Tropical Cyclone Marcus strengthened into the equivalent of a Category 5 hurricane on the Saffir-Simpson Scale as it churned northwest of Australia on Wednesday.  At 5:00 p.m. EDT on Wednesday the center of Tropical Cyclone Marcus was located at latitude 15.8°S and longitude 108.0°E which put it about 600 miles (960 km) northwest of Learmonth, Australia.  Marcus was moving toward the west-southwest at 14 m.p.h. (22 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 918 mb.

After completing an eyewall replacement cyclone on Tuesday, Tropical Cyclone Marcus began to intensify quickly again on Wednesday.  Marcus exhibited a circular, symmetrical circulation.  There was a tiny circular eye at the center of circulation.  The strongest winds were occurring in a ring of thunderstorms that surrounded the small inner eye.  Recent satellite images suggested that another eyewall replacement cycle may be beginning.  A larger, outer eyewall appeared to have encircled the small inner eye.  Bands of showers and thunderstorms were revolving around the concentric eyewalls.  Storms in the core of Marcus were generating very well developed upper level divergence which was pumping mass away in all directions from the tropical cyclone.

As frequently happens during eyewall replacement cycles, Tropical Cyclone Marcus increased in size after Tuesday’s eyewall replacement cycle was completed.  Winds to hurricane/typhoon force extended out about 50 miles (85 km) from the center of circulation.  Winds to tropical storm force extended out 180 miles (290 km) from the center.  The Hurricane Intensity Index (HII) for Tropical Cyclone Marcus was 35.0.  The Hurricane Size Index (HSI) was 18.7 and the Hurricane Wind Intensity Size Index (HWISI) was 53.7.

Tropical Cyclone Marcus is in an environment that is very favorable for strong tropical cyclones.  Marcus is moving over water where the Sea Surface Temperature is near 30°C.  It is moving through an area where the upper level winds are weak and there is little vertical wind shear.  Although Tropical Cyclone Marcus is in a very favorable environment, another eyewall replacement cycle would cause it to weaken.  The wind would begin to converge into the outer eyewall, which would cause the storms in the inner eyewall to weaken.  The wind speeds would decrease as the inner eyewall weakens.  Then the strongest winds would be found in the outer eyewall and the circulation would increase in size.

Tropical Cyclone Marcus is nearing the western end of a subtropical ridge over Australia.  The ridge is steering Marcus toward the west-southwest, but the tropical cyclone will turn more toward the south when it reaches the end of the ridge.  Tropical Cyclone Marcus will make a gradual turn toward the southeast during the next several days.  On its anticipated track Tropical Cyclone Marcus will remain west of Western Australia during the next 36 to 48 hours.