Observed Characteristics of Mesoscale Banding in Coastal Northeast U.S. Snow Storms

Locally enhanced snowfall within mesoscale snow bands yields spatial gradients in accumulation that are difficult to forecast. While frontogenesis is associated with > 250 km long single snow bands, the physical mechanisms that yield sets of smaller multi-bands are less clear. Observations and reanalysis from 110 snow storms in the northeast United States between 1996 and 2016 are used to examine the context and occurrence of features within the precipitating portion of the cyclone. Radar data from six National Weather Service operational radars are combined to characterize the spatial distributions and variability of precipitation bands and newly identified velocity wave trains.

Storms were analyzed in a Lagrangian framework centered on the cyclone low. Most snow band features were observed to develop and remain in the northern part of the cyclone. Snow storms with multi-bands were found to be the most common followed by snow storms with coexisting single and multi-bands, snow storms with no band features, and snow storms with only single bands. In some storms, multi-bands move radially away from the low and can converge with a longer band. In other storms, the multi-bands are relatively stationary with respect to the low pressure center and to each other.

Within the snow storm, weaker, stratiform precipitation tends to form first, then areas of intense snowfall within bands develop and later fade. This time sequence differs from a typical deep convective system where heavy precipitation forms first and then weakens to form stratiform precipitation. On average, the storm total band area was found to represent < 10% of the total precipitation area. Hence, the duration of mesoscale bands over a specific location greatly influences snowfall accumulation.

We examined Doppler radial velocity fields to investigate dynamical features associated with single bands and multi-bands. No clear, sustained convergence signatures were found associated with multi-bands suggesting that these bands do not consist of persistent, active updrafts. For the first time, we document observed velocity wave trains across multiple radar domains within snow storms. These velocity waves are roughly aligned with the snow bands but typically moved faster than the multi-bands and appear to originate outside of the precipitation echo area.