In Wyoming, the shadow path makes the transition from the rugged Rocky Mountains to the smoother topology of the Nebraska High Plains (Figure 1). That transition comes with an important change in climatology as mountain-moderated Pacific moisture in the western part of the state is replaced in the east by Plains moisture that flows northward from the Gulf of Mexico and southward from Canada. When winds blow from the east, the High Plains along the Nebraska border are frequently immersed in low-level Gulf moisture. Because this moisture is typically only a few hundred metres deep, the humid air is unable to penetrate deeply into the rising terrain of the Wind River Basin, much like a rising shoreline that limits the spread of ocean tides.
Wyoming is basin-and-range country: a complex jumble of mountain ranges (all part of the Rockies) separated by flat dryland or desert basins. Since the general lie of the mountains in western Wyoming is perpendicular to the prevailing westerly flows, they create a barrier that forces the atmosphere to deposit much of its moisture—what is left of it—on the west-facing slopes, leaving the central and eastern parts of the state with a semiarid climatology. August is the driest month of the year, and cloud conditions for the eclipse may be as favorable as those in Idaho and Oregon.
Western Wyoming: Mountain Country
The Wyoming Range and the Teton Mountains guard the western border of the state, presenting a formidable barrier to the limited amount of Pacific moisture that approaches across the Snake River Plain in Idaho. As the flow through the Plain reaches the Tetons, air is forced to an abrupt rise, converting atmospheric humidity into mountain clouds. According to the satellite measurements in Figure 2, the Tetons add 15 to 20 percent to the average morning cloudiness compared to amounts measured at lower elevations near Idaho Falls. Beyond the Tetons, however, the air drops abruptly into Jackson Hole, the valley that contains the resort towns of Jackson and Teton Village. As fast as the cloud formed on the way up, it dissipates on the way down (Figure 3), giving the valley a more promising climate than the surrounding peaks. This orographic drying is only partly captured by Figure 2, as the Hole is too narrow to be reflected in the 25-km resolution of the satellite measurements.
The observations from Jackson Hole Airport presented in Table 1 suggest that the valley is a prime eclipse-watching site with an average cloudiness of 34 percent, only a few points below locations in the Snake River Plain in Idaho. As well, the valley and mountains are a photographic gem because of the grandeur of the Tetons. Precipitation is modest in the valley—about 33 mm of rain spread across nine days of the month, almost entirely from afternoon convection that spreads across the valley from the slopes.
Because of the higher average cloudiness, observation sites on the peaks above Teton Village (reached by chairlift) should be avoided if the forecast on eclipse day calls for early showers or thundershowers. When thunderstorms approach, the chairlifts are closed, and watchers must remain on the peaks, in the limited shelter available, until the threat of lightning has passed, eliminating any possibility of movement. If the weather is good, however, the peaks will provide a spectacular eclipse vista.
The central axis of the eclipse lies north of Jackson – right through the middle of Jackson Hole airport – where the valley opens to its widest extent, further enhancing the effects of the descending air flow that dries out the atmosphere. From this location, there is a grand view of the Tetons to the west, and open terrain east that provides a generous space for viewing. If the day should prove cloudy, Highway 26 leads 256 km to the east side of the Wind River Range near Riverton, where exceptionally sunny August skies may be found. Riverton is shielded from Pacific moisture by just about the whole of the Rocky Mountains, but, at the same time, lies in a long east-west gap in the terrain that allows Great Plains moisture to reach deep into Wyoming on easterly winds. Fortunately, the altitude of Riverton is itself a barrier to the Plains moisture – a barrier that becomes less and less effective as the elevation along the track descends past Casper to Nebraska. From a climatological point of view, the centre line between Riverton and Shoshoni is not only the best location in Wyoming but also one of the best for the whole eclipse track.
The mountain slopes in Wyoming respond very strongly to daytime heating – possibly more than any other part of the eclipse track . With enough moisture and instability, late morning or early afternoon will see small convective clouds building over the dark forested slopes of the Tetons and the Wind River Range. These may remain as small puffy clouds that dissipate as the temperature falls with the approach of the lunar shadow, or may grow into larger showers and thundershowers depending on the instability and moisture supply. Deep morning buildups are uncommon, but not impossible if upper level disturbances are crossing the state on eclipse day. That heavy cloudiness is rare in this part of Wyoming in August can be seen in Figure 2 where the red 10:30 a.m. cloudiness curve almost exactly coincides with the time of shadow passage on eclipse day (at 11:40 a.m. local daylight time).
Casper and Eastward to the Great Plains
Toward Casper, the satellite-measured cloud cover increases again, mostly because of the cloud-building presence of the Laramie Mountains to the south of the city. Though small in extent, the Laramie Mountains are one of the cloudier ranges in Wyoming, feeding on plains moisture that is drawn to the slopes by afternoon heating. Surface measurements of cloudiness at Casper are more revealing than those from satellite, showing an average amount of just over 40 percent (Table 1), which, by comparison with the sunshine measurement at Lander, probably means a percent of possible sunshine (and the chances of seeing the eclipse) around 75 percent.
During a 2013 visit, we were drawn to Fort Caspar in the western suburbs of Casper (the spellings are correct). The large open spaces and the pioneer ambiance lend themselves to a comfortable eclipse-day experience.
August thunderstorms are most frequent on the High Plains east of Casper and one of the most prolific thunderstorm generators is the Laramie Mountains, as noted above. Typically, small convective clouds will form on the mountains in the late morning, growing into large towering cumulus and thunderstorms by mid-afternoon (Figure 6). At that point, the storms may move off of the mountains, tracking east and northeastward onto the plains of Nebraska. Such storms may dissipate as they leave the mountains, or blossom into large severe-storm systems that continue into late evening as they move eastward onto the Great Plains. For the most part, they are creatures of the late afternoon and won’t be a concern on eclipse day. More unsettling will be large-scale, high-level weather systems that cross the mountains without significant dissipation. The 30 percent of skies that are broken or overcast in the airport measurements should encourage the adoption of a “Plan B” for eclipse day.
Casper records an average of 7 thunderstorm days and Lander 6.5 in August. The state is not noted for severe convective weather, but does have a significant historical record of hail damage and occasional tornadoes, particularly on the Front Range east of Casper. June is the peak month for tornadoes in the state, with an average of 42 in the month; August is much quieter with just under 10. An average of about 29 large hail events occur in the summer, concentrated in the southeast near Laramie. In the area of the eclipse track, severe hail events occur about once per season, from May until September.
Beyond Casper, the terrain flattens and descends steadily toward Nebraska, a descent of about 300 m to the border. The vegetation along the highway testifies to a dry climatology, and even though moisture is available from the plains to the east, it is usually subdued compared to the humidities that can develop over Nebraska and Kansas. For the most part, the cloud record is composed of high-level systems that cross over the Rockies and the occasional early thunderstorm that forms on the hills. The afternoon storms are not reflected in the statistics of Table 1, as those cloud data are for the late morning hour of the eclipse.
One feature of the whole of the western portion of the track and sometimes beyond, is the frequency of forest fires and the presence of smoke from those fires. The dry summer season almost always sprouts a large number of forest fires in the western mountains, anywhere from the Mexican border to Northern Canada (Figure 7). When winds are right, the smoke will be carried over the eclipse track, putting a haze across the sky and possibly masking the dimmest parts of the corona. As a rule, the reduction in visibility is only serious if the fire is relatively close, in which case it may be avoided by a short movement elsewhere. The extent of the smoke is best detected in late afternoon and early morning visible-light satellite images.
Wyoming provides a generous amount of open space in which to seek out and view the eclipse. Highway 26, from Casper to Riverton, follows the shadow axis for 160 km (100 miles), passing through an open, high-plains countryside. West of Riverton, another 120 km (80 miles ) of highway takes the traveller to the shoulders of the Wind River Range, all within the depths of the shadow path and all with a favorable cloud climatology. East of Casper, for 110 km (70 miles), Interstate 25 never deviates from the central axis of the path by more than 16 km (10 miles). The generally favorable weather, ease of movement, and the wide-open skies make Casper a very enticing home base from which to plan an eclipse expedition.
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Updated December 2016