The first total solar eclipse since 1979 will cross the continental United States on August 21, 2017, when the shadow of the Moon sweeps across the Earth, travelling from the North Pacific, south of the Aleutian Islands, to the Eastern Atlantic near the Cape Verde Islands in a little over three hours (Figure 1). After a 38-year eclipse drought, this one arrives over the United States to the open arms of a friendly August climatology.
The eclipse will be visible across all of North and Central America and the northern half of South America. A total eclipse, in which the Sun is completely covered by the Moon, will be seen only along a 100 km-wide band that stretches from Oregon to South Carolina (Figure 2). Along this axis, the Sun’s bright surface is completely blocked by the Moon, revealing the sweeping grandeur of the solar corona and the brilliant red opulence of the atmosphere’s lowest layer. Outside of this narrow track, observers will see a partial eclipse in which only a part of the Sun is masked by the Moon .
An Overview of Weather Prospects
This eclipse arrives at a propitious time: the summer thunderstorm season is winding down and retreating southward; the Arizona monsoon is breaking; and the storm-carrying jet stream has not yet begun its journey southward from Canada. The dry and generally sunny fall season is about to begin. The best climatological prospects for a sunny day are found in the west and midwest, but weather forecasting has now reached a level of accuracy that movement to a favourable area can be planned several days or a week in advance from any location in America.
The westerly winds that bring weather systems onto the North American continent first have to cross the mountain barriers that border the Pacific coast. The moisture-laden air is forced to rise up the windward slopes, giving seaward-facing Oregon a reputation for cloud and precipitation. It’s not a heavy cloudiness – there’s still a generous amount of sunshine – but better locations are only a short distance away, on the east side of the coastal mountains.
By the time that the prevailing westerlies have crossed the Coast and Cascade Ranges, much of the humidity has been wrung out of them. The moderately cloudy climate of the Pacific coast gives way to much sunnier weather in Oregon’s Columbia Basin and later in the Snake River Plain of Idaho (Figure 3). Several locations—central Oregon, southwest and southeast Idaho, and central Wyoming— offer the best weather prospects along the entire track (Figure 4).
As the lunar umbra speeds eastward, cloud cover follows the ups and downs of the terrain, until, beyond Wyoming, the shadow breaks free of the mountains and heads out across the Great Plains. For the most part, the best weather prospects are found to the lee of the western mountains peaks. Central Oregon is sheltered by the Cascade Mountains, Idaho’s best spots are in the valley of the Snake River, and Wyoming’s sunniest prospects are to the lee of the Wind River Range and to the Rocky Mountains generally.
The images above were acquired by the Aqua and Terra satellites in their daily passes across the United States. To the left of the red line, images show the cloud cover at approximately 10:30 am local standard time; to the right, at 1:30 pm local time. White sections of the images are where data are missing. Images: SSEC University of Wisconsin-Madison/NASA.
As the Moon’s shadow crosses the Missouri River, average August cloud cover begins a steady climb that culminates on the heights of the Blue Ridge Mountains of North Carolina and the Piedmont Plateau of South Carolina. The eclipse track is by now far enough south that sub-tropical moisture is almost always available, ready to feed convective clouds that form on the slopes of the mountains on most days. Larger weather systems are often “pinned” by the Appalachians, delaying their crossing to the Atlantic side and adding an extra day of bad weather to Tennessee and the Carolinas. There is even a small threat of hurricane weather along the Atlantic coast at this time of year.
August is a sunny month everywhere in the continental United States, and measurements of sunshine hours at locations along the track show at least a 60 percent probability of seeing the eclipse. At the best sites in Oregon and Idaho, that probability rises to 85 percent. In most places along the track, a little attention to the next-day weather forecast and a bit of travel will greatly increase the odds of a spectacular August 21 (Figure 5).
For more detailed assessment of climatological prospects for states along the eclipse path, see the additional studies attached to this introduction.
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Updated December 2016