NASA-NOAA's Suomi NPP satellite passed over the center of Ida at 4:05 a.m. EDT (0805 UTC) on Friday, Aug. 27. The Visible Infrared Imaging Radiometer Suite (VIIRS) instrument provided forecasters with a visible-light image that showed Ida was elongated with the bulk of its thunderstorm activity and rainfall to the east of its center. Some thunderstorms were wrapping into the circulation, particularly on its western side. VIIRS provided cloud-top temperatures and an image that uses infrared data combined with visible light to measure the height of clouds.
"Tropical cyclones that have very cold cloud tops in the central inner-core region and surrounding the eye-wall have been shown to be more intense and potentially generate more rainfall," said Brad Pierce of NASA's Goddard Space Flight Center.
VIIRS data can be useful for measuring the growth and development of tropical cyclones, as well as helping to determine if they are intensifying or weakening. According to Rob Gutro, a tropical cyclones researcher at NASA's Goddard Space Flight Center, "The colder the cloud tops within the eye and eyewall, the greater the potential for strengthening. Additionally, the extent to which the cirrus canopy (ice cloud) extends outward from the cyclone center provides useful information to the forecaster about the potential vertical wind shear and potential for strengthening or weakening."
VIIRS data is used by forecasters in conjunction with other satellite data to determine if a storm is intensifying or weakening.
Another NASA instrument that captured an image of Ida was the MODIS instrument aboard NASA's Aqua satellite on Friday, Aug. 27. The image revealed Ida was elongated and stretched from southwest to northeast. VIIRS data collected at the same time showed the strongest thunderstorms were located northwest of the center and east of the center.
Wind Shear Taking a Toll
On Aug. 27, vertical wind shear was impacting the storm. Wind shear is the difference in wind speed and direction between two levels in the atmosphere. High shear can disrupt a cyclone's organization, and strong vertical shear can push precipitation away from the center.
On Aug. 26, the Global Precipitation Measurement mission or GPM core observatory satellite passed over Ida. GPM's Microwave Imager (GMI) and Dual-Frequency Precipitation Radar (DPR) instruments provided information on the rainfall around the center of the storm. GPM's radar (DPR Ku Band) made observations that revealed rain rates within Ida's intense thunderstorms were falling at a rate of over 1 inch (2.5 cm) per hour.
Forecaster Outlook
On the National Hurricane Center's (NHC) 8 a.m. EDT update, the center of Tropical Storm Ida was located near latitude 21.5 degrees north and longitude 75.1 degrees west. Ida is moving toward the west-northwest near 12 mph (19 kph). A west-northwest to northwestward motion is expected through Saturday, followed by a turn toward the west-southwest late Saturday or Sunday.
Maximum sustained winds are near 65 mph (100 kph) with higher gusts. Some additional strengthening is possible before the wind shear becomes too strong and inhibits significant development or organization. Weakening is forecast to begin over the weekend. Tropical-storm-force winds extend outward up to 90 miles (150 km) from the center.
Ida is still expected to produce up to 15 inches of rainfall over east-central Florida. The NHC noted on its 11 a.m. advisory that "these rains could produce widespread urban and river flooding, along with mudslides."
