Amphan to push the massive storm towards eastern India and Bangladesh
above: GeoColor image of Cyclone Amphan at 0640Z (2:40 AM EDT) Monday, May 18, 2020. (RAM/CIRA/CSU)
After a stunning bout of rapid intensification on Sunday, Tropical Cyclone Amphan poses a serious storm surge threat to the highly vulnerable coast in the upper Bay of Bengal. The Joint Typhoon Warning Center (JTWC) upgraded Amphan to the equivalent of Category 5 strength at 2 a.m. EDT Monday, with one-minute sustained winds estimated at 140 knots (160 mph). to update: As of 5 p.m. EDT Monday, the JTWC had rated Amphan’s maximum winds at 130 knots (150 mph), making it a strong Category 4 on the Saffir-Simpson scale.
On the scale used by the Indian Meteorological Department (IMD), it was Amphan promotion 3 a.m. EDT Monday to the highest possible level: a supercyclonic storm. Only a handful of storms — about one every decade — reach this level, which corresponds to a three-minute average wind speed of 120 knots (140 mph). Hurricane ratings made by the National Hurricane Center and JTWC are based on a one-minute average, which averages the wind speed of a given storm.
Amphan took advantage of very favorable conditions in the southern Bay of Bengal to consolidate strength with incredible speed over the weekend. Based on very warm sea surface temperatures of 31 °C (88 °F), high ocean heat content, and light wind shear, Amphan Withdraw From minimum tropical storm force (35 knots or 40 mph) to Category 5 equivalent strength (140 knots or 160 mph) in just 48 hours – and from minimum hurricane force (65 knots or 75 mph) To Class 5 equivalent strength in just 24 hours.
Amphan Forecast: Some strong winds will likely weaken before landfall, but the storm threat will remain serious
Steering currents are expected to take Amphan on a direct path just east of the north. On this path, Amphan is likely to make landfall somewhere between Kolkata, India, and Chittagong, Bangladesh, on Wednesday afternoon or evening local time.
Even when Amphan reached Category 5 strength, there were already signs of a potential eyewall replacement cycle. Such a process would halt the strength of Amphan and perhaps weaken it slightly, although it may also tend to expand the field of cyclonic winds. As it moves towards the northern Bay of Bengal on Tuesday local time, wind speeds will increase from around 10 knots to 20-25 knots, and winds will pump somewhat drier air into the storm.
All of these factors suggest that Amphan’s strong winds may gradually decrease in strength before it reaches shore, although perhaps not to the extent that JTWC’s 09Z on Monday predicted that Amphan will be a Category 1 storm when it makes landfall. Monday’s 00Z run of the high-resolution HWRF model, one of the best at storm intensity, indicates Amphan will maintain Category 4 strength until landfall. to update: Monday’s 15Z forecast from JTWC has Amphan approaching the coast as a Category 3 force, as shown above.
The most serious threat posed by Amphan is the potential for a catastrophic storm. Even if the strong winds in Amphan weaken, the storm threat will remain serious. Amphan is a major cyclone that actually pushes a huge amount of water north into the Bay of Bengal, affecting cyclones moving north. Any slight weakening of Amphan winds as a result of the eyewall replacement cycle will have little immediate impact on mitigating the storm threat, which has already been put into practice (literally). There is a lot of momentum in the water pushed by large, powerful storms even after they weaken, as demonstrated by 2008’s Hurricane Ike in Texas and 2012’s Hurricane Sandy in New Jersey and New York.
The IMD warns of a possible surge of up to 4-6 meters (13-20 feet) over parts of West Bengal, with 3-4 meters (10-13 feet) possible over Bangladesh.
The tragic history of storm surges in the northern Bay of Bengal
Some of the most destructive and deadly cyclones in world history hit the northern Bay of Bengal. The cyclone of 1876 brought the highest known storm wave to Bangladesh – 13.0 meters (43 ft). The powerful hurricane killed an estimated 200,000 people. The deadliest storm in world history, the 1970 Bhola Cyclone, killed an estimated 300,000–500,000 people when it made landfall in Bangladesh along the mouth of the Meghna River near Bhola Island on November 12, 1970. The cyclone brought a storm surge estimated at 10.4 metres. (34 feet) to the coast.
Just last year, the Bay of Bengal experienced a severe cyclone in May: Category 4 Tropical Cyclone Fani, which made landfall in eastern India in the state of Odisha on May 2 with sustained winds of 155 mph. Fani killed 89 people and caused $8.1 billion in damage in India and Bangladesh, according to insurance company Aon, making it one of the five costliest Indian cyclones on record. Fanny’s landfall on the western side of the Gulf meant that the height of the storm to the right of center was lower than the landfall at the top of the Gulf.
Before Fani, the most recent major tropical cyclone to hit India was the 1999 Odisha cyclone, which struck northeastern India in the Indian state of Odisha (formerly called Orissa) near the city of Bhubaneswar as a Category 4 storm with winds of 155 mph. October 29, 1999. A powerful storm surge caused a storm surge of 26 feet (8 m) on the coast, then stalled just inland, dropping heavy rain. The disaster claimed the lives of 9,658 people and caused damage worth $2.5 billion (in 1999 dollars). It is the most expensive and fourth most dangerous tropical cyclone in India in the past 100 years.
Warning systems have improved significantly in India and Bangladesh over the 20 years since the Odisha cyclone. However, the region’s geography and high population means it remains exceptionally vulnerable, and amfAR must be taken seriously by all concerned. There is no doubt that the novel coronavirus pandemic will complicate the evacuation and sheltering process in countless ways.
Tropical Storm Arthur passes near the Outer Banks of North Carolina
A tropical storm warning remains in effect for eastern North Carolina ahead of Tropical Storm Arthur, which is on track to move east of the Outer Banks later Monday. Impacts to the North Carolina coast are supposed to be minimal, and computer models have come to a better agreement on the eastward curve that will take Arthur out to sea by Tuesday. For the latest forecasts, see this frequently updated Weather.com article.
Dr. Jeff Masters contributed to this post.