Tropical Cyclone Amphan Intensifies Into Equivalent of Cat. 5 Hurricane

Tropical Cyclone Amphan intensified into equivalent of a Category 5 hurricane on the Saffir-Simpson Scale over the Bay of Bengal on Monday.  At 5:00 a.m. EDT on Monday the center of Tropical Cyclone Amphan was located at latitude 13.9°N and longitude 86.5°E which put it about 635 miles (1025 km) south-southwest of Kolkata, India.  Amphan was moving toward the north at 7 m.p.h. (11 km/h).  The maximum sustained wind speed was 160 m.p.h. (260 km/h) and there were wind gusts to 195 m.p.h. (315 km/h).  The minimum surface pressure was 909 mb.

The circulation around Tropical Cyclone Amphan appeared to develop a concentric eyewall structure on Monday.  The inner end of a rainband wrapped around the original small eye and eyewall.  The inner eye had a diameter of 10 miles (16 km).  A ring of strong thunderstorms surrounded the inner eye and the strongest winds were occurring in that ring of storms.  An outer eyewall with a diameter of 50 mile (80 km) surrounded the inner eye.    Bands of showers and thunderstorms were revolving around the concentric eyewalls.  Storms in the core of Amphan were generating strong upper level divergence which was pumping mass away from the tropical cyclone in all directions.

Tropical Cyclone Amphan had a large circulation.  Winds to hurricane/typhoon force extended out 60 miles (95 km) from the center of circulation.  Winds to tropical storm force extended out 200 miles (325 km) from the center.  The Hurricane Intensity Index (HII) for Tropical Cyclone Amphan was 35.0.  The Hurricane Size Index (HSI) was 16.5 and the Hurricane Wind Intensity Size Index (HWISI0 was 51.5.  Tropical Cyclone Amphan was capable of causing catastrophic damage.

Tropical Cyclone Amphan will move through an environment very favorable for a strong tropical cyclone during the next 36 hours.  Amphan will move over water where the Sea Surface Temperature is near 31°C.  It will move under an upper level ridge where the upper level winds are weak.  There will be little vertical wind shear.  The eyewall replacement cycle will stop the intensification of Tropical Cyclone Amphan.  Amphan will weaken, at least temporarily, when the inner eyewall dissipates.  When that occurs the strongest winds will be occurring in the outer eyewall.  The eyewall replacement cycle could cause the circulation to increase in size.

Tropical Cyclone Amphan will move around the western end of a high pressure system over Southeast Asia.  The high will steer Amphan toward the north-northeast.  On its anticipate track the center of Tropical Cyclone Amphan will approach the coast around the northern Bay of Bengal in 36 hours.  Amphan will be a large dangerous tropical cyclone when it approaches the coast.  Tropical Cyclone Amphan will generate a significant storm surge along the coast of Bangladesh.  Amphan will cause major wind damage.  It will drop locally heavy rain over northeast India and Bangladesh.  Freshwater flooding will occur.