Holiday drive weather

Cajon Pass, at 3,777 ft between San Bernardino and the High Desert, closes a few times each winter when snow and spinouts pile up; the CHP closes I-15 at the bottom on both sides.

Tejon Pass is at 4,144 ft. Caltrans starts Operation Snowflake when forecasters expect heavy snow at 4,500 ft or lower; once snow sticks, the CHP paces traffic over the pass, and if that is not enough I-5 closes.

Two NWS offices cover the drive: Seattle from King County through Lewis County, and Portland from Cowlitz County (Kelso) south to Portland. A warning can be in effect for one end of the drive and not the other.

Three NWS offices cover the drive: Phoenix for the Valley and Wickenburg, Flagstaff for the high desert north of Congress, and Las Vegas from the Bagdad area through Kingman to Las Vegas.

Two NWS offices cover the drive: Salt Lake City for all of Utah, including Cedar City and St. George, and Las Vegas from the Arizona Strip through the Virgin River Gorge to Las Vegas.

Sherman Hill summit at 8,640 ft between Cheyenne and Laramie is the highest point on I-80; the Laramie–Rawlins stretch past Elk Mountain gets sustained crosswinds over 50 mph.

Four NWS offices cover the drive: Chicago (northern Illinois), Milwaukee/Sullivan (Madison), La Crosse (Mauston and Black River Falls) and Twin Cities (from Menomonie to Minneapolis).

Three NWS offices cover the drive: New York (New York City), Mount Holly/Philadelphia (New Jersey and Delaware) and Baltimore/Washington (Maryland and DC). A storm's rain–snow line can sit anywhere along the route.

The rain–snow line matters more than totals: a few miles inland or north can mean sleet and ice instead of rain.

TDOT describes the Monteagle interchange (exit 134) as the last westbound exit before an approximately 800-foot descent along steep grades.