A clash between summer and winter across the central U.S. is causing an autumn outbreak of snow where the air is colder and severe storms, along with tornadoes and flash flooding, where it's warmer.
There's a "big temperature contrast across the country as a battle of the seasons commences," said the Weather Prediction Center.
Cooler-than-average temperatures have infiltrated the west, while above-average temperatures are occupying the east.
Between the two competing air masses is where the extreme weather will battle.
Autumn severe weather is very similar to severe outbreaks we see in early spring -- as colder air clashes with the oncoming warming of spring, says CNN meteorologist Chad Myers.
"The shorter fall severe weather season occurs when the warm air of summer is pushed away by the incoming colder air of winter."
A storm system is already delivering welcome snow across the Rockies. Already a foot in the mountains of Montana.
"The snow is expected to be quite heavy to the west of the storm track over the northern High Plains tonight," says the prediction center. "Winter Storm Watches and Warnings are currently in effect where snowfall totals of 1 to 2 feet are possible."
These winter weather alerts stretch from Utah to Montana.
"It's definitely not a rare thing to be getting snowstorms in October, that's when it starts becoming more likely to be getting snow. I know the average for Denver is Oct. 18 for the first snowfall and it definitely varies depending on where you are in the mountains," NWS Boulder meteorologist Caitlyn Mensch told CNN meteorologist Jennifer Gray on Monday.
The possibility for tornadoes
In the warmer, moist air on the southern side of the storm system, severe thunderstorm gusts, large hail and tornadoes are possible mainly on Tuesday evening into the night, across parts of the central and southern Plains, said the Storm Prediction Center.
"A supercell or two may pose a threat for significant (EF2+ damage) tornadoes, in addition to very large hail," said the SPC.
The greatest threat for tornadoes is in western Oklahoma and Kansas. There is an enhanced risk -- level 3 of five -- for severe storms.
Areas of slight risk -- level 2 of 5 -- include more of Oklahoma and Kansas and a small portion of Texas and Nebraska.
A marginal risk -- level 1 of 5 -- of severe storms stretches from southern Texas into South Dakota.
In addition, a dangerous plume of moisture will move in across central Texas, including Dallas.
"A potentially significant heavy rainfall event begins to unfold today through Thursday over the Central/Southern Plains," tweets the WPC.
Moisture from a landfalling hurricane named Pamela will stream into Texas and combine with this strong storm, said CNN meteorologist Dave Hennen.
This surge of tropical moisture from the Pacific will lead to widespread storms likely to produce several inches of rain from very heavy and intense downpours, according to the WPC.
Flood watches stretch from the Mexico, Texas border to the Texas, Oklahoma border.