A steep temperature drop, a blizzard and a downpour could assail the millions expected to attend Barack Obama's presidential inauguration on January 20, officials said.
"Just given a sunny pleasant 50-degree day (10-degree Celsius), just having that many people around town will be a problem, so any sort of weather issue on top of that is only going to exacerbate that," warned Christopher Strong, a meteorologist with the National Weather Service.
"No matter what happens, we'll be as well prepared as we can be given the enormous potential for crowds."
Average late January temperatures in Washington fluctuate between minus six and plus four degrees Celsius (21 to 39 degrees Fahrenheit), while the median temperature sits at about 2.7 degrees Celsius (37 degrees Fahrenheit).
But bone-chilling freezes could be hastened by winds that could reach 16 kilometers (10 miles) an hour, according to weather services.
Statistics showed there is a five-percent chance that snow will powder spectators on the Mall, a long tree-lined strip of land leading up to the US Capitol where Obama will be sworn in -- and a 12-percent chance they will be soaked in rain.
But weather services said there is a 30-percent chance of snow before the ceremony. Local authorities have prepared a fleet of 200 snow plows and salt trucks ready to hit the main arteries in case of a snowfall.
Old-timers remember Ronald Reagan's 1981 inauguration as the sunniest in recent memory at a balmy 13 degrees Celsius (55 degrees Fahrenheit).
But with temperatures never rising above nine degrees Celsius (16 degrees Fahrenheit), his second inaugural in 1985 was forced to move inside and the inaugural parade was cancelled that day.
Cold weather on Inauguration Day has also had dramatic consequences.
William Harrison took the oath of office coatless and in freezing temperatures on March 4, 1841 while delivering the longest inaugural address in American history, lasting an hour and forty minutes. He then rode through the streets on his horse in the inaugural parade.
A few days later, Harrison fell with pneumonia and died the next month.
John F. Kennedy's 1961 inauguration was remembered for terrible traffic jams caused by a heavy snowfall while that of Franklin D. Roosevelt in 1937 was drenched with rain.
Snow or not, Inauguration Day temperatures will probably be low, very low this year. "Watch for signs of frostbite ... watch for signs of hypothermia -- shivering and numbness, confusion ... slurred speech," the local government warned on its website.
