Kevin Myatt is The Roanoke Times' weather columnist.

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• Sprinkles or flurries possible Tuesday, but maybe something bigger for the weekend?
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• Coldest morning of winter so far likely across much of Southwest Virginia; Tuesday precipitation looking doubtful
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Our most wintry periods of cold, snow and ice require three large features to be in place.

(1) High pressure near Greenland, blocking the west-to-east flow of the jet stream, causing it to buckle southward far enough for cold air to filter into our region.

(2) High pressure near the West Coast, forcing the jet stream to dig even farther southward toward the Gulf Coast.

(3) A southern branch of the jet stream undercutting high pressure in the West, bringing wet storms from the Pacific across the southern tier of the United States.

Later this week, the first of those three will take hold.

High pressure has been building westward from Europe toward Greenland, and when it sets up later this week, the jet stream will dip southward and we will see colder weather as 2009 begins.

The longer term, more mysterious question is whether the other two pieces will develop.

At this stage, it doesn't look as if it will happen quickly. Low pressure in the northern Pacific will continue to sweep relatively mild upper-level winds inland across the country from west to east. Meanwhile, there's no solid sign of the jet stream splitting off a discernible southern branch.

Still, the change over the Atlantic will introduce enough cold so that wintry precipitation will become possible with a new storm system over the weekend. As of now, it looks more like a rain episode possibly starting as some ice. But there is a week's worth of details to sort out, and there is at least some chance of our first widespread winter storm of the season by the weekend.