A nasty lake effect snow event is cranking up over western New York this afternoon, threatening to dump several feet of snow on communities from Buffalo to Watertown. Towns south of Buffalo could dig out from three feet of snow from the system, while eastern Lake Ontario could see up to five feet of snow in 36 hours.

Bands of lake effect snow are developing on all five of the Great Lakes this afternoon, but the heaviest snowfall totals are expected to occur across western New York near Buffalo and Watertown. The latest forecast from the National Weather Service shows that up to three feet of snow will fall south of Buffalo (with the city proper seeing eight to ten inches). The snow will be even heavier east of Lake Ontario, where orographic lift will allow for up to five feet of snow to fall across northern Oswego and southern Jefferson Counties.

Lake effect snow is nature's greatest snow machine—as we saw back in November, it can produce unimaginable amounts of snow in an extremely short amount of time. In order for lake effect snow to develop, you need three major ingredients: warm lake water unobstructed by ice cover, very cold air moving over the lake, and a long fetch (wind moving over the water).

Water has a higher heat capacity than air; it takes much longer for the lakes to warm up and cool off than the air above them. As a result, the Great Lakes often stay relatively warm well into the winter months. Even though temperatures have dipped far below freezing (and even far below zero) more than a few times in the past couple of months, this morning's ice cover analysis shows that only 18.7% of the five Great Lakes have ice on them, which is a little above average for January 8 in the past six years. As you can see in the above map, water surface temperatures range from the upper 40s on eastern Lake Ontario to near freezing on Lake Superior.

According to a temperature analysis performed at 2:00 PM EST, air temperatures are downright frigid across the Great Lakes region, with readings in the single digits above the northwestern part of Lake Superior, gradually warming into the upper teens and lower 20s as you cross Lakes Erie and Ontario.

As bitterly cold air passes through the region, the relatively warm water heats up the air directly above the surface of the lake through conduction. This lake-warmed air begins to rise as a result of convection—and if there's enough moisture available—the convection will manifest as heavy snow showers.

The third factor involved in lake effect snow is the fetch, or the length of water over which the wind blows. A long fetch—wind blowing lengthwise across one of the lakes—is conducive to the development of single-band lake effect snow, or one heavy band that can persist for a long time (up to a day or more in the most intense scenarios). A short fetch—wind blowing across the narrow width of the lakes—supports multi-band lake effect snow, or numerous smaller bands that produce modest snow totals over a much larger area.

Thanks to the good folks over at nullschool dot net, we have a great visualization of which way the winds are blowing this afternoon. The orientation of surface winds is providing a short fetch across Lakes Superior, Michigan, and Huron, a mixed (?) fetch over Lake Erie, and a long fetch over Lake Ontario.

Here's the result:

Folks who live in western Michigan have a widespread, steady snowfall thanks to numerous, small bands feeding ashore from Lake Michigan. Lake Superior also has some multi-banding going on, as does Lake Erie since the winds are not quite lined up with the length of the lake at the moment. Snowfall totals near Lakes Michigan and Superior will be conversational in nature, with just a couple of inches expected along the northern shore of the U.P. and western shore of the mitten.

Now we get to the fun stuff. The wind is lined up perfectly to produce a long fetch over Lake Ontario, and we can see that by looking at the radar site in Montague, New York:

Like a cold, wintry fire hose, the single band coming off Lake Ontario is blasting a narrow corridor between Watertown and Pulaski with snowfall rates of up to three inches per hour. This is where we'll see the heaviest snowfall accumulations by tomorrow night. The band will drift north and south (and probably change orientation a bit as winds shift), and whichever unlucky communities get caught under the band the longest will see snowfall totals of about one child deep.

Once surface winds finally align along the length of Lake Erie, snow should switch from multi-band to one of these single-band events that sets up over or just south of Buffalo, providing the region with two to three feet of snow.

Odds are that if you're reading this and you're not experiencing lake effect snow right now, you can't let your guard down just yet. Even though temperatures are warming up after this week's deep freeze, most people who live in the Ohio Valley, Mid-Atlantic, and Northeast have a few shots at seeing frozen precipitation next week. It's too early for details right now, but it could be interesting. Keep an eye on your forecasts this weekend.

[Images: NASA, NWS, GLERL, WeatherBELL, earth.nullschool.net, Intellicast, Gibson Ridge]

You can follow the author on Twitter or send him an email.