IAP 12.310

January 2008
 Significant U.S. Weather Events

Ocean Effect Snow over the Cape (Jan 2, 2008)

How is it possible for Cape Cod to get 2 inches of snow and Boston not see a drop of liquid?

Similar to the physical processes generating lake-effect snow, when cold air (~<20 F) at the surface interacts with the warm ocean waters (>~38 F), there is instability.  This causes the snow showers to "pop-up" seen in the following radar image. 

(Look for Boston and Cape Cod.  The contoured colors give an indication on the precipitation strength.  Values less than 10 dBZ are generally considered "noise" and hence no precipitation; however values between 10 and 25 are "weak"; 25 to 45 are "moderate" and 45 and above are "strong" to "severe.")

From the radar image, notice how the snow showers developed mainly  over the ocean water.

One might be able to predict this "storm" simply by looking at the current conditions shown above.  The winds were generally going from North to South over Cape Cod.  This allows the cold air over the land (say from Maine) to interact with the warm ocean water, generate snow showers and move over the Cape.

So, how is it possible for Cape Cod to get 2 inches of snow and Boston not see a drop of liquid?  Well, there can be several scenarios. But on this particular night, because the upstream air mass for Boston was over the cold land, there was no instability and hence no generation of snow showers.  Meanwhile over the Cape, the upstream air mass was from the ocean waters.  When the warm waters detects the cold air mass, (similar to boiling a pot of water) instability occurs and snow showers develop.

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