Nebraska and Kansas

Nebraska
Figure 1: The track of the Moon’s umbral shadow across Nebraska and Northeast Kansas.

On leaving Wyoming, the Moon’s shadow comes to the flatter landscape of Nebraska, descending  more than 1200 m from the High Plains in the west to the Central Lowlands along the Missouri River (Figure 1).

Protected from Pacific flows by the Rockies, and from easterly weather by the prevailing winds and the distant Appalachians, the “Great Plains” or “Prairies” become an easy conduit for air masses moving northward from the Gulf of Mexico or southward from Canada. Southerly flows bring high humidities and warm, sultry days. Northerly flows bring cooler and drier weather, usually preceded by a cold front that gives birth to towering thunderstorms as it collides with the entrenched Gulf air. Westerlies bring dry weather, while the uncommon easterlies are usually found in the bad weather behind a departing low-pressure system.

The clash of these airmasses gives the Great Plains its reputation for severe convective weather—especially tornadoes—but August is several months later than the peak of the severe-weather season and the climate is more subdued than in May and June. Nebraska has an average of 21 tornadoes in June, but only 2 in August.

cloudgraph-modis
Figure 2: Graph of average cloud cover along the central axis of the eclipse based on observations from satellites over a 15 to 17-year period. Data NASA/GSFC.

The major source of moisture on the west side of the Appalachians is a low-level air stream from the Gulf of Mexico that tends to flow north and eastward, following the trend of the prevailing winds aloft.  As a consequence, there is a steady increase in available moisture along the eclipse track from west to east across Nebraska, and eventually, all the way to the Appalachians in Tennessee.

The route of the eclipse through Nebraska and Kansas is not across a landscape of obdurate flatness, but instead passes over undulating hills, grassy dunes, occasional buttes and escarpments, rugged Badlands,  and the rich ecology of intermingled lakes and wetlands. There are no cloud-making mountains or dry valleys, and so the weather is generated by passing large-scale weather systems that slide overtop the Rockies, frontal systems from Canada, and the growth and decay of convective clouds on days with suitable instability. In middle and upper layers, the westerly flows carry cool, dried-out remnants of Pacific air across the mountains and onto the Plains. If these westerlies cross a humid, low-level flow of Gulf air, the combination creates the temperature and moisture profile that helps initiate the thunderstorms for which the Prairies are so well known.

Table 4 Nebraska
Table 1: Surface-station climate data at the time of the eclipse for selected stations in Nebraska along the shadow track.

On average, Nebraska sunrise tends to be the cloudiest time of day, but as the morning progresses, the low-level cloud left over from the night burns away in the summer sunshine. Later in the day, convective clouds take over, bringing a gradual increase in average cloud cover for the afternoon. In Figure 2, we see that the morning cloud has largely dissipated by 10:30 a.m. By 1:30 p.m., the first convective clouds of the day are forming. The eclipse occurs about noon (standard time) in the state and so cloud prospects can be estimated by selecting a mid-point between the two curves.

Table 1 and the cloud graphs of Figure 2 give the nod to Alliance in western Nebraska for the most favourable cloud climatology. Alliance is favoured by the drying downslope flow from the Rockies and from the relatively infrequent influx of Gulf moisture into the region. In southeastern Nebraska and northeast Kansas where the eclipse reaches the Missouri River, the terrain is wide open to sub-tropical moisture reaching northward from the Gulf of Mexico. In addition, the air is humidified by extensive evapotranspiration of soil moisture from fields of grain, sorghum, and corn planted on the Prairies.  A consequence of the gradual increase in moisture from west to east is  the 10 to 15 percent increase in average August cloud cover as the eclipse track crosses the state. This is further emphasized by the amount of precipitation , which ranges from a low of about 7 mm in central Wyoming to 100 mm along the Kansas-Missouri border in August.

Direct measurements of sunshine (Table 1) along the track are a little less pessimistic than the cloud measurements, showing that North Platte, in central Nebraska, receives 75 percent of the maximum possible sunshine, while Lincoln and Topeka are only slightly less content with an average of 70 percent. No sunshine measurements are available for the Alliance area.

Nebraska
Figure 3: Nebraska – wide, open spaces and the weather isn’t too bad either.

Nebraska favors the well-prepared expedition, as highways in the state permit rapid movement to clearer skies, which are often not too distant when thunderstorms are responsible. Good weather information will be essential—satellite images work particularly well in the short range, but a forecast from the day before will probably prove more convenient.

Carhenge - a bit whimsical, but under Nebraska's best skies.
Figure 4: Carhenge – a bit whimsical, but under Nebraska’s best skies. Carhenge lies a short distance north of Alliance, NE.

 

A good home base for eclipse expeditions is Alliance, NE, or nearby Antioch, both located in the best weather zone on the plains. Highway 2 from Alliance offers 320 km of in-the-track travel toward the east if cloud avoidance is necessary and a westerly escape can go all the way to Casper and still stay in the umbral region. In central and eastern Nebraska, Interstate 80 allows rapid relocation, cutting diagonally across the track, from Lincoln to Lexington, a distance of 240 km. Nebraska and northeast Kansas don’t offer any cloud-eating terrain, so flight is the best option if a new viewing location is wanted.

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Updated December 2016

Climatology and weather for celestial events