PORTLAND, Ore. -- One thing could obstruct the viewing experience for millions of people planning to watch the solar eclipse in August.


A *recent report looked at the historical trend of cloud conditions on August 21 in the path of the eclipse and found that viewers on the Oregon coast could potentially experience a cloud-obstructed view of the eclipse.

The west and east coasts both carry the highest probability of heavy cloud conditions. In addition, many areas east of the Mississippi River may experience cloud cover as the eclipse passes.

MAP: Average historical cloudiness for August 21

The darker the dot, the greater chance for cloudiness at the hour of peak viewing during the total solar eclipse on August 21, 2017. (Map source: Deborah Riddle, CICS-NC and NOAA NCEI)

The intermountain west will likely have clear skies on August 21. On the Oregon coast, there is a 56 percent chance viewers in that area will experience cloud obstruction. But moving east, in McMinnville and Salem, the percentage chance of cloud cover falls to 31 and 33 percent, respectively.

The following is an incomplete list of cities and towns on the path of totality from west the east. The list includes local times for viewing of the eclipse, followed by the average cloudiness percentage. A higher percentage means a lower chance of cloud cover. So for example, a 67 percent viewability percentage means a 33 percent chance of clouds.

CHART: Eclipse viewability percentage

The historical data comes from 10-year hourly climate averages from 2001-2010. The information was measured at weather stations across the country each August 21 as near the hour of the eclipse as possible.

The calculations are historical, which provide a basis for comparison but no guarantee of what viewers will experience this August 21.

MAP: State capitols in path of totality

State capitols in the path of the total solar eclipse: Salem, Ore.; Lincoln, Neb.; Jefferson City, Mo.; Nashville, Tenn.; and Columbia, S.C. The optimal viewing time is given for each. (Map source: Deborah Riddle, CICS-NC and NOAA NCEI)

This year's eclipse is expected to last about 90 minutes, making its first appearance over the Pacific Northwest in midmorning and ending on the South Carolina coast. The last time a total eclipse crossed the United States was in 1979. The last time an eclipse traveled coast to coast was 1918.

* The report was published June 6 by the National Oceanic and Atmospheric Administration's National Centers for Environmental Information and the Cooperative Institutes for Climate and Satellites-North Carolina.