Where Will Clouds Foil the Big Eclipse?

All it would take is a single passing shower—or even a large cumulus cloud—to produce epic disappointment for people who have spent months or years planning to view the total solar eclipse sweeping across the United States on August 21. Millions more around the contiguous U.S. will get to experience a partial solar eclipse, again assuming that weather permits. It’s easy to see why the craving for an eclipse-oriented weather forecast could get insatiable over the next few days.

If this were January, it would be easier to target large areas where clear skies or thick clouds might prevail. During winter, midlatitude storm systems are typically larger and more intense, and they often emerge in forecast models a week in advance, even if their exact location can’t be nailed down. However, it’s now August—a good-news/bad-news month for eclipse lovers. The good news: the sun shines on average for 60% to 80% of all daylight hours in August over most parts of the country. This means there’s roughly a two-in-three chance of sunshine in many places based on solar climatology alone. Maps of the average amount of sky covered by clouds at midday in August don’t look quite as favorable for eclipse viewing, especially in the Southeast U.S. This may be because cumulus clouds are more likely to develop as the day goes by, and the Southeast will be the last part of the nation to experience totality.

The big downside of August for eclipse watching is that it’s a month filled with convection—showers and thunderstorms, some of which may come and go within hours. Even if we knew with 100% confidence that a particular spot in the zone of eclipse totality was going to get convection next Monday (which we don’t), it would be impossible to nail down whether those storms would be present during the less-than-three-minute window of totality, which is what matters most.

The graphic below is a sequence of cloud-cover forecasts for midday Monday from the GFS model, generated over a four-day period. The amount of seeming randomness makes it clear that trusting in any week-ahead cloud forecast for a specific point is a fruitless exercise, although we can expect certain patterns to emerge as we get closer to eclipse day.

Figure 1.  Forecasts of average total cloud cover (in percentage of sky covered) for the central U.S., produced by the GFS model at 12-hour intervals from 7 pm CDT Sunday to 7 am Tuesday. Each forecast is for conditions at 1 pm CDT on Monday, August 21, 2017, when the zone of totality will be translating eastward near Grand Island, Nebraska. Image credit: College of DuPage NeXT Generation Weather Lab.  Thanks to David Ewoldt for compiling and providing these images.

Cloud forecasts from the European model (ECMWF), similar to those above from the GFS, are being posted daily at greatamericaneclipse.com, with the eclipse path juxtaposed on the image. The GFS maps are updated regularly at the College of DuPage’s NeXT Generation Weather Lab site; they can be accessed in the Weather Analysis Tools > Numerical Models section. Maps for regions throughout the eclipse path are available.

With the above caveats on long-range forecasting in mind, here are some thoughts (as of Tuesday evening) on what we might expect next Monday.

—Pop-up showers and storms from Kentucky to South Carolina. East of the Mississippi, the pattern looks quite seasonable for mid-August, which means warm, humid conditions and a chance of showers and thunderstorms. These may be quite localized, making it impossible to know exactly which spots might see their eclipse views thwarted. Larger areas of high cloud may extend from individual storms or storm clusters, and these high clouds can persist long after the storms dissipate.

—Possibly widespread clouds near a frontal zone from Wyoming to Missouri. It’s been a remarkably cool August so far across nearly all of the U.S. east of the Rockies (see Figure 2 below). Upper-level troughs diving into the region from Canada have brought delightfully mild summer conditions to many parts of the central and eastern U.S. But these troughs have also triggered widespread clouds and convection, including very heavy rains, especially across the southern and central Plains and Mississippi Valley (see Figure 3 below).

Figure 2. Temperature departures from average (degrees F) for the period August 1-13, 2017. Image credit: NOAA/NWS/CPC.

Figure 3. Precipitation for the period August 1-14, 2017, as a percentage of the long-term average for this period. Image credit: NOAA/NWS/AHPS.

A large-scale shift in the pattern should bring gradually warmer weather next week across much of this unusually cool region. Monday looks to be a transition day, though, with a frontal zone draped across the central Great Plains, likely in Kansas or Nebraska. Global forecast models are suggesting that a surge of moisture carried by weak upper-level winds from the south may be approaching this boundary on or around Monday. Such a pattern would be ripe for scattered showers and thunderstorms—more concentrated in some areas than others—as well as the potential for widespread high clouds and/or a deck of low clouds north of the front. If there’s going to be any large-scale eclipse blockage on Monday, Wyoming and Nebraska may have the best chance in the country of being Ground Zero for it.

—Seasonably scattered clouds in Oregon and Idaho, with a chance of smoke. This region tends to be quite sunny at midday in mid-August, and right now I’d expect this to be the case on Monday. The main exception is the narrow coastal zone in Oregon west from the Cascades to the Pacific, where low marine clouds can always interfere with the sky view. The biggest risk may be layers of smoke from wildfires in the U.S. Northwest and British Columbia. The dense smoke that afflicted Portland and Seattle a few days ago has been shunted into Canada for the time being, and there are no signs of it returning to the same degree by Monday barring any large new fires. Any such smoke is unlikely to completely obscure the sun, unless the smoke is coming from a nearby fire. Some thin high clouds are possible, but these would be unlikely to fully block the eclipse.

“The bottom line:  it looks good over BOTH sides of the Cascades, except for the immediate coast,” said Cliff Mass (University of Washington) in a detailed outlook for Oregon posted on Tuesday at his weather and climate blog. “I believe that model solutions are relatively stable now--but by Friday we should be very confident in Monday's forecast," he added.

More on the eclipse

See our post from last week for more eclipse background, including safety and logistics tips.

At weather.com, Jon Erdman gives a full national outlook for eclipse day, including areas outside the zone of totality.

The website Eclipsophile has a superb, detailed review of climatological factors along the eclipse path, including state-by-state breakdowns.

In a Forbes post, Marshall Shepherd (University of Georgia) ponders the scenario—unlikely for Monday, but not impossible—of a total eclipse passing over an Atlantic tropical cyclone.