National Weather Service United States Department of Commerce

Heavy precipitation for parts of the Pacific Northwest

Moisture will continue to stream into the Pacific Northwest for the next several days. Heavy rain will continue along the coast, while heavy snow will impact the highest elevations of the Cascades. In the northern Plains and Upper Great Lakes, more arctic air will bring periods of snow, blustery winds and cold wind chills. Light snow is also possible in New England and the Ohio Valley. Read More >

Lake effect clouds and snow are a common wintertime occurrence downwind of the warm waters of the Great Lakes as cold air moves over the warm water surface. The corresponding air is warmed via convective processes and moistened via evaporation. This additional heat and moisture can be sufficient to generate cumuliform clouds downwind of the moisture source. If the temperature difference between the lake water and the air temperature at 850mb (known as delta T) is greater than about 15C, then clouds and snow often occur downwind of the Great Lakes.

Occasionally, this same process occurs over Lake Oahe in central South Dakota.  The lake is sufficiently large (especially now that it is filled nearly to capacity) such that given the right atmospheric conditions and wind trajectory, clouds and snow showers can form downwind of the lake.

The combination of a very cold and unstable airmass, northwesterly winds in the low levels of the atmosphere, and an upper level disturbance produced several bands of “enhanced” snowfall downwind of Lake Oahe on Thursday, 3 December 2009. That day, the Oahe Dam reported water temperatures around 45F (7C), with model guidance indicating 850mb air temperatures around -15C (resulting in a delta T around 22C).  

Below is a visible satellite picture from 1215pm CST Friday, 4 December 2009.  The white over much of central South Dakota indicates areas light snow...with the brighter white “streaks” areas of heavier snowfall.  Notable locations where somewhat heavier snowfall may have fallen were to the south/southwest of Pierre, and to the north/northeast of Mobridge. In these areas, the winds aligned parallel to the “fetch” or length of the lake such that more heat and moisture could be exchanged with the atmosphere.  Those two areas are quite remote so unfortunately no snowfall reports where available.