SAN FRANCISCO — With assistance from computer models that rely heavily on satellite data, meteorologists were able to forecast accurately the intensity of Hurricane Sandy and the path it was likely to follow once it reached the Eastern Seaboard, giving local communities approximately five days to prepare before the storm reached land.

That feat was even more remarkable given Sandy’s unusual path, which included a sharp turn west when it merged with a winter storm sweeping across the United States, said Dennis Hartmann, a professor of atmospheric sciences at the University of Washington.

Even as they evaluate the accuracy of recent forecasts, atmospheric scientists warn that a looming gap in weather satellite coverage threatens the nation’s ability to predict the course of future storms. To produce accurate long-term forecasts, meteorologists need to feed frequent global observations of temperature, atmospheric pressure and wind speed into sophisticated weather models that use supercomputers to determine probable outcomes.

Most of the data used by the U.S. National Oceanic and Atmospheric Administration (NOAA) to forecast the path of a storm several days ahead of its impact are gathered by polar-orbiting satellites, including the European Organization for the Exploitation of Meteorological Satellites’ Metop-B spacecraft launched Sept. 17, which crosses the equator in the morning over local areas, and NOAA’s Polar Orbiting Environmental Satellite, which crosses the equator in the afternoon.

The problem, according to atmospheric scientists, is that U.S. polar-orbiting satellites are likely to stop functioning years before their successors are capable of gathering the observations needed for accurate weather prediction. NOAA launched its most recent polar-orbiting satellite, NOAA-19, in 2009 and NASA launched another, Suomi NPP, in 2011.

Recent studies, including a 2011 report by the National Research Council (NRC), suggest the instruments onboard those spacecraft will stop functioning before their replacements on the first spacecraft in the Joint Polar Satellite System are prepared to take over. NOAA-19 sensors were designed to last at least three years and NPP instruments were designed to operate for five years.

A 2011 NASA inspector general’s report highlighted technical issues that could limit the life of NPP instruments to three years, meaning the United States could lose its primary operational weather satellite in polar orbit from 2015 until the first JPSS spacecraft is launched — an event scheduled to occur sometime before spring 2017, NOAA spokesman John Leslie said.

The anticipated gap is likely to be extended by the amount of time needed to calibrate new instruments. Once on orbit, government officials will spend between six and 12 months calibrating various sensors and verifying the accuracy of their data before using the information to predict severe weather, NOAA officials said.

The failure of NPP before its replacement is ready “would have a detrimental effect on weather forecast quality,” said Hartmann, who chaired the NRC panel that reviewed NASA’s Earth science program in 2011.

To address that risk, NOAA officials are conducting an extensive assessment of alternative sensors that could supply data for weather models and ways to incorporate that data in forecasts. “Our goal is to ensure that NOAA’s National Weather Service has the critical data it needs to sustain the accuracy and timeliness of our weather forecasts and warnings,” NOAA Administrator Jane Lubchenco wrote in a Sept. 18 memorandum that describes numerous steps agency officials plan take to enhance oversight of its weather satellite programs.

Debra Werner is a correspondent for SpaceNews based in San Francisco. Debra earned a bachelor’s degree in communications from the University of California, Berkeley, and a master’s degree in Journalism from Northwestern University. She is...