In an earlier article, we discussed the dynamics and thermodynamics of lake effect snow. In this post, we discuss the meteorological differences between single-band and multiple-band lake-effect snow events. Keep in mind that all lake-effect snow events are fueled by moisture and latent heat flux from the relatively warm lake water; these fluxes are already assumed for this discussion. Again, refer to the previous posts for more details about the ice microphysics and atmospheric thermodynamics involved.
Single Snow Bands
Before we begin the discussion, I wanted to show a rapid 42-hour time-lapse of the Buffalo, NY radar during the historic 2014 lake-effect event. There are two features worth noting that will be discussed below: 1) the evolution and persistence of the single snowband that forms over Lake Erie, and 2) the sudden northward movement of the snowband toward the end of the time-lapse, which occurred as the upper-level trough axis approached from the west and the angle between the lake’s major axis and the prevailing wind increased.
Time-lapse radar loop of the historic lake-effect snow event near Buffalo, NY in November 2014 (single band)
Time-lapse video of the historic lake-effect snow event near Buffalo, NY in November 2014 (single band)
Single-snowband events develop when the prevailing flow is closely aligned with the major axis of a lake or any elliptically shaped body of water that is large enough to provide a sufficient fetch. Initially, these bands of snow tend to develop over the center of the lake. Their development is controlled by the convergence of the land breeze along the shores. In single-band events, the bands can be 10-30 miles wide and 30-120 miles long. Snowfall rates can be as high as 4 inches per hour. Single snowbands tend to be approximately oriented along the thermal wind vector as outlined in the earlier article.
Single Bands in a Benign Weather Pattern with Light Winds
Low-level convergence is normally the impetus for the formation of significant snowbands (discussed in the previous articles here and here); however, singular snowbands can also develop when the base-state winds are light. Under certain circumstances (e.g., within a very cold Arctic airmass, high pressure, weak flow), when the winds are relatively light due to a weak low-level pressure gradient, thermal convergence dominates. Thermal convergence results from horizontal temperature gradients (e.g., sea breeze fronts and land breeze fronts). As shown in the image above (and in the animated gif below), a snowband may develop over the lake due to this type of convergence.
Snow bands that form in a benign environment are considerably less intense for the following reasons:
- Lack of convective instability
- Lack of large-scale forcing
- Presence of large-scale subsidence
- Lack of low-level horizontal convergence (wind convergence)
- Shorter fetch
All of these factors were discussed in the previous articles as important contributors to significant lake-effect events.
These bands are usually relatively short in length (spatially) because subsidence* tends to suppress convective activity. However, because single snowbands typically have high snow-to-liquid ratios, they can still produce snow accumulations in excess of 6 inches, despite their environmental limitations. The large-scale subsidence that these “light wind” snowbands typically form under and their dependence on the shoreline configuration, make these bands difficult to forecast.
*Note: The presence of subsidence is assumed for this scenario because of the relatively light winds, which commonly occurs in benign weather patterns characterized by the lack of large-scale forcing.
Multiple snowbands tend to develop when the prevailing wind is oriented along a shorter fetch of water or nearly perpendicular to the major axis of an elliptically shaped body of water. The reduced fetch limits the latent heat and moisture fluxes from the lake. As a result, the snowbands that develop in multiple-band events are usually quite shallow and are not as intense as single bands that form over a long fetch. By definition, they tend to cover a larger area; but, they are generally weaker and have lower snowfall rates than long-fetch bands. Each band within a multiple-snowband event can be on the order of a few miles to 15 miles wide and 15-30 miles long.
At the heart of the discussion of multiple snow-band events is a mesoscale phenomenon known as a horizontal convective roll (HCR). According to the Glossary of Meteorology, HCRs are defined as:
“Counter-rotating horizontal vortices that commonly occur within the convective boundary layer; their major axes are aligned with the mean boundary layer wind shear vector. The depth of the roll circulations is consistent with the depth of the boundary layer; the wavelength, measured from updraft to updraft in the counter-roll direction, is about three times the boundary layer depth.”
Horizontal convective rolls develop in response to daytime heating and are roughly aligned with the vertical wind-shear vector. These rolls can act to initiate deep convection ahead of a front.
Example of Single and Multiple Bands During One Event (November 18, 2014)
The following radar image shows examples of both multiple bands and single bands. In this case, multiple bands are located downwind of Lake Superior and Lake Michigan. Just by looking at this radar image, you get a feel for the deep-layer prevailing wind and its fetch over each (see the 500-mb data below). Over the western Great Lakes (i.e., Superior and Michigan), the deep-layer winds are more northwesterly, resulting in a NW-SE fetch (and ultimately to the formation of multiple, relatively light, bands of snow). Further east, over Lake Erie, the deep-layer wind is more westerly/southwesterly and aligned with the major axis of the lake, resulting in a persistent and more intense, single band.
This is the 500-mb map for the same day as the radar image above (11/18/14), valid just four hours prior (12Z or 6 am EST).