Lake effect snow has been ongoing for the past few days across the Great Lakes. In this blog, we’ll explain what lake effect snow is and was causes it. Lake effect systems are systems in which the lowest levels of the atmosphere are modified by lakes. In the United States, the downwind areas Great Lakes are best known for lake effect snow, and it usually begins during the late fall or early winter.
The clouds associated with lake effect precipitation can be very deep (on the order of a few miles and large upward velocities. In some cases, some lake effect snow events can contain lightning. As with thunderstorms, lake effect snow is driven by the release of CAPE, but unlike thunderstorms, lake effect snow events require much less CAPE. The CAPE in lake effect snow events is generated as polar or arctic air moves over a warm lake. As a result, most lake effect snow occurs behind a cold front. The larger the temperature differential between the air and the lake water, the more destabilization that occurs.
There are several factors in the development of lake effect convection. The lapse rates between the surface and 850 mb should be at least dry adiabatic (10°C/km). The air temperatures at 850 mb should be at least 13°C colder than the underlying water. In addition, the length of the fetch is also important. A fetch parallel to the major axis is important in lake effect snow. In general, the fetch must be at least 75 km to produce lake effect snow.
Lake effect precipitation can also be influenced by wind speed and shear. The angle at which the wind crosses the lake can be important in the organization of lake effect precipitation. The direction of the mean wind and the mean wind shear usually differ by less than 20 degrees in lake effect events. In the heaviest lake effect events, strong winds blow parallel to the major axis of an elliptically shaped lake. Bands of lake effect precipitation can be 15-30 miles wide and 30-120 miles long.
Below is a radar image of a lake effect snow event. While we’ve discussed what causes lake effect snow, we haven’t discussed the orientation of the bands. To discuss the orientation of the snow bands, we need to discuss the concepts of geostrophic wind and thermal wind.
Radar image of Lake Effect Snow
Geostrophic wind is a theoretical wind in which the Pressure Gradient Force exactly balances the Coriolis force. This usually happens in the upper atmosphere where frictional forces are negligible. On a constant pressure surface, the geostrophic wind is parallel to the height contours. To look at the geostrophic wind during this event, below is the 500 mb chart from the morning of the event. The geostrophic wind is the red vector over Western New York.
The thermal wind is the vector difference of the geostrophic wind at an upper level and the geostrophic wind at a lower level. When there’s little or no change in the direction of the geostrophic wind, the direction of the thermal wind is roughly the same as the direction of the geostrophic wind. Looking at the wind profile from the Buffalo, NY sounding, there’s some change in wind direction, but is predominately southwesterly. Therefore, the thermal wind is roughly oriented from southwest to northeast. As a result, you can see on the radar image above that the snowbands are oriented from southwest to northeast.