Total Solar Eclipse Weather: Here’s the Cloud Cover Forecast for August 21

solar eclipse
The Earth, moon and sun must align perfectly for a total solar eclipse to happen. Inclement weather could still spoil the best opportunity to see the total eclipse in generations. NASA's Goddard Space flight Center/SVS

On August 21, a total eclipse will be on display for the first time on the United States mainland since 1979. As it travels across the breadth of the U.S., passing 14 states through a 70-mile-wide corridor, starting in the Pacific Northwest and ending in the Carolinas, thousands will be hoping to catch a glimpse of the moon passing directly in front of the sun.

However, inclement weather could spoil the best opportunity to see the total eclipse in generations if cloud cover blocks the view.

In his weather blog, Forbes meteorologist Jon Erdman explains that in summer the greatest threats to seeing the solar event are short-lived individual thunderstorms rather than larger-scale storm systems.

Related: Total solar eclipse: New Mexico Chaco Canyon rock art shows ancient eclipse

An individual thunderstorm will blow itself out in the two to three-hour period the eclipse happens over in some places. But an ill-timed storm at the moment of the eclipse could obscure the view during the two to three minutes of the moon passing directly in front of the sun.

A greater threat to seeing the eclipse, also common in summer, are mesoscale convective systems—essentially a complex of thunderstorms that becomes organized on a larger scale than one-off storms. These can last up to an hour, lingering in a given location and causing cloud cover.

However, the timing of the eclipse is in the viewers’ favor. These weather systems normally occur in the early morning and at night, and the eclipse will take place during the early afternoon.

It is still too far off, with more than a week to the eclipse, to make accurate predictions about cloud cover, but general trends show the best conditions will be in the West. Afternoon thunderstorms are likely from the Rockies to the desert Southwest; however, these will arrive after the eclipse.     

The only possible wrinkle could be provided by some low clouds on the West Coast. In areas with onshore winds along the coastline, some clouds could remain in place into the late morning.

In the central states, thunderstorms are more of a threat to visibility. States in this area are closest to the jet stream, and if current patterns hold, thunderstorm clusters may occur. Further south, in the Plains, thunderstorms will be less likely but are currently predicted to be higher than average on August 21.

In the eastern states, the last areas to see the eclipse, showers and storms are likely as the jet stream takes a southward dip. Scattered thunderstorms look probable also from the Appalachians to the northern Gulf Coast as well as across the Florida Peninsula.

Because of the time of day the eclipse will hit the East, sun watchers in these states have the highest chances of having their view blocked by errant thunderstorms.