Envision this setting: mid-day lunch-break on a brisk, bright December afternoon in Rochester. It’s one of those days where you think the blinding sun would warm the air slightly, however the air remains bitter. Now fast-forward an hour, you take a quick glance out your window to find blizzard conditions. This rapid change of conditions that Rochesterians are far too familiar with is lake-effect snow.
Although we likely won’t experience lake-effect snow for another month or so, that doesn’t mean upstate New York won’t be impacted by lake effect precipitation. Lake effect isn’t all about snow, as lake effect rain can occur in September and October.
Generally, cool air temperatures traveling over a much warmer body of water causes lake effect precipitation. Strong, chilly winds blow across a lake, picking up moisture from the water. The weather nerds call that latent heat flux. As the warmer air near the surface rises, it begins to cool and as a result is able to hold less moisture. This drop in temperature and overload of moisture causes the vapor in the air to undergo condensation (to liquid water) or deposition (to ice) forming clouds. When the water droplets or ice crystals grow to a large enough size precipitation falls from the clouds onto the downwind shores.
Upstate New York cities like Rochester are situated in the perfect position for lake effect precipitation. Throughout the year, there is a prevailing wind from the west or northwest over Lakes Ontario and Erie. Since Rochester is downwind, this region is continually pounded with lake effect precipitation from late fall, when the temperatures begin to cool, until March, which is when air temperatures come in line with the temperature of the lakes again.
One might wonder what determines if Rochester receives lake effect rain or lake effect snow. This solely depends on the air temperature. Temperatures greater than the freezing mark during a lake effect event will produce rain, or perhaps sleet; accordingly air temperatures below 32° Fahrenheit will produce snow. As a weather nerd in Upstate New York, lake effect precipitation is one of the most thrilling weather phenomena. Since lake effect precipitation is so localized, predicting where it will hit is almost an art. For example, Boonville, NY averages approximately 220 inches of snow annually, while Utica, NY averages just over 100 inches a year – a difference of 100 inches of snow in just 40 miles that mostly results from lake effect.
These drastic differences in annual snowfall occur near regions of high intensity lake-effect snowfall known as snowbelts. These are regions directly south and east of the body of water, essentially the kill zone for lake effect snow. When passing through these snowbelts traveling on I90, one will often experience snow bands with visibilities at times reduced to near zero. Take some advice from the weather wonk and always expect the unexpected when driving during lake-effect snow season.