Summer days are usually warm or hot in the lowland areas and cool in
the higher mountains; the nights are cool.
Spring arrives early in the lowland areas and progressively later at
higher elevations. Weather conditions are particularly favorable in
autumn, with less rainfall at that season. Winters are variable, both
as to severity and length, but they normally are mild in the lowlands.
Rain, snow, and fog often are encountered in the mountains during the
winter. Roads are kept open all year, but the transmountain highway
sometimes is closed for a few hours during heavy snow or sleet storms
when snowplows cannot keep it open. Snows are infrequent in the park,
but you should take chains if you plan to drive through in winter.
Roads are posted if driving conditions are unfavorable.