Climbing the Sierra Madre Occidental, the eclipse track moves onto the Interior Plateau of mainland Mexico. The height of the Sierra, 3200 meters high, keeps all but the largest Pacific systems from bringing precipitation. These Pacific disturbances, borne on westerly winds in the upper atmosphere, are primarily a winter feature, and in May are becoming increasingly rare. Cloud cover increases slowly as the track travels from Hermosillo to El Paso in Texas. Some of this increase in cloud cover is a result of heating of the eastward facing mountain slopes. Bubbles of warm air cool as they rise, becoming saturated and turning into puffy cumulus. At worst, only scattered cumulus can be expected since eclipse time is around 10:15 AM CST in northern Mexico. Cloud development doesn't peak until 2 PM when temperatures are approaching their maximum.
Morning fog and stratus is more likely to be a problem in the jumble of mountainous terrain. High and dry elevations cool rapidly overnight. Cool air collects in the valleys where temperatures may fall low enough to allow the air to become saturated. Overnight lows average 59 F at Hermosillo in May. In comparison to the frequency of foggy mornings along the outer Baja coast, it's a minor problem in the mountains, and can be avoided by moving a short distance. Most overnight fogs will have likely burned off by eclipse time. Fog may redevelop as the ground cools during the eclipse, but the temperature decline in an annular eclipse will be smaller than that during a total. The outcome will depend on the moisture available.
Whether in dry or wet climates, mountains generate cloudiness. The Interior Plateau is no exception. Satellite images from May of 1992 show that the Sierra Madres are often spotted with patchy morning clouds, though usually not too heavily. These clouds are often at middle and high levels - 6000 feet or more above ground. Flatter areas away from the mountains (i.e. - up toward Ciudad Juarez and El Paso, Texas) are probably better sites, but for the most part the cloud which forms over the Sierra will blow toward them anyway. The effects of this cloud can be seen in Figure 8 as the area within the 50% contour which surrounds Temosachic.
During the spring months the sub-tropical jet stream can usually be found over northern Mexico, arcing across the Baja Peninsula and into the southern United States. The jet stream is occasionally marked by a band of cirrus and altostratus cloud, sometimes thick, but usually thin and wispy. Depending on the weather patterns of the day, the jet may lie south of the eclipse track, or a little to the north, though the former is more likely. In general, it's found close to the track, and may be difficult or impossible to avoid.
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