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THE MAP ROOM
The Ocean-Effect Snowstorm of 14 January 1999
On 14 January 1999, a very unusual snowstorm occurred across eastern Massachusetts. It was not associated with synoptic-scale low pressure, or even a frontal boundary, yet up to a foot fell across many of the southern suburbs of Boston. This was an ocean-effect snowstorm, with some equally intriguing snow enhancement mechanisms added to make it unusually prolific.
Ocean effect can be considered a close cousin to lake-effect snow. Ocean-effect snow can form when a cold continental polar air mass moves offshore. The associated sensible heat and moisture flux from the ocean surface results in considerable boundary layer instability as the cold air moves over the relatively warmer waters. Similar to lake effect, a subsidence inversion associated with the continental polar air mass limits the depth of the convective boundary layer.
Along the shoreline, differential diabetic heating induces frontogenesis, while shore-parallel surface winds result in frictional convergence. This convergence and frontogenesis enhances and helps organize the convective activity in the unstable boundary layer, which is typically in the form of horizontal rolls.
Capes and peninsulas that jut out into the water body can also enhance convergence and thereby produce a locally stronger band.
In addition to the New England coast, ocean-effect snow has also been observed in the Gulf of St. Lawrence, the Bay of Fundy, the mouths of Delaware and Chesapeake Bays, and occasionally even in New York Harbor. Coastal New England typically experiences two to four ocean-effect events per winter. The orientation of the New England coastline favors a north, or north-northeast low-level flow (sometimes northeast as well, especially if the bands are already established). Snowfall from ocean effect is rarely more than a couple of inches. However, on rare occasions localized snowfall of 6-8 inches has been observed with persistent, longer-lived bands, particularly across locations where the land extends farther out into the ocean, such as Cape Ann (the peninsula north of Boston and just south of the New Hampshire line) and Cape Cod.
Figure 1 depicts the snowfall (in inches) recorded from the evening of 13 January through 14 January 1999. A band of 10-16 inches of snow fell south of Boston (BOS), with a maximum amount of...