Mahasen not Strengthening; Eastern Pacific's First Tropical Depression Forming

Posted by: Dr. Jeff Masters, 1:34 PM GMT on May 15, 2013

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Tropical Cyclone Mahasen in the Bay of Bengal continues to show little change as it steams north-northeastward at 14 mph towards the Bangladesh coast just north of the border with Myanmar. Though the storm is no longer expected to reach hurricane strength, Mahasen's storm surge and heavy rains represent a significant threat to people living in low-lying areas along the Bangladesh and Myanmar coasts. At least 70,000 people have been asked to evacuate, and a boat carrying refugees capsized on Monday, killing eight and leaving 50 missing. The ocean bottom is shallow and allows storm surges to pile up to great depths on the Bangladeshi coast, and Mahasen is expected to bring a storm surge of 3 - 5 feet (1 - 1.5 meters) to the Bangladesh coast on Thursday, according to the Regional Specialized Meteorological Center in Delhi, India. The Bangladesh Meteorological Department is warning of a 5 - 7 foot (2 meter) storm surge for their coastal districts. The greatest storm surge will occur to the right of where the center crosses the coast, in northern Myanmar. The Myanmar Department of Meteorology and Hydrology is warning of a 6 - 10 foot (2 - 3 meter) storm surge there. Accompanying the surge will be torrential rains of 3 - 7 inches that have the potential to cause dangerous flooding. At least seven people have been killed in Sri Lanka due to landslides triggered by Mahasen's heavy rains.


Figure 1. MODIS image of Tropical Cyclone Mahasen taken at 07:45 UTC Wednesday May 15, 2013. At the time, Mahasen was a tropical storm with 50 mph winds. The storm appears disorganized due to wind shear keeping the heavy thunderstorms to the west of the center. Image credit: NASA.

The 10 am EDT Wednesday advisory from the Joint Typhoon Warning Center put Mahasen's top sustained winds near 50 mph. Satellite loops show that Mahasen continues to struggle with wind shear. The low-level circulation center has become exposed to view several times, with an intense area of thunderstorms with very cold cloud tops just to the west of the center. The cloud pattern is not well-organized, with little spiral banding. Wind shear remains a moderate 10 - 15 knots, but Mahasen is now over cooler waters with a much reduced total heat content, and it appears unlikely that the storm will change much in intensity before landfall. Landfall is expected to occur near 18 UTC on Thursday.


Figure 2. Storm-total rainfall from Tropical Cyclone Mahasen as predicted by the 12 UTC May 14, 2013 run of the HWRF model. Rainfall amounts of 3 - 7" are expected along a wide swath of Bangladesh and Myanmar. Image credit: India Meteorological Department.


Figure 3. Bathymetry of the Bay of Bengal. The shallow waters of the Continental Shelf (mostly shallower than 200 meters) are shaded whitish-grey. From Kolkata, India to Cox's Bazar, Bangladesh, the ocean bottom is shallow and allows storm surges to pile up to great depths. Fifteen of the twenty deadliest tropical cyclones in world history have been Bay of Bengal storms that have hit Bangladesh, India, or Myanmar, bringing catastrophic storm surges. Image credit: geomapapp.org.

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


Figure 4. Latest satellite image of Invest 90E.

First tropical depression of the year forming in the Eastern Pacific
The official start of hurricane season in the Eastern Pacific is Wednesday, May 15 (today!), and Mother Nature is playing along with this idea. The first "Invest" of the year, Invest 90E, has become well-organized on satellite loops, and NHC has set in motion the process to name this system Tropical Depression One-E (or possibly Tropical Storm Alvin) at 11 am EDT. Wind shear is a low 5 -10 knots, and is predicted to remain low for the next five days. Ocean temperatures are a warm 29 - 30°C, and it is possible that this could become Hurricane Alvin late this week. The storm is moving west-northwest into the Central Pacific, and is not a threat to any land areas.

I'll have a new post late this morning or early this afternoon on yesterday's remarkable heat wave in the Midwest. Can you believe 106° in Iowa and 108° in Nebraska, after unprecedented May snows were falling less than two weeks ago? Unbelievable!

Jeff Masters