Little Change to Tropical Storm Mahasen; First Eastern Pacific Tropical Disturbance

Tropical Cyclone Mahasen in the Bay of Bengal continues to show little change, and the storm is running out of time to undergo rapid intensification into a dangerous major hurricane. The 10 am EDT Tuesday advisory from the Joint Typhoon Warning Center put Mahasen's top sustained winds at 50 mph, with a motion northeast at 8 mph towards Bangladesh. Satellite loops show that Mahasen has been struggling with wind shear over the past day. 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. The upper-level outflow channels have degraded some since Monday, even though wind shear has decreased to a moderate 10 - 15 knots. Ocean waters have cooled some, to 29 - 30°C (84 - 86°F). The warm ocean waters extend to great depth, though, 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 05:35 UTC Tuesday May 14, 2013. At the time, Mahasen was a tropical storm with 50 mph winds. Wind shear has caused the low-level center to become exposed to view on the east side, with a clump of heavy thunderstorms to the west of the center. Image credit: NASA.


Figure 2. Storm-total rainfall from Tropical Cyclone Mahasen as predicted by the 00 UTC May 14, 2013 run of the HWRF model. Rainfall amounts of 3 - 7" 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. Image credit: India Meteorological Department.

Forecast for Mahasen
The most favorable conditions for Mahasen to intensify would appear to be today, as the storm will be moving over cooler waters with a much reduced total heat content beginning about 06 UTC on Wednesday. Wind shear is expected to remain in the moderate range through Wednesday, so there is still time for Mahasen to reach Category 1 strength, as the official JTWC forecast calls for. There is a lot of hot, dry air to the storm's northwest over India, though, and if this dry air gets wrapped into Mahasen's circulation, it could put the brakes on intensification. Landfall is expected to occur in Bangladesh near the Myanmar border on Thursday near 18 UTC, and wind shear is predicted to increase on Thursday, so the opportunity to Mahasen to undergo rapid intensification into a major Category 3 or stronger hurricane is growing short. I give a 10% chance that Mahasen will undergo rapid intensification to a Category 3 or stronger storm before landfall. There is a precedent for this sort of event: in October 2010, Tropical Cyclone Giri, which was approaching the coast of Myanmar near Bangladesh, underwent explosive deepening from a 60 mph tropical storm to a top-end Category 4 hurricane with 155 mph winds in just 30 hours. Even a Category 1 storm has the potential to bring a destructive storm surge to the coast of Bangladesh, where the ocean bottom is shallow and allows storm surges to pile up to great depths. Heavy rainfall will present a major flooding threat regardless of the storm's final intensity at landfall. The 00Z May 14 run of the HWRF model predicts that the Mahasen will dump a significant area of heavy rains of 3 -7" over Maynmar and Bangladesh. The storm surge, high winds, and heavy rains of Mahasen are a particular concern for the 140,000 Myanmar refugees living near the coast in makeshift camps. Evacuations of 70,000 of these refugees has been recommended, and a boat carrying refugees capsized on Monday, killing eight and leaving 50 missing.


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

Weather Whiplash in the Midwest: a 70° warm-up in just one day
It's been a "Topsy-Turvy Temperature Regime for U.S. this May" says underground's weather historian Christopher C. Burt in his latest post. After record May cold and snows hit the Great Lakes over the weekend, a ferocious May heat wave is in full bloom today. Several cities are poised to experience their greatest 1-day May temperature swing on record today. Chicago bottomed out at 36° on Monday morning, and this afternoon's high is predicted to be 88°--a spectacular 52° change in temperature in just one day. The all-time record for a one-day warm-up in the Windy City during May is 50°, set May 1, 1992. A 50°+ temperature swing is also expected in Minneapolis, where the high today is predicted to be 94°, coming on the heels of a 41° low Monday morning. The most dramatic "Weather Whiplash", though, came in Aberdeen, South Dakota, where the mercury hit 22° on May 12, then shot up to 92° on May 13--an astonishing 70° rise in just one day!

Alvin coming? First "Invest" of the year forms in the Eastern Pacific
The official start of hurricane season in the Eastern Pacific is Wednesday, May 15, and it looks like Mother Nature is playing along with this idea, with the appearance yesterday of the year's first "Invest" (Invest 90E) in the Eastern Pacific. Invest 90E is moving west-northwest into the Central Pacific, and is not a threat to any land areas. Satellite loops show a modest area of disorganized heavy thunderstorms. Wind shear is a low 5 -10 knots, and is predicted to remain low for the next four days. Ocean temperatures are a warm 29 - 30°C, and I give a 30% chance that 90E will become a tropical depression by Thursday morning, as predicted by the GFS model. If the system reaches tropical storm strength, it would be called Alvin.

What is an "Invest"?
When a National Hurricane Center forecaster sees a tropical disturbance that may be a threat to develop into a tropical depression, the forecaster may label the disturbance an "Invest" and give it a tracking identification number. There is no formal definition of what qualifies as an "Invest". Declaring an "Invest" is merely done so that a set of forecasting aids like computer model track forecasts can be generated for the disturbance. The "Invest" is given a number 90-99, followed by a single letter corresponding to the ocean basin--"L" for the Atlantic, or "E" for the Eastern Pacific. Other warning agencies assign "Invests" for the other ocean basins--"W" for the Western Pacific, "A" for the Arabian Sea, etc. Detailed microwave and traditional satellite images are available for all "Invests" across the globe at the Navy Research Lab web site.

Jeff Masters