By Mark Guarino

CHICAGO (Reuters) - As colder-than-average temperatures locked large swaths of the United States into a deep freeze Tuesday, snow was likely in an unlikely place - Las Vegas.

The National Weather Service (NWS) said cold air pressure from the northern High Plains would move south starting Tuesday, producing light snow and sleet over parts of western Texas that will expand into parts of the Southwest Tuesday night into Wednesday. Light rain is expected to develop over parts of Southern California and the desert Southwest on Wednesday.

The NWS said Las Vegas temperatures fell below freezing Monday and would continue through Thursday at an average low of 30 degrees. Up to 3 inches of snow was expected by Tuesday night.

"Many tourists who come to Las Vegas may be unprepared for the true winter-like conditions this storm could bring with it," the NWS said in a statement. "Travel conditions could be difficult, if not impossible, on area roads."

Las Vegas does not have snowplows but the Nevada Department of Transportation said it has six snowplows at the ready if the snow poses a significant threat to roads. The Las Vegas Convention and Visitors Authority is expecting about 340,000 visitors for New Year’s Eve celebrations, it said.

Cold arctic air will spread throughout the Upper Midwest, where temperatures will fall 20 to 30 degrees below normal Tuesday and Wednesday, producing light snow in some parts. The falling temperatures will be accompanied by dangerous wind chill values between 20 to 30 degrees below zero, which will make the cold feel unseasonably brittle.

In Milwaukee, a low temperature of about 5 degrees was expected Tuesday with the high on Wednesday predicted to reach only 18 degrees, the NWS reports.

Farther east, light snow is expected over the Central Appalachians Tuesday with light rain to develop over the Mid-Atlantic down to parts of Florida late Tuesday through Wednesday night.

(Reporting by Mark Guarino; Editing by Bill Trott)