At the time of this writing, the northeast United States is in the grip of a frigid air mass in the wake of a winter storm which brought widespread snowfall, along with a variety of mixed precipitation, to the Eastern Seaboard. While today’s winds and associated below-zero wind chill factors are a topic for a future blog post, today we focus on the physical mechanisms which are responsible for creating various types of frozen precipitation.
First of all, let’s review some facts about precipitation. Most precipitation in the midlatitudes, where we live, is formed as a result of rising and cooling air in the atmosphere. It is cold enough high above the surface that, even in the summer, much of the precipitation which forms in clouds initially forms as snow crystals. So, what determines whether we see rain, snow, freezing rain or sleet at the ground? The most important factor is the temperature profiles in between the ground and cloud base. Rain forms as precipitation falls out of the cloud and completely melts before hitting the ground. If precipitation falls as snow, that means that the entire column of air underneath the cloud is below freezing, and so snow crystals remain frozen all the way to the ground. The graphic below nicely illustrates this concept:
Let’s discuss the two more complicated precipitation types, freezing rain and sleet. In each of these two cases, there is a warm (above freezing) layer of air somewhere between the cloud and the ground. This allows for partial or total melting of ice crystals as they fall out of the cloud. What happens next depends on how much cold (below freezing) air is at the surface. If there is a deep enough cold layer, the partially melted drops have time to refreeze before hitting the ground, and sleet results. These ice pellets accumulate in a very similar manner to snow. Freezing rain occurs when the cold air at the surface is not deep enough to allow for total refreezing of the precipitation. It then falls as supercooled rain droplets and freezes on contact with surfaces on the ground. Freezing rain can be a major public hazard due to the sheer weight of water as it accretes on trees and power lines.
Now, back to our most recent storm of January 19-20, 2019. Folks in upstate New York and western New England may recall that National Weather Service forecasts called for between 12 and 18 inches of snow during the days prior to the storm. Further east into Massachusetts, it was apparent that some mixed precipitation would occur. Many of us located in and around the Albany area will recall hearing that unmistakable ‘ping’ of heavy sleet during the early morning hours of Sunday (January 20). A transition from snow to sleet during a storm can drastically cut down on the amount of accumulation that results. On January 20, extended periods of sleet fell across the Capital Region. Thus, the storm total accumulation was on the lower end of the forecast range. The warm air aloft worked its way slightly further to the north and west, and most computer models did not pick up on this until about 12 hours before the onset of precipitation in the region. Forecasting accumulations during a mixed precipitation event can be very challenging. The newest radar technology, known as ‘dual-polarization’, has greatly helped forecasters determine where in a storm the precipitation is falling all snow, and where sleet and/or freezing rain may be mixing in.
The National Weather Service relies on networks of volunteer weather observers to provide ‘ground truth’ in real time during winter storms. These individuals can help forecasters determine exactly how much precipitation, and what type, has fallen. The Community Collaborative Rain, Hail and Snow (CoCoRaHS) network is one such network. Any interested individual can become a volunteer weather observer. For more information, visit their website.
Here in New York’s Capital Region, we ended up with a respectable snowfall, even with the sleet which mixed in during the morning of January 20. In and around the Hudson Valley, between 12 and 18 inches of snow fell. Farther north and west where precipitation remained all snow, upwards of 18 inches fell.
The computer models indicate that the weather pattern will remain active this upcoming week. Be sure to stay connected with us via Facebook, where we disseminate National Weather Service watches and warnings, as well as other helpful hazardous weather tips!