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Lake-Effect Snow is also unheard of in Texas, but it produces the largest snow totals ever observed east of the Rockies. At its root, lake-effect snow is just a form of convective precipitation, like thunderstorms. But unlike Texas thunderstorms, which involve warm, moist air moving into an unstable environment, lake-effect snow involves cold, dry air becoming warm, moist, and unstable.
Lake-effect snow is most common in the late fall and early winter in the Great Lakes region. The Great Lakes take a long time to cool off, and usually don't freeze until midwinter. Meanwhile, a series of Arctic cold blasts (technical term) sweep across the relatively warm Lakes. If the path of the air takes it across a large expanse of open lake, it is warmed and moistened as it goes, becomes convectively unstable, and produces shallow but vigorous snow showers downstream.
While the principle is simple, actual lake-effect snow is quite variable. It may take the form of a single strong snow band or several weaker snow bands. The particular location of the snow depends on the wind direction, with places downstream of the largest expanse of water subject to the most snow. If there are hills or mountains downstream of the lake, the upslope flow enhances the lake effect. Sometimes, the lake effect snowbands can survive several hundred kilometers downstream.
To forecast lake-effect snow, keep in mind the following:
- Lake-effect snow will be seriously under-forecasted by the models. So if the models produce lake-effect snow, you can be confident lake-effect snow will be there.
- To determine the wind direction, examine the 850 mb analysis or forecast. The surface pressure field can be affected by the lakes themselves, and the surface winds are often influenced by convergence over the lakes or into the rain bands.
- The colder the air, the stronger the lake effect. Air which is warmer than the lakes will do nothing. You can use the 850 mb temperatures for this: just add 13 to the 850 mb temperature (to correct for the 1300 m height difference between the 850 mb level and lake level) and compare to the lake water temperatures, as reported on buoys or sea surface temperature maps.
- Cyclonic curvature favors lake effect snow. If the 850 mb height contours curve to the left as you go downstream, strong lake-effect snow is more likely.
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