Travel in Southern New England (Massachusetts, Rhode Island, Connecticut, southern Vermont and southern New Hampshire) can be severely disrupted by short duration, intense snowfall. In the worst cases, large and sometimes fatal traffic accidents occur, as drivers experience low visibility and sudden slippery roads. Such events are locally called “snow squalls” (Lundstedt, 1993) a term that is not defined in the American Meteorological Society’s Glossary of Meteorology. Similar occurrences have been reported in Pennsylvania (DeVoir, 2004), Virginia (DeVoir, 2004), Illinois (Pettegrew et al., 2009), and in Canada (Milrad et al, 2011 and 2014). Some scientists have referred to these events as “snow bursts” or “snowbursts”. The essential characteristics of these events are intense snowfall, a sudden loss of visibility, and gusty winds, lasting a short amount of time. The amount of snowfall is often less than 5 cm. In rare cases, thunder may be observed with the snowfall (Market et al., 2002).
Using these characteristics, we define a snow squall event as a line of snow showers (may be a broken line) at least 100 km in length, in which the visibility measured at an official National Weather Service observing site drops to 2.0 miles or less during the snowfall, and wind gusts are reported. Following this definition, we analyzed all of the Meteorological Terminal Aviation Routine Weather Reports (METARs) in Southern New England from November, 1997 through March, 2016, and found 31 events.
Of the seventeen events in the first thirteen years of this period, 1997 through 2009, fourteen formed in New York State as a line or lines, and moved into Southern New England. One event emerged out of a lake effect snowstorm on Lake Ontario, and the other two events were characterized by an area of short lines spread over Southern New England. Eleven of the linear cases involved a single line, while the other four had two or more. Line widths were between 18 and 25 km, lengths ranged from 180 to 700 km, and the maximum reflectivity in the lines was as small as 25 dBZ and as large as 48 dBZ. The height of the radar echoes ranged from 2.5 to 5.5 km. The speed of propagation was between 10 and 20 m/s.
Beginning in 2010, nine of the fourteen events were not characterized by a line or lines coming from New York State. In two cases, the lines formed in Eastern Massachusetts, while five of the events involved small lines that moved north to south, or in one case, south to north, along the coast. The overall frequency of snow squall events has remained constant, generally one or two per year, but in 2013 there were four events, all in March.
In all of the cases with eastward moving lines, there was an upper level trough axis that swung through Southern New England as the snow squalls moved through, and at the surface an arctic front or trough was present. The snow squall oriented themselves parallel to the front or trough, and there was often, but not always, a sharp temperature drop as the snow squall passed. The average temperature drop was 7ºC at the surface. For the snow squalls that moved north or south along the coast, there was also an upper level trough or low present, but it was located further to the east, and was frequently closed off.
This paper will show examples of the events, and highlight the differences between the types of events, and the forcing mechanisms at work in them. We will also address the apparent change in the typical mode of propagation that has shown up in the data.
References:
DeVoir, G. A., 2004: High impact sub-advisory snow events: The need to effectively communicate the threat of short duration high intensity snowfall. Preprints, 20th Conf. on Weather Analysis and Forecasting/16th Conf. on Numerical Weather Prediction, Seattle, WA, Amer. Meteor. Soc., P10.2. [Available online at http://ams.confex.com/ams/pdfpapers/68261.pdf.]
Lundstedt, W., 1993: A method to forecast wintertime instability and non-lake effect snow squalls across northern New England. NWS Eastern Region Tech. Attach. 93-11A, 13 pp.
Market, P. S., C. E. Halcomb, and R. L. Ebert, 2002: Aclimatology of thundersnow events over the contiguous United States. Wea. Forecasting, 17, 1290–1294.
Milrad, S. M., J. R. Gyakum, E.H. Atallah, and J. F. Smith, 2011: A diagnostic examination of the eastern Ontario and western Quebec wintertime convection of 28 January 2010. Wea. Forecasting, 26, 301–318.
Milrad, S. M., J. R. Gyakum, K. Lombardo, and E.H. Atallah, 2014:On the dynamics, thermodynamics, and forecast model evaluation of two snow-burst events in Southern Alberta. Wea. Forecasting, 29, 725–749.
Pettegrew, B. P., P. S. Market, R. A. Wolf, R. L. Holle, and N. W. S. Demetriades, 2009: A case study of severe winter convection in the Midwest. Wea. Forecasting, 24, 121–139.
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