Snow Band Movement and Rain Occurrence in Northeast U.S. Winter Storms

Forecasting snow accumulation in storms affecting the northeastern U.S. is a significant challenge. In these storms, locally heavy snow is associated with long (>200 km), narrow bands and groupings of smaller (<200 km) bands termed multi-bands. This research aims to improve understanding of the causes of formation, maintenance and movement of multi-bands. In particular, the causes of multi-bands are poorly understood. Another goal is to determine which areas and environments in the region tend to receive more rain than snow during these storms.

Data from six National Weather Service radars from Delaware to Maine were used to examine bands in over 100 storms that occurred between 1996 and 2016. Converting reflectivity to snow rate allows for better detection and visualization of snow multi-bands. The North American Regional Reanalysis (NARR) is used examine several characteristics of the storm environment including sea level pressure, temperature, wind, and geopotential height. We found that near-surface air temperatures tended to be several degrees higher over water than over land, so offshore areas were more likely to have rain at the surface than onshore areas.

The radar data were put into a Lagrangian framework relative to the low pressure center to characterize storm-relative band motions. Five types of band motion were found in the northwest and northeast quadrants of the storm. Band motion characteristics varied among storms and could also vary among different periods in the same storm. Often bands moved radially away from the low pressure center in a direction perpendicular to the long axis of the band. Sometimes multi-bands with this radial motion converged with a >200 km long single-band. Multi-bands could also be quasi-stationary relative to the low, move in a direction parallel to the long axis of the band, or nearby bands could move in several different ways at the same time. The 700 hPa horizontal wind field was found to generally be indicative of band motion.