Timing of This Summer's Solar Eclipse Is Ideal For Minimal Weather Issues

Jon Erdman
Published: May 12,2017

The odds are good that weather won't get in the way of this summer's first total solar eclipse in the Lower 48 states in over 38 years.
A narrow swath from Oregon to South Carolina will see the moon completely mask the sun on Aug. 21. Nearly all other parts of North America – as well as parts of South America, Africa and Europe – will also see at least a partial eclipse.
(MORE: 10 Best States to See the Total Solar Eclipse)
The eclipse will start mid-morning in the Pacific Northwest, around midday in the nation's heartland and early-afternoon in the Southeast, ending in the Lower 48 after 4 p.m. EDT.

The path of totality (gray line) and areal coverage of the Aug. 21, 2017 solar eclipse.

To be clear, it is far too early for us to make a credible, specific forecast for any location on Aug. 21. However, average August weather conditions over many years provide some perspective on the chance rain or cloudiness could block your view.

Rain Chance

Dr. Brian Brettschneider, an Alaska-based climatologist with the Western Regional Climate Center, constructed several maps which lend some insight into typical August weather.
The first map below shows which month of the year is typically the wettest.
Areas in the maroon shading, from parts of the Great Lakes and northern New England to the mid-Atlantic and Southeastern seaboard, including parts of Florida, to a swath of the Desert Southwest, southern Rockies and much of Alaska, typically are wettest in August.

The map above shows which month is the wettest, on average, based on 1981-2010 data from NOAA's National Centers for Environmental Information. Areas in dark maroon are typically wettest in August.

That doesn't sound like good news for eclipse viewing.
However, let's examine a better proxy for typical rain chances – namely, the average number of days with measurable rain in August.

Average number of August days with measurable precipitation with the Aug. 21,2017 solar eclipse's path of totality overlaid.

Given the summer dry season, the Great Basin and West Coast have the best chance of a dry August day. Even Texas and southern Oklahoma only see measurable rain once every six to 10 August days. Most of the rest of the central U.S. only records rain every three to six August days.
In the Great Lakes, southern Rockies, Appalachians, interior Northeast, parts of the Southeast and northern Gulf Coast, rain is a bit more common, occurring every two to three August days.
Therefore, in all these areas above, your chance is better you'll see a dry August day than a rainy one.
One exception to this, however, is the Florida Peninsula, where afternoon thunderstorms can almost be a daily occurrence in summer.

What About Those Summer Thunderstorms?

A single thunderstorm won't last long enough to affect the entire 2- to 3-hour viewing period of the eclipse.

An example of a mesoscale convective system, or cluster of thunderstorms, responsible for flash flooding along the northern Gulf Coast on April 30, 2014. Infrared satellite image and 15-minute lightning strike data (teal blue minus signs and red plus signs) is shown.
(UW-CIMSS)

However, one of these stray thunderstorms could be so poorly timed it occurs over part of the area seeing an eclipse for the roughly 2- to 3-minute period of totality.
This is likeliest on the far southeastern U.S. leg of the total eclipse area, where total coverage of the sun occurs between 2 and 3 p.m. EDT – roughly the time of day pop-up afternoon thunderstorms typically flare up.
(MORE: NASA's Eclipse Timing Maps)
Posing more of a threat to block out the eclipse, clusters of thunderstorms known as mesoscale convective systems – MCS, for short – are common in the summer. Their rain or cloud shield can linger over a given location for more than an hour.
However, these clusters are typically more active during the nighttime and early-morning hours – not necessarily during the early-afternoon hours, when the August eclipse will occur in the central and eastern U.S.

What About Cloud Cover?

Of course, all you need are stubborn, sufficiently thick clouds to block your view of the eclipse, even on an otherwise dry day.
There is some good news: according to Dr. Brettschneider's March 2015 blog, August is typically the least cloudy month for the overall Lower 48.
In most of the nation, average afternoon cloud cover in mid-late August is less than 50 percent.

Average cloud cover (percent) at 2 p.m. EDT from August 14-28, based on 1979-2016 ERA-Interim data from the European Centre for Medium-Range Weather Forecasting.

August doesn't typically feature the large, strong frontal systems known by meteorologists as extratropical cyclones, producing not just widespread precipitation but also clouds that cover sometimes a dozen states or more. These happen most often from late fall through the middle of spring.
Also, some of the typically cloudier locations in the U.S., such as the Great Lakes, New England and the Pacific Northwest, aren't as cloudy in August.
If this eclipse happened in November or December, for example, there would be a higher chance of eclipse fans disappointed by a stubborn cloak of clouds.
(MORE: America's Dreariest Cities)

Visible satellite image of Hurricane Irene on Aug. 28, 2011, at 8:32 a.m. EDT, just 28 minutes before landfall in Brooklyn, New York.
(NASA)

One wildcard to all this is tropical cyclones.
Upper-level winds can spread a canopy of clouds over areas far from the center of a tropical storm or hurricane.
Just the remnant moisture of a former tropical storm or hurricane can either trigger more numerous thunderstorms or can, again, block out the eclipse with its clouds.
This can not only occur in the eastern and southern U.S., but also with remnant eastern Pacific tropical cyclones in the typically sunnier Desert Southwest.
Again, we have no way to forecast exactly what weather features will be in place this far in advance of the historic total solar eclipse. However, this is about as close to the sweet spot of typically optimal viewing weather one can get for much of the country.
Be sure to check back frequently for complete coverage of the historic August 2017 total solar eclipse.
Jonathan Erdman is a senior meteorologist at weather.com and has been an incurable weather geek since a tornado narrowly missed his childhood home in Wisconsin at age 7. Follow him on Facebook and Twitter.
MORE: March 2015 Eclipse

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