DENVER (AFP) - Coloradans were ditching the suntan lotion on Tuesday (Sept 8) and pulling out warm gloves and boots as the western US state went from blazing summer heat to snowfall in just one day.

In the state capital Denver, the temperature was around 33 degrees Celsius on Monday afternoon.

By Tuesday morning, it had dropped to 2.2 deg C, and it could cool further as the day wears on, forecasters say.

"Drastic change to WINTER starts tonight!," the National Weather Service in Boulder tweeted on Monday.

"Prepare now, for this sharp change from summer to winter!"

Indeed, snow fell on Denver on Tuesday morning.

Forecasters said it could be one of the most abrupt temperature plunges ever in Colorado over such a short span of time.

It was caused by a blast of cold air that barrelled in from Canada.

And it came just three days after temperatures in Denver hit 37.2 deg C.

The bizarre early cold was forecast to last through Wednesday night, with temperatures expected to be back up to 25 deg C on Sunday.

For now, winter weather alerts were issued for southern Colorado, with authorities warning that branches still sporting the green leaves of summer could snap under the weight of snow.