Surprise Snow Event
A swift kick in the teeth came from Mother Nature during approximately a four hour period from 7:00am through 10:30am on Saturday the 18th when heavy rain changed to heavy wet accumulating snow. The snow zone extended from the Catskills, Mohawk valley, Capital Region through southern Vermont, Berkshire county, MA and extended east through northern Massachusetts and southern New Hampshire to the coast. Moderate to heavy rain was forecast for the region from a fast moving low that originated over the Central Plains states on Friday and tracked across the Ohio valley, then south of New England on the 18th. So why did it snow?
Snow developed with this system for a couple of reasons. First, the air mass over the Northeast was cooling as the storm approached and contained dry air at levels from the ground to the mid levels of the atmosphere. As precipitation fell into the dry air layers, evaporation occurred which served to further cool the atmosphere. However, cooling from evaporation alone was insufficient to cool the air down at ground level enough to support snow. The second factor was a narrow zone of very strong lift in the atmosphere that the storm and it's associated jet stream and parent upper air low were able to create. The zone of strongest lift, as it was passing over New York and New England, caused substantial cooling due to rapid accent. The accent also produced heavy precipitation, which initially fell in the form of heavy rain. However, as the rapid accent continued to cool the column of air, snow levels began dropping. As the snow began forming and falling in the atmosphere it initially melted in the above freezing environment. The melting process removed heat from the atmosphere causing it to cool further. Eventually air temperatures dropped to freezing all the way down to the ground allowing snow to reach the ground and accumulate. Evidence of the banded nature of the storm is apparent in the weather observed at Glens Falls during the event. While snow was falling at Albany, under the zone of strong accent, light rain was falling at Glens Falls, where the air aloft was not rising as rapidly.
It is not typical in May for physical processes in the atmosphere to cool the lowest levels enough to support heavy accumulating snow. The combination of factors that lead to the surprise snow were not readily visible even within an hour or two of the snow phase of the storm commencing. It only became apparent that cooling processes in the atmosphere were great enough to support snow as the snow began falling.
Fortunately, the very quick motion of the storm prevented widespread excessive snow amounts which precluded significant tree and power line damage. However, some higher elevations locations (2000 feet and higher) in the Catskills and southern Vermont received anywhere from 4"-8" of snow where some trees and power lines did come down. Officially Albany recorded 2.2" of snow making it the 2nd heaviest May snowfall, as well as the first time it has ever snowed on May 18. The event was also the latest occurrence of measurable snow at Albany. The latest occurrence of a trace of snow at Albany is May 28, 1902. The table below lists the top five monthly snow totals for May as well as the top five daily snowfall records at Albany.
The following table is a listing of snow accumulations as reported by the WRGB WeatherNet 6 weather watchers as well as Albany National Weather Service Cooperative Observers.