Kevin Myatt is The Roanoke Times' weather columnist.

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• Winter trying again to show up with snow
• We got graupel, but not on official record
• Moisture could get caught up in cold blast
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• Many looking past mild, quiet week toward possibly wild weekend
• Sprinkles or flurries possible Tuesday, but maybe something bigger for the weekend?
• For now, it looks like a quiet, mostly mild week ahead for SW Virginia
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It's looking a lot like a "crunchy storm" is on the way for Sunday.

That's the term I use to describe these mixed-precipitation winter storms we seem to get at least once a year in Southwest Virginia.

First, it snows. Then, it sleets. Then, freezing rain puts a layer of glaze ice on top.

The result is neither the fluffy frivolity of a pure snow nor the slick polish of an ice storm. Instead, we get a gritty white enamel that crunches as you step through it or drive into it.

Our crunchy storms vary from one another in the proportion of each precipitation type. Sometimes, there is a lot of snow, then a little sleet and ice. Sometimes, only a little snow, a lot of sleet and a little ice.

Exactly how those layers stack up remains the biggest open question about Sunday's potential storm.

With weather, nothing is certain until it actually happens, but two possibilities at this point seem highly unlikely: that we will miss wintry precipitation entirely on Sunday, or that it will be only rain.

There's just too much moisture being slung at us from the Pacific Ocean on a strong river of winds high in the atmosphere known as the jet stream. There's also too much cold air being pressed down on us from Canada.

To miss the storm entirely or only have it be rain would take some huge changes in the atmosphere over the next 36-48 hours.

But beyond the two questions of whether there will be moisture and whether it will be cold enough for wintry precipitation, everything else is still murky.

The problem is that the winds bringing in the moisture high in the atmosphere are also bringing warmer temperatures. As it warms up aloft while the surface remains cold, the more likely we are to switch from snow to sleet and freezing rain.

Snow requires temperatures several degrees below freezing at cloud level and, for the most part, temperatures at or below freezing all the way from the cloud to the ground. Snow can penetrate a thin layer of above-freezing temperatures and stay intact, explaining why it sometimes can snow even with temperatures in the low 40s. But it doesn't take a very wide layer of barely above-freezing temperatures to melt snow entirely.

When snow melts, it can fall to Earth as rain, or it can refreeze in lower cold layers into those bouncy little ice balls known as sleet. Sleet looks white like snow as it accumulates, but instead of being fluffy, sleet forms a hard layer.

If falling rain encounters a thin layer of cold air very near the surface, the rain freezes on objects at the surface. This is freezing rain, one of the most deceptively destructive forces in all of weather. A little glaze from freezing rain can make roads impassable but mostly just makes outside objects glittery. Start getting more than about a quarter-inch of ice, though, and things such as tree branches and power lines start breaking.

One possibility for this Sunday is that the winds aloft bring just enough moisture but not enough warmth to keep the precipitation entirely as snow. Snow fans would love this. We could get several inches of snow if that happens.

Another possibility is that the higher-altitude warm winds override the cold air and we very quickly begin getting freezing rain. I don't think anybody really wants this. It could lead to damaging ice accumulations and power outages.

The most likely scenario is just what I mentioned to start this column. First, it snows. Then, it sleets. Then, freezing rain puts a layer of glaze ice on top. That would probably leave 1-4 inches of crunchy muck by Monday morning.

I didn't even get into the various possibilities on timing and precipitation amounts. You can go to my Weather Journal blog on roanoke.com for the latest on that and other aspects of Sunday's winter storm threat.