Someone recently asked me “Can you explain to me how it is that we can get snow for 10 days straight, yet we never seem to get rain for 10 days straight?” Simply put, I can blame it on Lake Effect snow. In a nutshell, lake effect snow occurs whenever cooler, dry air passes over the warm lake, such as the Great Lakes. Last week, there was a thriving cold air setup with a strong northwesterly flow (winds coming in from the northwest caused by a low pressure system that had parked itself in eastern Canada and western New England for much of the week. That low pressure system was also responsible for the rather strong winds that were prevalent last week.

Basically, as that cold air moved over the lake, it picked up moisture off the lake, and carried it onto land in Pennsylvania and New York, where it deposited this built-up moisture as snow, and lots of it. Much of northwestern Pennsylvania saw several feet (in upwards of 40 inches) of snow, and these bands did extend far enough south to give much of southwestern Pennsylvania and the western Alleghenies decent accumulation as well (though nothing on that scale).

Because this low had stalled for several days, we were able to milk a lot of moisture out of the lake (think of it somewhat like a sponge) and get snowfall for several days. Lake-effect rain (especially earlier in autumn) is possible if conditions close to the ground are too warm to keep the precipitation snow, but the primary reason we don’t see prolonged periods of steady rainfall (on average) is that the cold air aloft required to produce “lake effect rain” is not in place, and the lake may not be as warm (Lake Erie’s temperature tends to peak in September) and normal rain-producing systems simply do not stay in one place long enough (again, this is a rule with exceptions). That’s not to say we can’t get a ton of rain in a short period of time in other times of the year, but not from the same mechanisms.

For more on lake-effect snow (and it’s associated counterparts), make the visit to Wikipedia (seriously, where else would you look?).