A weather front stretching 3,000 miles across America will drive near-record heat in parts while elsewhere braces for a foot of snow.
Halloween will feel more like an Indian Summer across Atlantic states as an early winter roars into the northeast.
But on the cold side of a giant front sweeping the nation, official warnings have been issued for plunging temperatures and heavy snow.
Thermometers will rise 35F higher than average for the time of year across the east and southeast as fierce thunderstorms pepper the frontal boundary.
Temperatures will surge across the east coast
The Weather Channel
A spokesman for the National Weather Service (NOAA) said: “A potent storm over the central and northern Rockies will produce moderate to heavy snow across the high elevations of Utah, Wyoming, and Colorado.
“An injection of colder air with significant moisture will bring snow accumulations up to a foot for many mountain ranges.
“High temperatures will soar into the 70Fs and 80Fs across a large portion of the eastern half of the country, with highs eclipsing 90Facross parts of the southern Plains.”
Volatile weather could threaten Election turnout with more than 20 states facing rain and thunder.
Most at risk will be a stretch of the country along the boundary of a huge weather front reaching from California to New England.
AccuWeather long-range expert Paul Pastelok said: “Voters and election officials should be prepared for the possibility of rain or thunderstorms across more than 20 states on Election Day.
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Most at risk will be a stretch of the country along the boundary of a huge weather front reaching from California to New England
The Weather Channel
“We are forecasting rain and thunderstorms across the middle of the country as a cold front sweeps across this area near or on Election Day, with the specific impacts depending on the timing of the front.
“There is a risk of significant rainfall and the threat of flash flooding starting Saturday through Election Day across parts of Texas, Oklahoma, Arkansas, Kansas, Missouri, Illinois and Iowa.”
A chilly election awaits voters in across Colorado, Nebraska, Wyoming, Montana, Idaho, Oregon, Nevada, Utah, and northern Arizona.
However, temperatures along the east coast will soar higher than average for the time of year.
Mr Pastelok said: “It should be a dry and pleasant Election Day along the I-95 corridor from Boston through New York, all the way down to the Carolinas and even much of Florida.
“We could see some snow west of Denver and north of Flagstaff on Election Day, and don’t be surprised if you see snow flying in the higher elevations of the Four Corners.”
Some parts of the US could see snow on election day
AccuWeather
In the meantime, trick or treaters will be collecting candies in possibly record-breaking warmth.
Temperatures in the east are expected to hold in the 70Fs through the evening, according to experts.
Weather Channel meteorologist Caitlin Kaiser said: “A winter coat will be a good addition to your Halloween costume if you live in the Northwest, Rockies or upper Midwest.
“However, trick-or-treaters could be sweating through their costumes in parts of the south and east with evening temperatures expected to linger in the 70Fs.”
Jim Dale, US meteorologist for British Weather Services and social commentator, said: “If Washington or surrounding regions get to 27C, we could be looking at records falling.
“It is going to be unusually warm for trick-or-treaters this year.”