on Thursday near 18 UTC. However, the model forecasts of Mahasen from the GFS, ECMWF, UKMET, GEM, NAVGEM, and FIM models continue to show wide disagreement on the future intensity, speed, and landfall location of the storm. It is possible that wind shear will keep the storm disorganized and below hurricane strength until landfall, as suggested by the ECMWF model. However, other model guidance, such as the 00 UTC May 13 forecast from the HWRF model, bring Mahasen to Category 2 strength by Tuesday. Mahasen is currently approaching a trough of low pressure to its northwest that is expected to recurve the storm to the northeast into Bangladesh. As the recurvature process progresses today through Tuesday, wind shear should relax to a low to moderate 5 - 15 knots, and a strong upper-level outflow channel will intensify to the storm's north, aiding intensification. There is a lot of hot, dry air to the storm's northwest over India, and if this dry air gets wrapped into Mahasen's circulation, it could put the brakes on rapid intensification, though. Considering all these factors, I give a 30% chance that Mahasen will undergo rapid intensification to a Category 3 or stronger storm by Wednesday. The storm should experience higher wind shear and less oceanic heat content in the waters beneath it in the 12 hours before landfall, which should cause some weakening. But even a weakening Category 1 storm has the potential to bring a devastating storm surge to the coast of Bangladesh, and torrential rainfall will be a major flooding threat regardless of the storm's final intensity at landfall. The 00Z May 13 run of the HWRF model predicts that the Mahasen will dump a significant area of heavy rains of 16 - 30 cm (6 - 12") over Maynmar and Bangladesh. The storm surge, high winds, and heavy rains of Mahasen are a particular concern for the thousands of Myanmar refugees living near the coast in makeshift camps, as reported by the New York Times.

MJO pulse that spawned Mahasen headed towards the Atlantic
Mahasen spun up in response to an active phase of the Madden Julian Oscillation (MJO) that has been moving through the Indian Ocean during the past week. The MJO is a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days. The strong MJO pulse coincided with a convectively coupled atmospheric Kelvin wave (CCKW), a wave of increased heat and moisture propagating along the Equator, which helped increase thunderstorm activity. The active pulse of the Madden Julian Oscillation is expected to reach the Western Caribbean (in a somewhat weakened state) May 21 - 25, and there will be a heightened chance of an early-season tropical storm forming in the Eastern Pacific and Western Caribbean during that time period.

Resources
Comparative model forecasts of Mahasen from the GFS, ECMWF, UKMET, GEM, NAVGEM, and FIM models

India Meteorological Department's tropical cyclone page

Radar out of Chennai, India

Bangladesh Meteorological Department Warning

Myanmar Dept. of Meteorology and Hydrology Warning

Jeff Masters