While a Category 4 hurricane on the Saffir-Simpson hurricane wind scale, NASA-NOAA's Suomi NPP satellite passed over Tropical Cyclone Marcus in the Southern Indian Ocean.
On March 22, 2018 2:30 a.m. EDT (06:30 UTC) the Visible Infrared Imaging Radiometer Suite (VIIRS) instrument aboard NASA-NOAA's Suomi NPP satellite captured a true-color image of Marcus. The VIIRS image showed an eye 18 nautical miles wide.
True-color imagery is created by combining data from the three color channels on the VIIRS instrument sensitive to the red, green and blue (or RGB) wavelengths of light into one composite image. Data from several other channels may also be included to cancel out or correct atmospheric interference that may blur parts of an image.
Animated enhanced infrared satellite imagery revealed warming cloud top temperatures on the western side. Warmer cloud tops mean weaker uplift, which indicates the storm does not have the power that it had previously.
The Joint Typhoon Warning Center noted that "Microwave imagery shows Marcus is undergoing an eyewall replacement cycle."
On March 23 at 11 a.m. EDT (1500 UTC) Marcus had maximum sustained winds near 143.8 mph (125 knots/231.5 kph). It was centered near 18.5 degrees south latitude and 106.2 degrees east longitude. That's about 522 nautical miles west-northwest of Learmonth, Western Australia. Marcus was moving to the south-southwest at 10.3 mph (9 knots/16.6 kilometers) per hour.
The Joint Typhoon Warning Center forecasts that Marcus will weaken to a tropical storm by March 24 as it encounters increasing vertical wind shear and cooler sea surface temperatures.