The total solar eclipse will cross from Oregon to South Carolina along a 70-mile-wide (110 kilometer) "path of totality." In North America, people outside of the path of totality will see a partial solar eclipse. Totality will last, at most, about 2 minutes and 40 seconds at the center of the path, so bad weather could potentially block the main event. However, the partial eclipse will last for much longer throughout the country, so partly cloudy skies are less of a threat to skywatchers.
Look for up-to-date weather forecasts for your area on the National Weather Service website. You can watch a live broadcast of the eclipse on Space.com's homepage, courtesy of NASA, starting at 12 p.m. EDT tomorrow (Aug. 21). You can also get live eclipse news updates from Space.com, plus helpful eclipse maps and safety viewing information, with the Eclipse Safari mobile app.
A stationary weather front running from northern Utah to northern Wisconsin will provide the focus for clouds, scattered showers and thunderstorms near and along it. For those within the eclipse track in Nebraska and Kansas, as well as northern Missouri, this could lead to problematic weather by eclipse time.
In contrast, high pressure building across the Pacific Northwest and the northern Great Plains will bring mainly fair conditions and mostly clear skies. For the Southeastern U.S., high pressure slowly building from the Atlantic Ocean will gradually dry out the atmosphere and bring sultry and sunny weather for the eclipse. The exception will be near and along the coastal plain of South Carolina, where the remnants of an old frontal line could lead to clouds and scattered showers.
State by state breakdown:
Oregon: With the sole exception of locations near and along the immediate Pacific coast, the weather looks perfect for eclipse viewing, with clear skies along much of the path of totality. Those near the ocean shore, however, will need to be concerned about low stratus clouds and fog. Typically, these clouds "roll back" away from the coast around 9 or 10 a.m. local time. However, as the sun becomes progressively covered by the moon, the temperature may start to fall, which in turn may slow or halt the retreat of the marine cloud layer, and possibly even cause the clouds and fog to push back onshore. Best advice is to relocate to a position at least several miles inland, away from the shore, to ensure clearer skies for the big show. Over western Oregon, where dewpoint temperatures will be in the lower 40s, look for temperatures to drop as much as 10 to 20 degrees around the time of greatest eclipse.
Idaho: Near perfect weather statewide is expected for the eclipse! A few high-level clouds may streak the skies over the southern sections of the state, but these should be mainly south of the totality zone. As was the case in western Oregon, dewpoints will be near 40 degrees, so temperatures are likely to drop at least 10 to possibly even 20 degrees around the time of maximum eclipse.
Wyoming: Across the northern and western sections of the state the eclipse could be veiled by a swath of mid-to-high level cloudiness. In the totality zone, this could include those in and around Grand Teton National Park. But farther to the south and east, skies are expected to be mainly clear, including over the city of Casper. As was mentioned for both western Oregon and Idaho, dewpoint temperatures will be hovering close to 40, meaning a 10 to 20 degree drop in temperature should be expected around the time of the peak of the eclipse.
Nebraska: Unfortunately it appears that much of the "cornhusker state" may be adversely influenced by a broken to overcast layer of clouds. There may even be an isolated pop-up shower or a strong-to-severe thunderstorm. Those in the path of totality have their best chance of getting a view of the sun over the western part of the state (Scottsbluff, Alliance, Bridgeport), where the clouds may be thinner and where there just might be a few fortuitous gaps in the cloud cover. Temperatures and humidities will also be oppressive.
Kansas: The northeast corner of the state is brushed by the totality path. Like Nebraska, it is expected to be oppressively warm and humid with broken to overcast skies and a marginal risk for severe thunderstorms. The southeast part of the state is expected be under much clearer skies, but unfortunately this is also outside of the totality path.
Missouri: The moon's dark shadow could fall on considerably cloudy skies as it enters Missouri from the northwest. Places such as Kansas City, St. Joseph and Excelsior Springs might be looking at more clouds than eclipsed sunshine; there is even a risk for scattered thunderstorms. However, places farther to the south and east, such as Columbia, Jefferson City, St. Louis and Cape Girardeau, could have clearer conditions, with broken to perhaps scattered clouds at the time of greatest eclipse. It will also be very warm and sultry with heat indices approaching 100 degrees.
Illinois: The northern and especially central part of the state will be plagued by broken to overcast skies, but the clearest part of the state is expected to be in the south, which includes the communities of Carbondale and Marion, which are in the totality path and are forecast to have 30 to 60-percent cloud cover when the last rays of sunlight are extinguished by the passing new moon. Heat and humidity will be prevalent as well.
Kentucky and Tennessee: Both states are crossed by the path of totality. It will be oppressively hot and humid but with sun-filled skies and just some scattered clouds making for very good viewing conditions as the moon's shadow rapidly sweeps on by.
Georgia: The northern half of the state, which includes the totality zone over the northeastern portion, should experience conditions similar to Kentucky and Tennessee. The southern half of the state will see quite a few more clouds.
North Carolina: The total eclipse path will pass over the mountainous terrain of western North Carolina, and much of the state will enjoy sunny skies on Monday. Typically, thanks to the very warm and humid weather that occurs in North Carolina in the summer, the sun's heat generates cumulus clouds that tend to grow during the mid and late afternoon. However, the eclipse will be reducing the amount of sunshine on Monday afternoon, leading any cumulus build-up to diminish as the eclipse progresses. So in this way, the eclipse may actually help reduce the amount of cloud cover.
South Carolina: It appears that the "Palmetto State" will be split between favorable eclipse weather and unfavorable eclipse weather. The northern and western half of the state should enjoy generally sunny to partly cloudy skies, while the southern and eastern half is expected to be mired under partly to mostly cloudy weather and even a threat of scattered showers and thunderstorms near and along the coastal plain. A simple breakdown could also go this way: Greenville-Spartanburg, good to excellent chances; Columbia, "iffy," so go to the north and west if you are mobile; Charleston, fair to poor chances. Watch out for possible showers and even thunder around eclipse time.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes abut astronomy for Natural History magazine, the Farmers' Almanac and other publications, and he is an on-camera meteorologist for Verizon Fios1 News in Rye Brook, New York.